The ability to generate random numbers can be useful in certain kinds of programs, particularly in games, statistics modeling programs, and scientific simulations that need to model random events. Take games for example -- without random events, monsters would always attack you the same way, you’d always find the same treasure, the dungeon layout would never change, etc… and that would not make for a very good game.

So how do we generate random numbers? In real life, we often generate random results by doing things like flipping a coin, rolling a dice, or shuffling a deck of cards. These events involve so many physical variables (e.g. gravity, friction, air resistance, momentum, etc…) that they become almost impossible to predict or control, and produce results that are for all intents and purposes random.

However, computers aren’t designed to take advantage of physical variables -- your computer can’t toss a coin, throw a dice, or shuffle real cards. Computers live in a controlled electrical world where everything is binary (false or true) and there is no in-between. By their very nature, computers are designed to produce results that are as predictable as possible. When you tell the computer to calculate 2 + 2, you *always* want the answer to be 4. Not 3 or 5 on occasion.

Consequently, computers are generally incapable of generating random numbers. Instead, they must simulate randomness, which is most often done using pseudo-random number generators.

A **pseudo-random number generator (PRNG)** is a program that takes a starting number (called a **seed**), and performs mathematical operations on it to transform it into some other number that appears to be unrelated to the seed. It then takes that generated number and performs the same mathematical operation on it to transform it into a new number that appears unrelated to the number it was generated from. By continually applying the algorithm to the last generated number, it can generate a series of new numbers that will appear to be random if the algorithm is complex enough.

It’s actually fairly easy to write a PRNG. Here’s a short program that generates 100 pseudo-random numbers:

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 |
#include <iostream> unsigned int PRNG() { // our initial starting seed is 5323 static unsigned int seed = 5323; // Take the current seed and generate a new value from it // Due to our use of large constants and overflow, it would be // hard for someone to casually predict what the next number is // going to be from the previous one. seed = 8253729 * seed + 2396403; // Take the seed and return a value between 0 and 32767 return seed % 32768; } int main() { // Print 100 random numbers for (int count=1; count <= 100; ++count) { std::cout << PRNG() << "\t"; // If we've printed 5 numbers, start a new row if (count % 5 == 0) std::cout << "\n"; } return 0; } |

The result of this program is:

23070 27857 22756 10839 27946 11613 30448 21987 22070 1001 27388 5999 5442 28789 13576 28411 10830 29441 21780 23687 5466 2957 19232 24595 22118 14873 5932 31135 28018 32421 14648 10539 23166 22833 12612 28343 7562 18877 32592 19011 13974 20553 9052 15311 9634 27861 7528 17243 27310 8033 28020 24807 1466 26605 4992 5235 30406 18041 3980 24063 15826 15109 24984 15755 23262 17809 2468 13079 19946 26141 1968 16035 5878 7337 23484 24623 13826 26933 1480 6075 11022 19393 1492 25927 30234 17485 23520 18643 5926 21209 2028 16991 3634 30565 2552 20971 23358 12785 25092 30583

Each number appears to be pretty random with respect to the previous one. As it turns out, our algorithm actually isn’t very good, for reasons we will discuss later. But it does effectively illustrate the principle of PRNG number generation.

**Generating random numbers in C++**

C (and by extension C++) comes with a built-in pseudo-random number generator. It is implemented as two separate functions that live in the cstdlib header:

**std::srand()** sets the initial seed value to a value that is passed in by the caller. srand() should only be called once at the beginning of your program. This is usually done at the top of main().

**std::rand()** generates the next random number in the sequence. That number will be a pseudo-random integer between 0 and RAND_MAX, a constant in cstdlib that is typically set to 32767.

Here’s a sample program using these functions:

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 |
#include <iostream> #include <cstdlib> // for std::rand() and std::srand() int main() { std::srand(5323); // set initial seed value to 5323 // Print 100 random numbers for (int count=1; count <= 100; ++count) { std::cout << std::rand() << "\t"; // If we've printed 5 numbers, start a new row if (count % 5 == 0) std::cout << "\n"; } return 0; } |

Here’s the output of this program:

17421 8558 19487 1344 26934 7796 28102 15201 17869 6911 4981 417 12650 28759 20778 31890 23714 29127 15819 29971 1069 25403 24427 9087 24392 15886 11466 15140 19801 14365 18458 18935 1746 16672 22281 16517 21847 27194 7163 13869 5923 27598 13463 15757 4520 15765 8582 23866 22389 29933 31607 180 17757 23924 31079 30105 23254 32726 11295 18712 29087 2787 4862 6569 6310 21221 28152 12539 5672 23344 28895 31278 21786 7674 15329 10307 16840 1645 15699 8401 22972 20731 24749 32505 29409 17906 11989 17051 32232 592 17312 32714 18411 17112 15510 8830 32592 25957 1269 6793

**PRNG sequences and seeding**

If you run the std::rand() sample program above multiple times, you will note that it prints the same result every time! This means that while each number in the sequence is seemingly random with regards to the previous ones, the entire sequence is not random at all! And that means our program ends up totally predictable (the same inputs lead to the same outputs every time). There are cases where this can be useful or even desired (e.g. you want a scientific simulation to be repeatable, or you’re trying to debug why your random dungeon generator crashes).

But often, this is not what is desired. If you’re writing a game of hi-lo (where the user has 10 tries to guess a number, and the computer tells them whether their guess is too high or too low), you don’t want the program picking the same numbers each time. So let’s take a deeper look at why this is happening, and how we can fix it.

Remember that each number in a PRNG sequence is generated from the previous number, in a deterministic way. Thus, given any starting seed number, PRNGs will always generate the same sequence of numbers from that seed as a result! We are getting the same sequence because our starting seed number is always 5323.

In order to make our entire sequence randomized, we need some way to pick a seed that’s not a fixed number. The first answer that probably comes to mind is that we need a random number! That’s a good thought, but if we need a random number to generate random numbers, then we’re in a catch-22. It turns out, we really don’t need our seed to be a random number -- we just need to pick something that changes each time the program is run. Then we can use our PRNG to generate a unique sequence of pseudo-random numbers from that seed.

The commonly accepted method for doing this is to enlist the system clock. Each time the user runs the program, the time will be different. If we use this time value as our seed, then our program will generate a different sequence of numbers each time it is run!

C comes with a function called time() that returns the number of seconds since midnight on Jan 1, 1970. To use it, we merely need to include the ctime header, and then initialize srand() with a call to std::time(nullptr) (or std::time(0) if your compiler is pre-C++11).

Here’s the same program as above, using a call to time() as the seed:

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 |
#include <iostream> #include <cstdlib> // for std::rand() and std::srand() #include <ctime> // for std::time() int main() { std::srand(static_cast<unsigned int>(std::time(nullptr))); // set initial seed value to system clock for (int count=1; count <= 100; ++count) { std::cout << std::rand() << "\t"; // If we've printed 5 numbers, start a new row if (count % 5 == 0) std::cout << "\n"; } return 0; } |

Now our program will generate a different sequence of random numbers every time! Run it a couple of times and see for yourself.

**Generating random numbers between two arbitrary values**

Generally, we do not want random numbers between 0 and RAND_MAX -- we want numbers between two other values, which we’ll call min and max. For example, if we’re trying to simulate the user rolling a die, we want random numbers between 1 and 6 (pedantic grammar note: yes, die is the singular of dice).

Here’s a short function that converts the result of rand() into the range we want:

1 2 3 4 5 6 7 8 9 |
// Generate a random number between min and max (inclusive) // Assumes std::srand() has already been called // Assumes max - min <= RAND_MAX int getRandomNumber(int min, int max) { static const double fraction = 1.0 / (RAND_MAX + 1.0); // static used for efficiency, so we only calculate this value once // evenly distribute the random number across our range return min + static_cast<int>((max - min + 1) * (std::rand() * fraction)); } |

To simulate the roll of a die, we’d call getRandomNumber(1, 6). To pick a randomized digit, we’d call getRandomNumber(0, 9).

**Optional reading: How does the previous function work?**

The getRandomNumber() function may seem a little complicated, but it’s not too bad.

Let’s revisit our goal. The function rand() returns a number between 0 and RAND_MAX (inclusive). We want to somehow transform the result of rand() into a number between min and max (inclusive). This means that when we do our transformation, 0 should become min, and RAND_MAX should become max, with a uniform distribution of numbers in between.

We do that in five parts:

- We multiply our result from std::rand() by fraction. This converts the result of rand() to a floating point number between 0 (inclusive), and 1 (exclusive).
If rand() returns a 0, then 0 * fraction is still 0. If rand() return RAND_MAX, then RAND_MAX * fraction is RAND_MAX / (RAND_MAX + 1), which is slightly less than 1. Any other number returned by rand() will be evenly distributed between these two points.

- Next, we need to know how many numbers we can possibly return. In other words, how many numbers are between min (inclusive) and max (inclusive)?
This is simply (max - min + 1). For example, if max = 8 and min = 5, (max - min + 1) = (8 - 5 + 1) = 4. There are 4 numbers between 5 and 8 (that is, 5, 6, 7, and 8).

- We multiply the prior two results together. If we had a floating point number between 0 (inclusive) and 1 (exclusive), and then we multiply by (min - max + 1), we now have a floating point number between 0 (inclusive) and (max - min + 1) (exclusive).
- We cast the previous result to an integer. This removes any fractional component, leaving us with an integer result between 0 (inclusive) and (max - min) (inclusive).
- Finally, we add min, which shifts our result to an integer between min (inclusive) and max (inclusive).

**Optional reading: Why don’t we use the modulus operator (%) in the previous function?**

One of the most common questions readers have submitted is why we use division in the above function instead of modulus (%). The short answer is that modulus tends to be biased in favor of low numbers.

Let’s consider what would happen if the above function looked like this instead:

1 |
return min + (std::rand() % (max-min+1)); |

Seems similar, right? Let’s explore where this goes wrong. To simplify the example, let’s say that rand() always returns a random number between 0 and 9 (inclusive). For our sample case, we’ll pick min = 0, and max = 6. Thus, max - min + 1 is 7.

Now let’s calculate all possible outcomes:

0 + (0 % 7) = 0 0 + (1 % 7) = 1 0 + (2 % 7) = 2 0 + (3 % 7) = 3 0 + (4 % 7) = 4 0 + (5 % 7) = 5 0 + (6 % 7) = 6 0 + (7 % 7) = 0 0 + (8 % 7) = 1 0 + (9 % 7) = 2

Look at the distribution of results. The results 0 through 2 come up twice, whereas 3 through 6 come up only once. This method has a clear bias towards low results. By extension, most cases involving this algorithm will behave similarly.

Now lets take a look at the result of the getRandomNumber() function above, using the same parameters as above (rand() returns a number between 0 and 9 (inclusive), min = 0 and max = 6). In this case, fraction = 1 / (9 + 1) = 0.1. max - min + 1 is still 7.

Calculating all possible outcomes:

0 + static_cast(7 * (0 * 0.1))) = 0 + static_cast (0) = 0 0 + static_cast (7 * (1 * 0.1))) = 0 + static_cast (0.7) = 0 0 + static_cast (7 * (2 * 0.1))) = 0 + static_cast (1.4) = 1 0 + static_cast (7 * (3 * 0.1))) = 0 + static_cast (2.1) = 2 0 + static_cast (7 * (4 * 0.1))) = 0 + static_cast (2.8) = 2 0 + static_cast (7 * (5 * 0.1))) = 0 + static_cast (3.5) = 3 0 + static_cast (7 * (6 * 0.1))) = 0 + static_cast (4.2) = 4 0 + static_cast (7 * (7 * 0.1))) = 0 + static_cast (4.9) = 4 0 + static_cast (7 * (8 * 0.1))) = 0 + static_cast (5.6) = 5 0 + static_cast (7 * (9 * 0.1))) = 0 + static_cast (6.3) = 6

The bias here is still slightly towards lower numbers (0, 2, and 4 appear twice, whereas 1, 3, 5, and 6 appear once), but it’s much more uniformly distributed.

Even though getRandomNumber() is a little more complicated to understand than the modulus alternative, we advocate for the division method because it produces a less biased result.

**What is a good PRNG?**

As I mentioned above, the PRNG we wrote isn’t a very good one. This section will discuss the reasons why. It is optional reading because it’s not strictly related to C or C++, but if you like programming you will probably find it interesting anyway.

In order to be a good PRNG, the PRNG needs to exhibit a number of properties:

First, the PRNG should generate each number with approximately the same probability. This is called distribution uniformity. If some numbers are generated more often than others, the result of the program that uses the PRNG will be biased!

For example, let’s say you’re trying to write a random item generator for a game. You’ll pick a random number between 1 and 10, and if the result is a 10, the monster will drop a powerful item instead of a common one. You would expect a 1 in 10 chance of this happening. But if the underlying PRNG is not uniform, and generates a lot more 10s than it should, your players will end up getting more rare items than you’d intended, possibly trivializing the difficulty of your game.

Generating PRNGs that produce uniform results is difficult, and it’s one of the main reasons the PRNG we wrote at the top of this lesson isn’t a very good PRNG.

Second, the method by which the next number in the sequence is generated shouldn’t be obvious or predictable. For example, consider the following PRNG algorithm: `num = num + 1`

. This PRNG is perfectly uniform, but it’s not very useful as a sequence of random numbers!

Third, the PRNG should have a good dimensional distribution of numbers. This means it should return low numbers, middle numbers, and high numbers seemingly at random. A PRNG that returned all low numbers, then all high numbers may be uniform and non-predictable, but it’s still going to lead to biased results, particularly if the number of random numbers you actually use is small.

Fourth, all PRNGs are periodic, which means that at some point the sequence of numbers generated will eventually begin to repeat itself. As mentioned before, PRNGs are deterministic, and given an input number, a PRNG will produce the same output number every time. Consider what happens when a PRNG generates a number it has previously generated. From that point forward, it will begin to duplicate the sequence between the first occurrence of that number and the next occurrence of that number over and over. The length of this sequence is known as the **period**.

For example, here are the first 100 numbers generated from a PRNG with poor periodicity:

112 9 130 97 64 31 152 119 86 53 20 141 108 75 42 9 130 97 64 31 152 119 86 53 20 141 108 75 42 9 130 97 64 31 152 119 86 53 20 141 108 75 42 9 130 97 64 31 152 119 86 53 20 141 108 75 42 9 130 97 64 31 152 119 86 53 20 141 108 75 42 9 130 97 64 31 152 119 86 53 20 141 108 75 42 9 130 97 64 31 152 119 86 53 20 141 108 75 42 9

You will note that it generated 9 as the second number, and 9 again as the 16th number. The PRNG gets stuck generating the sequence in-between these two 9’s repeatedly: 9-130-97-64-31-152-119-86-53-20-141-108-75-42-(repeat).

A good PRNG should have a long period for *all* seed numbers. Designing an algorithm that meets this property can be extremely difficult -- most PRNGs will have long periods for some seeds and short periods for others. If the user happens to pick a seed that has a short period, then the PRNG won’t be doing a good job.

Despite the difficulty in designing algorithms that meet all of these criteria, a lot of research has been done in this area because of its importance to scientific computing.

**std::rand() is a mediocre PRNG**

The algorithm used to implement std::rand() can vary from compiler to compiler, leading to results that may not be consistent across compilers. Most implementations of rand() use a method called a Linear Congruential Generator (LCG). If you have a look at the first example in this lesson, you’ll note that it’s actually a LCG, though one with intentionally picked poor constants. LCGs tend to have shortcomings that make them not good choices for most kinds of problems.

One of the main shortcomings of rand() is that RAND_MAX is usually set to 32767 (essentially 15-bits). This means if you want to generate numbers over a larger range (e.g. 32-bit integers), rand() is not suitable. Also, rand() isn’t good if you want to generate random floating point numbers (e.g. between 0.0 and 1.0), which is often useful when doing statistical modelling. Finally, rand() tends to have a relatively short period compared to other algorithms.

That said, rand() is perfectly suitable for learning how to program, and for programs in which a high-quality PRNG is not a necessity.

For applications where a high-quality PRNG is useful, I would recommend Mersenne Twister (or one of its variants), which produces great results and is relatively easy to use.

rand() may return the same first value on successive calls on some compilers
The implementation of rand() in Visual Studio and a few other compilers has a flaw -- the first random number generated doesn’t change much for similar seed values. This means that when using time() to seed your random number generator, the first result from rand() won’t change much in successive runs. This problem is compounded by calling getRandomNumber(), which compresses similar inputs into the same output number. The results of successive calls to rand() aren’t impacted. However, there’s an easy fix: call rand() once and discard the result. Then you can use rand() as normal in your program. Recommendation: After calling srand(), call rand() once and discard the first result to avoid this issue. |

**Debugging programs that use random numbers**

Programs that use random numbers can be difficult to debug because the program may exhibit different behaviors each time it is run. Sometimes it may work, and sometimes it may not. When debugging, it’s helpful to ensure your program executes the same (incorrect) way each time. That way, you can run the program as many times as needed to isolate where the error is.

For this reason, when debugging, it’s a useful technique to set the random seed (via srand) to a specific value (e.g. 0) that causes the erroneous behavior to occur. This will ensure your program generates the same results each time, making debugging easier. Once you’ve found the error, you can seed using the system clock again to start generating randomized results again.

**Random numbers in C++11**

C++11 added a ton of random number generation functionality to the C++ standard library, including the Mersenne Twister algorithm, as well as generators for different kinds of random distributions (uniform, normal, Poisson, etc…). This is accessed via the <random> header.

Here’s a short example showing how to generate random numbers in C++11 using Mersenne Twister (h/t to user Fernando):

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 |
#include <iostream> #include <random> // for std::mt19937 #include <ctime> // for std::time int main() { // Initialize our mersenne twister with a random seed based on the clock std::mt19937 mersenne(static_cast<unsigned int>(std::time(nullptr))); // Create a reusable random number generator that generates uniform numbers between 1 and 6 std::uniform_int_distribution<> die(1, 6); // Print a bunch of random numbers for (int count = 1; count <= 48; ++count) { std::cout << die(mersenne) << "\t"; // generate a roll of the die here // If we've printed 6 numbers, start a new row if (count % 6 == 0) std::cout << "\n"; } return 0; } |

You’ll note that Mersenne Twister generates random 32-bit unsigned integers (not 15-bit integers like std::rand()), giving a lot more range. There’s also a version (std::mt19937_64) for generating 64-bit unsigned integers!

There’s so much functionality in <random> that it really warrants its own section. We’ll look to cover that in a future lesson in more detail.

5.10 -- std::cin, extraction, and dealing with invalid text input |

Index |

5.8 -- Break and continue |

The comment next to this piece of code confused me:

static const double fraction = 1.0 / (RAND_MAX + 1.0); // static used for efficiency, so we only calculate this value once

It's not the static that makes it calculate once, but the const. If you just declared a static variable without the const it would still be calculated each time you call the function.

Hi Nir!

No, it's as Alex said. Static variables are initialized once, meaning your initializer is only calculated once. const only means that the value cannot be changed after the variable has been initialized.

Example

Thanks a lot. That clears it up!

Hi Alex!

Maybe you can update the use of std::time(0) in the examples to be std::time(nullptr). The parameter in this function is a pointer, and as you know the modern way of having a null-pointer is nullptr, not 0 or NULL.

I get a warning in my compiler (MinGW and Clang in Qt 5.11) when using 0 as a pointer now, so I have to fix these old style ways of writing code!

Regards, Nils

Good call. Updated! Thanks.

"hard for someone to casually predict what the next number is"

Is this supposed to say causally instead of casually?

No, I did mean casually. If you were motivated, given a sequence of "random" results from a PRNG, you could probably write a program that could extrapolate the PRNG parameters. Then you'd be able to predict the next number, which makes it not so random.

But that's a lot of work, and most people won't do that.

Hello. I am having trouble with rand(). Trying your function above for 100 rolls of a six sided die, I am getting the same first number each time I run the program. Is it one of the shortcomings you mentioned above, something with my compiler, or am I doing something wrong?

Hi Max!

Read the box titled "rand() may return the same first value on successive calls on some compilers"

Thanks. That answers my question. Sorry I missed it.

Alright, so, I'm having some problems implementing that last line of code.

When I try to implement it, I run into the problem that I can't figure out how to forward declare it so that other files besides the initial one its declared in can use it.

When I define it in multiple files, the compiler gets mad because I'm defining a function multiple times across the multiple files.

When I don't define it in multiple files, it gets mad because it can't find the generate function I've created there.

And I'm not sure how to do the forward declaration, as the the generate function isn't created the same way most of the other functions I've used so far have been.

It works great in single file programs by the way, no problem there.

I had another friend answer my question, but for future posterity, the answer we figured out was to put

into one of the header files. That worked like a dream.

But regardless, thank you for giving me the mersenne code piece, it is very nice to not have to use the usual PRNG and it's occasional obnoxiousness.

For anyone looking for more information about "extern", it's covered in lesson 4.2.

References

Lesson 4.2 - Global variables and linkage

Here is write a code in codeblocks that is same as you but i get many errors. plz try to fix it out

and errors are these

error: no match for call to '(std::random_device) (int)'|

random.h|1616|note: candidate: std::random_device::result_type std::random_device::operator()()|

random.h|1616|note: candidate expects 0 arguments, 1 provided|

main.cpp|12|error: expected primary-expression before '<<' token|

and i have also one other doubt that how we can check that which c++ language version is using

Hi Vitrag!

Your code is different in line 8.

ok thanks, but ans my second doubt .How we can check that which language we are using in codeblock

c++ or c++11 etc...

I don't have codeblocks, according to this picture ( https://images.duckduckgo.com/iu/?u=https%3A%2F%2Fwww.diprimio.com%2Fimages%2Fpictures%2FBlog%2Fhowto%2Fcodeblocks-compiler-settings.png&f=1 )

you can select the version you want to use.

thanks for the help 😉

Hi Alex,

I know this is a little off topic but somewhere in my distant memory (I think from C days when PCs ran MS-DOS) I recall always using

for input / output functions. Are these now all covered in the

header or are they still necessary sometimes?

Hi Nigel!

stdio.h and stdlib.h are C headers, you should use cstdio and cstdlib in C++.

Both headers, and iostream, are still used, have a look at the references down below to see what they contain.

References

<cstdio> - http://www.cplusplus.com/reference/cstdio/

<cstdlib> - http://www.cplusplus.com/reference/cstdlib/

<iostream> - http://www.cplusplus.com/reference/iostream/

Thanks, it will help me avoid making mistakes in the future then.

Hello!

Isn't the number of seconds from January 1970 or whatsoever, already an integer? Why do we use a static cast over here?:

Hi Sid!

@std::time returns an @std::time_t type. @std::time_t is implementation specific.

References

* std::time - https://en.cppreference.com/w/cpp/chrono/c/time

* std::time_t - https://en.cppreference.com/w/cpp/chrono/c/time_t

Hello!

I don;t understand why we do this over here:

How does this return a value between 0 and 32767?

Hi Sid!

The modulus operator is explained in lesson 3.2.

References

* Lesson 3.2 - Arithmetic operators, section "Modulus (remainder)"

Thanks!

Hey Alex! Huge thanks for the amazing tutorials!

I have a question:

When I try to put the Mersenne Twister inside a function, it generates the same numbers when I'm trying to print 10 numbers using a for loop.

It generates different 10 number sequences each time I run the program, but the numbers are still the same, like this:

2 2 2 2 2 2 2 2 2 2

7 7 7 7 7 7 7 7 7 7

3 3 3 3 3 3 3 3 3 3

How can I fix that? Thank you!

Hi Gleb!

You're only calling @generateRandomNumber once, storing the result in @x and printing @x 10 times. But @x isn't changing in between the cycles. You need to call @generateRandomNumber inside the loop.

Also,

* Don't use "using namespace"

* Use uniform initialization

* @mersenneTwister and @range should be static

Hey nascardriver!

Thanks a lot for the answer, it works now!

About "using namespace std;": does it matter in this particular case?

It doesn't in this case, but you might run into name collisions in some other project and getting rid of a bad habit is harder than doing it correct in the first place.

Got it, thanks again!

Hello Alex,

I think you've made a typo on line 2 in the third piece of code. It should be "std::rand()", right? 🙂

Thank you very much for these tutorials, I'm very happy that I've found them.

Yes, I lost an 'r'. Thanks for the typo fix!

Dear Alex,

Your website is really great. You don't know how much this site has helped me in learning c++.

But I do have 1 question.

you said all headers and functions live in the standard library. So why should I do this std::cout, std::cin, std::endl etc. but in the examples above you write srand() and rand() but not std::srand() and std::rand()??

Oversight on my part. I've updated the examples to use std::rand(), std::srand(), and std::time() rather than their non-std:: counterparts.

why does it work without the std:: prefix tho?? that's what I wanted to know.

Hi Hexx!

The <*.h> headers guarantee declaration in the global namespace, the <c*> headers don't guarantee it but most implementations still declare the functions both globally and in the @std namespace.

Use the <c*> headers and the namespaced functions to avoid name collisions and improve compiler compatibilty.

Hi Teachers, so I was trying the die thing and this is my function:

and this is the main:

Why does the program print the same number every time I run it? Am I doing something wrong?

PD. This are my includes:

Hi ArmaIG!

See "A note for Visual Studio users (and possibly others)" and try if it changes anything. Your code works for me (g++ 8.1.0).

Oh yes! Thank you, it worked! Had to discard rand() twice tho...

Hi Alex,

Many thanks for this fantastic resource!

Question - why is it that we do a static_cast to an unsigned int on the return value of ctime time()?

srand(static_cast<unsigned int>(time(0)));

Could we simplify it with?:

srand(time(0));

I would have assumed (incorrectly) that time would have returned an unsigned int anyway?

Cheers!

Adam

Technically, time() returns a value of type time_t. Most often, this is implemented as an integral type, but not always. The explicit cast is used to explicitly specify that we want it to convert it into an unsigned int, so we can use it with srand().

Hey Alex. I was writing a function to generate numbers and whenever I put srand(time(0)) inside the random number function, it just keeps giving me same numbers. Only when I put srand(time(0)) inside the main function was it solved.

int generateRandomNumber(int min = 1, int max = 100)

{

//srand(time(0)); // kept causing me problems until I commented it out

static const double fraction = 1.0 / (RAND_MAX + 1.0);

return min + static_cast<int>((max - min + 1)*(rand() * fraction));

}

int main()

{

srand(time(0));

for (int count = 1; count < 100; ++count)

{

cout << generateRandomNumber() << "\t";

if (count % 5 == 0)

cout << '\n';

}

}

Thing is, I still don't get why srand(time(0)) shouldn't be inside the generateRandomNumber() function. Could you share me any insight on this?

Hi Davis!

"srand() sets the initial seed value to a value that is passed in by the caller. srand() should only be called once at the beginning of your program."

"rand() generates the next random number in the sequence."

time(0) will return the same number every time you call it, because your loop doesn't take one second to execute. You're seeding the random number generator with the same seed before every call to @rand. @rand starts out with the same number every time so you get the same result with every call.

When calling @srand only once @rand can modify the number used to calculate the next number in the sequence so you get different results.

Thanks a million! It was causing me so much trouble.

hi i want to to creat a client seed to hit 0 number everytime in dice game.n its numbers from 0 to 100.... can u help me ... plz reply me in my mail .. aliyousaf723@gmail.com

Hi ali!

If you want to get 0 every time, you can't use a random number, because it will change. If you want 0 once, you'll have to look at how your system implements the random number generation and solve the equation for 0.

But since you want 0 every time, just don't use random numbers at all.

In using the mersenne twister example above, the output was identical on multiple iterations of running the program. What does the "std::random_device rd;" statement do? And why would that be used instead of using the system clock?

std::random_device is supposed to be a uniformly distributed integer random number generator. It's supposed to be non-deterministic, however, on some compilers (e.g. MinGW) it apparently generates the same sequence. Given that, it may be better to just use the system clock.

Hey Alex. I noticed when doing the chapter 5 quiz in CodeBlocks that the same problem as in Visual Studio occurs, where the first number generated doesn't change much over consecutive runs. It wasn't until I discarded the first call of rand() in my program that the first random number would be different to the previous run's. Might be worth mentioning that in the section mentioning it (after validating it of course, I'm not perfect and just a clueless beginner), seeing as it's one of the IDEs you recommend in the beginning of the tutorial.

Cheers!

this has been mentioned in the tutorial,

Hey Alex,

I bumped into this searching for a program I was doing"

it actually produced a very high quality random number set but I have no clue about what this is?

can you elaborate and maybe introduce some resources on how to learn more?

thank you

sorry didn't check your website recently, please feel free to delete my comments.

A few comments/questions/ideas

• First program line 14: unnecessary parentheses.

• Perhaps a word about the best choices for seeds and other constants. Here we make the constants primes:

• You may wish to consider the use of the phrase “uniform distribution” rather than “even distribution.”

• Section Optional reading: Why don’t we use the modulus operator (%) in the previous function?

If you want a uniform distribution using the modulus function, then one way to get it would be to set RAND_MAX to one less than a multiple of 7, such as 13 (assuming that doing so is possible). In your example, you would then get the perfectly uniform distribution you desire:

0 + (0 % 7) = 0

0 + (1 % 7) = 1

0 + (2 % 7) = 2

0 + (3 % 7) = 3

0 + (4 % 7) = 4

0 + (5 % 7) = 5

0 + (6 % 7) = 6

0 + (7 % 7) = 0

0 + (8 % 7) = 1

0 + (9 % 7) = 2

0 + (10%7) = 3

0 + (11%7) = 4

0 + (12%7) = 5

0 + (13%7) = 6

Of course, if you are creating your own PRNG, it will be possible to set the equivalent of RAND_MAX.

Suppose, instead, you wanted to use the built in rand() function instead of creating your own. Since RAND_MAX is typically 32,767 = 7*31*151 we would get exactly 4,681 values for each of the possible returned values modulo 7 except for one more zero because both 0 and 32,767 modulo 7 is zero. By eliminating any return of zero before the modulus operation, we can make the distribution perfectly uniform and we don’t need any floating point operations at all.

• Visual Studio rand() implementation bug. Could you explain the bug details? Has it still not been fixed?

• Debugging PRNG idea: In some cases you might want to consider using something even simpler for debugging, such as an incrementing integer.

Thanks for all the great feedback. The lesson has been tweaked per your comments.

The rand() bug is still present in visual studio as of Visual Studio 2017. srand() is used to seed the random generator -- however, when using the system clock to seed (which is by far the most common way to seed), the first call to rand() produces a similar (often identical value). It's not until the second call to rand() that these diverge. In my opinion, that's a bug.

Regarding the statement in the first program that reads, "Due to our use of large constants and overflow, it would be very hard for someone to predict what the next number is going to be from the previous one." To clarify, I suggest adding the following phrase to this sentence: "without knowing what those constants are."

Thanks for the feedback. Instead, I changed the term "predict" to "casually predict".

Hello guys!

First of all, thank you so much for such a good tutorial. The quizzes and code questions are challenging and nice to work on them. Thank you also for promptly answering our questions and helping us with all these stuffs.

Now, may I ask you to compare the solution above(mersenne) with this one:

Don't get me wrong, I'm asking you this because it was taught to me without major explanations. Besides, I compared the results generated by both solutions and, at least to me, they were pretty similar, and I think this code is simpler, but I'm not sure it's more efficient and what's the difference between them.

Thx.

Your solution is strictly better, as it uses more functionality from the standard library instead of a user-created function. I've updated the example in the lesson accordingly, and have given you a hat tip. Thanks!

In the getRandomNumber() function, I'm confused with why you are doing a manual type casting to the constant RAND_MAX at the following statement.

static const double fraction = 1.0 / (static_cast<double>(RAND_MAX) + 1.0);

I do get warnings from the complier without it, but I have no idea why.

Hi Qing!

I don't know why Alex added a cast here, RAND_MAX + 1.0 will evaluate to a double anyway.

My compiler is not producing any warnings with this code

Which compiler are you using and what's the output?

You're right, the cast isn't needed. It was a leftover artifact from a previous version of this function.

what does these 2 lines does?

Creates a mersenne twister random number generator, seeded using a std::random_device object (you can use time(0) instead if you want to seed based on the system clock).

Hey Alex,

I wanted to think of my own way to limit generated numbers (I use the marsenne twister) to a range between two numbers (e.g. 0 to 100)

I thought of two things:

1) return min + (mersenne() % (max - min)); //Here I limit the max number with the % operator.

2) while(number > max) {number /= 10;} //Here, in case the number is bigger than max, divide it by 10 until it's not.

Also a quick note: I noticed that you already responded to some people about the remainder operator, so no need to explain what the problem with method 1 is. In conclusion:

1)Problem with method 1 is less evenly distributed numbers across the range.

2)Method 2 problem is extra iterations (So with a big number the loop would possibly have to run like 5,6 times to get the number, which is less efficient.)

My questions are:

1)Is method 1 OK, even though it will not distribute numbers as well?

2)Is method 2 OK, or is it too inefficient?

3)Or are both not OK and I should use the exact method you use in this section with the fraction?

Final note: I know there are some other flaws, but I am currently just concerned with those 3 questions.

Thanks for your time.

My opinions:

#1 is pretty bad as a general solution. In extreme cases, low numbers may appear twice as often as high numbers. That's a pretty severe skewing. So I'd say no, this isn't okay.

#2 is also pretty bad as a general solution. If you wanted to generate the results of a coin flip (e.g. a random number that is either 0 or 1) this could take thousands of iterations before you randomly landed on a 0 or a 1. So I'd say no, this isn't okay either.

I think it's great that you're thinking about these on your own and understanding the limitations. But why would you want to use a suboptimal solution when a better solution is being handed to you?

Hi, I'm confused, why are you using this "static const double fraction" formula in all advanced methods instead of the modulus operator like in the first function? Is this used to create a perfect distribution uniformity?

Could someone explain me this function?

I updated the lesson to talk about how the non-mersenne version of this function works. The mersenne version works the same way, just using the mersenne generator instead of rand().

If you are working with a uniform distribution can you use % instead to get a random number in the desired range like randomnumber = min + ( rand() % max-min+1) ?

You can, but it will result in an uneven distribution, particularly if max and min have a wide range.

I updated the lesson to talk about this a bit.

Dear Alex,

Thanks for the note on the Visual Studio fault with using rand! I kept getting the same three "random" numbers no matter when I ran the program. Now my program works like a charm!

Since I was doing my program for college class, I need to cite all sources of any help I obtained on the web. Your article only says "Alex" for the author. Do you have a link that has the information I would need for an APA citation on this website? I want to give credit where credit is due, especially for such a nice article that was easy for a newbie to understand.

Thanks again for your help!

Sorry for the slow response on this. Try http://www.citationmachine.net/apa/cite-a-website/manual with "Alex" as the last name. The rest of the information (title, publish date, etc...) you can glean from the website.

Lack of a full name shouldn't matter as long as it's clear what site it's from. After all, Voltaire didn't use a last name either.

I am wondering how I can adjust the getRandomNumber to not use rand() and still get a value between a given range, and not outside of it.

I am working on an assignment, and need to do a priority scheduler without the rand() or system time. I am wondering is there a way where I can generate between to given numbers 0-9?

You could write your own PRNG like in the first example. It probably wouldn't be very good, but it would be better than nothing. Alternatively, if your result doesn't need to be random, you could use a static variable to create just increment through the numbers 0 through 9.

Outside of that, I'm not really sure.