# 2.6 — Boolean values and an introduction to if statements

In real-life, it’s common to ask or be asked questions that can be answered with “yes” or “no”. “Is an apple a fruit?” Yes. “Do you like asparagus?” No.

Now consider a similar statement: “Apples are a fruit”. Is this statement true or false? It’s clearly true. Or how about, “I like asparagus”. Absolutely false (yuck!).

These kinds of sentences that have only two possible outcomes: yes/true, or no/false are so common, that many programming languages include a special type for dealing with them. That type is called a boolean type.

Boolean variables

Boolean variables are variables that can have only two possible values: true (1), and false (0).

To declare a boolean variable, we use the keyword bool.

To initialize or assign a true or false value to a boolean variable, we use the keywords true and false.

Just as the unary minus operator (-) can be used to make an integer negative, the logical NOT operator (!) can be used to flip a boolean value from true to false, or false to true:

Boolean values are not actually stored in boolean variables as the words “true” or “false”. Instead, they are stored as integers: true becomes the integer 1, and false becomes the integer 0. Similarly, when boolean values are evaluated, they don’t actually evaluate to “true” or “false”. They evaluate to the integers 0 (false) or 1 (true).

Consequently, when we print boolean values with std::cout, std::cout prints 0 for false, and 1 for true:

Outputs:

```1
0
0
1
```

If you want std::cout to print “true” or “false” instead of 0 or 1, you can use std::boolalpha:

This prints:

```1
0
true
false
```

You can use std::noboolalpha to turn it back off.

A first look at booleans in if-statements

One of the most common uses for boolean variables is inside if statements. If statements typically take the following form:

```if (expression) statement1;
```

or

```if (expression) statement1;
else statement2;
```

When used in the context of an if statement, the expression is sometimes called a condition or conditional expression.

In both forms of the if statement, expression is evaluated. If expression evaluates to a non-zero value, then statement1 is executed. In the second form, if expression evaluates to a zero value, then statement2 is executed instead.

Remember that true evaluates to 1 (which is a non-zero value) and false evaluates to 0.

Here’s a simple example:

prints:

```The condition is true
```

Let’s examine how this works. First, we evaluate the conditional part of the if statement. The expression “true” evaluates to value 1, which is a non-zero value, so the statement attached to the if statement executes.

The following program works similarly:

prints:

```b is false
```

In the above program, when the condition evaluates, variable b evaluates to its value, which in this case is false. False evaluates to value 0. Consequently, the statement connected to the if statement does not execute, but the else statement does.

Executing multiple statements

In a basic form of an if statement presented above, statement1 and statement2 may only be a single statement. However, it’s also possible to execute multiple statements instead by placing those statements inside curly braces ({}). This is called a block (or compound statement). We cover blocks in more detail in lesson 4.1 -- Blocks (compound statements).

An if or if-else using multiple statements takes the form:

```if (expression)
{
statement1a;
statement1b;
//  etc
}
```

or

```if (expression)
{
statement1a;
statement1b;
// etc
}
else
{
statement2a;
statement2b;
// etc
}
```

For example:

This prints:

```The condition is true
And that's all, folks!
```

A slightly more complicated example

The equality operator (==) is used to test whether two integer values are equal. Operator== returns true if the operands are equal, and false if they are not.

Here’s output from one run of this program:

```Enter an integer: 4
The value is non-zero
```

Let’s examine how this works. First, the user enters an integer value. Next, we use operator== to test whether the entered value is equal to the integer 0. In this example, 4 is not equal to 0, so operator== evaluates to the value false. This causes the else part of the if statement to execute, producing the output “The value is non-zero”.

Boolean return values

Boolean values are often used as the return values for functions that check whether something is true or not. Such functions are typically named starting with the word is (e.g. isEqual) or has (e.g. hasCommonDivisor).

Consider the following example, which is quite similar to the above:

Here’s output from one run of this program:

```Enter an integer: 5
Enter another integer: 5
5 and 5 are equal
```

How does this work? First we read in integer values for x and y. Next, the conditional expression “isEqual(x, y)” is evaluated. This results in a function call to isEqual(5, 5). Inside that function, 5 == 5 is evaluated, producing the value true (since 5 is equal to 5). That value is returned back to the caller. Since the conditional evaluated to true, the statement attached to the if executes, producing the output “5 and 5 are equal”.

Boolean values take a little bit of getting used to, but once you get your mind wrapped around them, they’re quite refreshing in their simplicity!

Non-boolean conditionals

In all of the examples above, our conditionals have been either boolean values (true or false), boolean variables, or functions that return a boolean value. What happens if your conditional is an expression that does not evaluate to a boolean value?

You already know the answer: If the conditional evaluates to a non-zero value, then the statement associated with the if statement executes.

Therefore, if we do something like this:

This will print “hi”, since 4 is a non-zero value.

Inputting boolean values

Inputting boolean values using std::cin sometimes trips new programmers up.

Consider the following program:

```Enter a boolean value: true
You entered: 119
```

Wait, what?

It turns out that std::cin only accepts two inputs for boolean variables: 0 and 1 (not true or false). Any other inputs will cause std::cin to silently fail. In this case, because we entered “true”, std::cin silently failed and didn’t assign a value to b. Consequently, when std::cout printed a value for b, it printed whatever uninitialized value was in variable b. Garbage!

Quiz

1) A prime number is a whole number greater than 1 that can only be divided evenly by 1 and itself. Write a program that asks the user to enter a single digit integer. If the user enters a single digit that is prime (2, 3, 5, or 7), print “The digit is prime”. Otherwise, print “The digit is not prime”.

Hint: Use if statements to compare the number the user entered to the prime numbers. Use a boolean to keep track of whether the user entered a prime number or not.

 2.7 -- Chars Index 2.5 -- Floating point numbers

### 476 comments to 2.6 — Boolean values and an introduction to if statements

• Jeff

So I'm superbly confused, I notice in the answer to the question he initializes a statement of:

Why is this necessary? When I write the code as follows it executes correctly, despite not having that statement included... I'm just confused as to any pros/cons with including or not including that statement. This is my code that ran properly:

• Alex

Using the prime variable isn't necessary. It was a stylistic choice that I employed to try and make each statement a bit simpler to understand.

If I were writing this code for myself, I would have written it as you did -- without the intermediary variable.

• yitzi

Hi Alex
I don't think this quiz is fair
because you have not taught us yet about multiple "if's"

• Alex

Why would you assume that you can only use one if statement per program or function?

A function is just a sequence of statements -- those statements can be if statements, or any other kinds of statements.

• R310

Hi :)!

What does ''trip up'' means? Is it to scare?

Why don't you use endl in those?

• nascardriver

Hi R310!

> What does ''trip up'' means?

> Why don't you use endl in those?
@std::cin::operator>> waits for the user to press enter. When the user presses enter, a line feed is inserted into the console already. Outputting a line break would result in an empty line.

• R310

I get it.
In the article you wrote that it trip up new programmers!

• Alex

"trip up" means to "cause someone to get stuck or make a mistake"

• R310

Oh sorry I saw this one after...

• jörg

hi, is this solution valid?

• nascardriver

Hi Jörg!

No it's not. The , operator is not an "or". Use ||.

• jörg

Okay, thank you

Hello !

“Consequently, when we print boolean values with std::cout, std::cout prints 0 for false, and 1 for true:”

In the  program that you wrote below the above statement I noticed that you did not initialize any variables with the data type “bool”. Then how does std::cout know that it has to print 1 when in the cout statement we only wrote true ? Is it possible that if we wrote std::cout <<“true”<< instead of just std::cout <<true<<,  std::cout would print true and not 1?

• nascardriver

When you write

The "true" is treated as a string and it will be printed as such.
When you write

The "true" is a bool, @std::cout will print a bool and @std::cout does so by printing the numeric value (0 or 1).

As mentioned in the lesson, you can also use @std::boolalpha to print bools as "false" and "true".

Right ! I get it now . Thank you !

• Kage-Yami

Something interesting I've noted... Looks like I don't get garbage back when I enter a non-boolean value as input. Using the latest (as at posting) version of MingW/g++.

It appears to be converting the input string into a number first, then doing the (zero-or-empty == false) thing. I'm somewhat experienced with PHP, and know that it is implemented in C/C++, so given the way that PHP casts strings into numbers, and non-booleans into booleans, these results are actually unsurprising to me...

Input    == Output
------------------
0        == 0
1        == 1
2        == 1
a        == 0
-5       == 1
+8       == 1
45674513 == 1
asdfasdl == 0
456asdf  == 1
asdf456  == 0

Un-related, looks like the edit box on this site has a bug - it ignore the [code] tags, breaking the code formatting.

• nascardriver

Hi Kage-Yami!

In order to figure out what's actually happening and what's supposed to happen one would have to dig through the standard and implementations, I don't feel like doing that now.
A bool is an integral type, so number extractions into a bool works, but they leave the input stream in a failed state (You can check it with @std::cin.fail()).
Code tags display only after refreshing the site if they were added in an edit.

• Kage-Yami

Interesting... thanks for the tip!

• Jhayar

Here is how I solved the quiz. Any suggestions?

• nascardriver

Hi Jhayar!

• derbutzemann

Looks similar to others solutions.
It is good to have it here, some people can find it interesting.
@nascardriver this time I did initialize all of them :), have a good day!

• Brian Gaucher

As you know, boolean expressions are expressions
You are checking if

will be true. If it is true you are returning false, why not try something like this

That way if it is prime, it will be true. Then you can do this

to invert. Or if you like compact code.

Then also, you must remember that

, so

id the same as

becomes

which can become

means "if x divided by i returns 0 it's not prime/is false"
This makes the code much more concise. And remember that comments at the code level should explain why, so a comment like "If it's modulus is 0 it's not prime" makes the intent of the code clear.

• Levi

I though it be fun to make it check for any user inputed integer to see if it's a prime.
It uses some not yet explained things like a for loop and and modulo but should make enough sense.
So here is my aproach to the assignment (from my testing it works). and hope it is use to someone who is trying something similar 😀
also was a fun little program to write.

• nascardriver

Hi Levi!

Some suggestions:

• Levi

Thanks man These are some sweet surgestions. it makes sense to only count to the square root as that is the largest division possible.
also cleaning up some of the code was nice shows me how to make my code nicer in the future.

• Brian Gaucher

A small adjustement over nascardrivers's code
I removed his comments to emphasize mine

• nascardriver

After compilation it's the same. I prefer the first variant, because it's easier to understand what the condition is.

• Shri

@alex, @nascardriver, I wrote below mentioned program to tell if a number is prime or not and we can check any number lesser than max size of int.

I understand that Nascardriver might not have used structs/recursion/function in your program because these concepts are not introduced yet. But I thought that my code should have been faster to detect large prime numbers, instead, @nascardriver's program is way faster than mine. I do not quite understand why.

Reason I think my code should have been faster for large prime number:
I divide the large prime number by 2,3,5,7 and so on and calculate the largest prime number which could divide the number user has input to be 1/2, 1/3, 1/5, 1/7 and so on. Thereby reducing the amount of checks that I do.

Whereas Nascardriver is checking till number_to_be_checked/2, which can be a lot of numbers when the number_to_be_checked is large.

So my question is, I do not understand why Nascardriver's code takes less amount of time than mine, is it because I'm using many variables and function calls?

my code:

• Hi Shri!

The biggest slow-downs in your code are recursion and dynamic lists. Both can be terribly slow. If you're using dynamic lists, you should reserve enough space beforehand so the underlying array doesn't have to be resized.

• Shri

Hey Nascardriver,

Thanks for the reply! Thanks for clarifying that recursion and dynamic lists require lot of time.

1 point I wanted to clarify with you is regarding your comment:
If you're using dynamic lists, you should reserve enough space beforehand so the underlying array doesn't have to be resized.

From a bit of googling around I found that std::list (which I have used) provides us with doubly linked list. I have not studied Data structures yet, but from what little understanding I have, I think doubly linked lists do not require reserving memory beforehand unlike say dynamic array.

So by this comment of yours:
If you're using dynamic lists, you should reserve enough space beforehand so the underlying array doesn't have to be resized.

Which array are you referring to that I have used in my code?

• > I found that std::list (which I have used) provides us with doubly linked list
I didn't know that. In that case there's no need to reserve memory beforehand. Still, a doubly linked list needs to store the value and two pointers for each element, meaning that 20 bytes (assuming 64 bit) have to be allocated for every prime number found. Overall your logic seems to be overly complex for the task you're trying to achieve.

• Jack

I wouldn't have

I would rather have a variable and set that to false, and the break out the loop. That way we have a single exit point, which makes debugging much easier. If there were multiple instances where false could be returned, then this can be confusing as to what return statement was executed.

Here is mine:

• Hi, i know we not covered the for loops but i want to know if my code it's ok, thank you.
You are doing a great job!!

• nascardriver

Hi Stefan!

From lesson 2.1: "If you’re using a C++11 compatible compiler, favor uniform initialization".
Your loop only needs to run up to sqrt(x).

@static_cast<int> converts the given value to an int. I do this, because @std::sqrt returns a double.
You need to include <cmath> to use @std::sqrt.

References
* Lesson 2.1 - Fundamental variable definition, initialization, and assignment
* Lesson 4.4a - Explicit type conversion (casting)
* @std::sqrt http://www.cplusplus.com/reference/cmath/sqrt/

• Max

Do you teach  System() commands in this tutorial??

• nascardriver

Hi Max!

You can check out the @std::system documentation here http://en.cppreference.com/w/cpp/utility/program/system .
Other than that there's not much to talk about, because all it does is forward a command to the OS which has nothing to do with C++.

• Max

Thanks

• Tyler S.

I just spent the past hour ringing my brain for an answer to this quiz on this section. I built so many solutions and none of them would function properly.

It was only when I finally gave up and clicked to see the answer that I realized you left a hint after the question...

So TIL how that you can add multipart if statements to a bool function and that you should always read the entire quiz before answering the first question.

Thank you, Alex.

• Samira Ferdi

Why you use return false in bool isPrime function?

• nascardriver

Hi Samira!

@isPrime has to return a value. If @x isn't 2, 3, 5 or 7 @isPrime won't return true and it does not return anything automatically.

• Samira Ferdi

Oh, I see! Thank you nascardriver. But can I use else statement instead of return? which one is the best practice? return or else statement?

• nascardriver

If you used else you'd still have to return. No matter what happens in a non-void function, you have to return (You'll learn about exceptions in the future).

• Bennhyon

Hi! I've been learning C++ here the last 2 days and i think i'm really making good progress thanks to you.
I wanted to share the code i did for the exercise if you could tell me if that's a good thing for a first time.

• nascardriver

Hi Bennhyon!

• I think you should put the "you silly" comment right after they enter a number, and make them fix it.

• Bennhyon

If you want to know how to do loops in code, you can look for "while loops c++" on google, or directly on this site. http://www.learncpp.com/cpp-tutorial/55-while-statements/

• Thanks. I'm actually going through the whole tutorial on this page right now and just haven't gotten to loops yet, but I'll get there.

• Tony

Hi Alex,

What do you think of this solution?

bool isPrime(int x)
{
return (x == 2 || x== 3 || x== 5 || x== 7);
}

int main()
{
int x;
std::cout << "Enter a single digit integer: ";
std::cin >> x;

if (isPrime(x))
std::cout << x << " is a PRIME number!";
else
std::cout << x << " is NOT a prime number!";

return 0;
}

I always forget that you can assign a function to a variable, so the only way I found out to do the exercice was this one^.

Is that okay?

• Alex

This code looks great. I didn't use logical OR in my solution because it's not covered until next chapter.

• Tony

Thanks for the fast reply, Alex!

Great tutorials, by the way. The best for c++ since it explains everything well and gives so many exercices (which many tutorials don't offer). These exercices are a freaking crucial point in learning this seemingly hard programming language. 😀

• Gabe

Hi me again,
I don't really know where to ask this so...
I want to be able to make my function return a text and I don't know how.

• nascardriver

Hi Gabe!

I'll have to use content from future lessons for this since it hasn't been covered yet.

Option 1:

Option 2:

Or don't return a string at all

• Gabe

Hello, I'm back
I took a break from this because it was frustrating. I realized I could make my life easier and have qOne() return a boolean and have that boolean decided what main would output. But I can't seem to make qOne run then call the return value to use in main.

• nascardriver

Is this what you're looking for?

References:
Lesson 1.4 A first look at functions and return values (Section "Reusing functions")

• Gabe

Thanks!

• Getty

Hi,

My code looks some what different to the solution. Running from a previous lesson, I thought that it was bad practice to have anything within the main function, other than calls to other functions? I thought that it should be seperated. Here is my code:

what would be the best practice / what would be expected if you was a software engineer?

Cheers,
G

• Alex

There's little point in having a main do nothing but call a single other function. In that case, you might as well put the logic in main.

Most often in production code I've seen, main looks something like this:

But since our programs don't tend to have initialization and cleanup needed, our main() can just be the contents of doWhatever().

• Ali Dahud

Hi Alex,

why do I have to return false at the end of the function?

• nascardriver

Hi Ali!

I assume you're talking about @isPrime.
@isPrime has been declared to return a bool

No matter what happens inside the function, it has to return a bool in every case (except when an exception is thrown)!
If we were to remove the "return false;" and none of the numbers were matched @isPrime would not have a return value, this in turn would cause undefined behavior which, in the best case, causes a crash. Worst case, it works fine for you but crashes on other machines and you'll have a hard time figuring out why it's working for you but not for others.

• Ali Dahud

And why do I have to return true in every other case?

• nascardriver

Because @isPrime is supposed to tell you if @x is a prime number.
Returning true means that @x is a prime, returning false means @x is not a prime.

• Ali Dahud

Oh now I  get it thank you 🙂

• nascardriver

Hi siriusbuisness!
Your compiler will generate the same output for both variants so there's no difference.
However, when using an intermediary value you can view the variable in a debugger which is not as easy when using the return value of isPrime directly.

• sirius buisness

Hi Alex (and others)

In my solution for this chapter’s quiz I used

whereas you used an intermediary value

Is the intermediary value better convention for if statements?

Thank you for your time and the tutorials.

• nascardriver

• sirius buisness

Thanks for your swift answer. I tried editing but for some reason the code wouldn't format properly afterwards so I re-posted.

• Alex

No, in most cases I'd write it as you did (unless I needed to save the result of the isPrime() call for later).

I used an intermediary bool here just because it's a bit easier for learners to understand.

• Karston

Here's how I did it.
What's the different between adding curly braces and no curly braces around the statement after if?

• nascardriver

Hi Karston!
The curly braces are required if you want to execute multiple lines:

Output:
cats

Output:
like
cats

(x / x == 1) Is true for any number, doesn't have an effect.
(x / 1 == x) Is true for any number, doesn't have an effect.
(x / 2 == 1) Is equivalent to ((x == 2) || (x == 3)) // 3, because (3 / 2) == 1. An int cannot hold 1.5 so it's floored to 1.

Your program doesn't accept 5 or 7 as prime numbers, have another try.

• Karston

Hello,

Thank you for your correction, fixed!

If that condition"if" has only one statement, then there is no need of using braces there..

But let's assume that u have more than a single statement, if you want them to execute only if the condition is satisfied, then u should go for curly braces..

For a single statement it is not mandatory to keep braces

• Riux

Hey Alex!
So I've tried to make a program that checks if a number is a prime.

The problem seems to be that in checkPrime() it ignores the for loop that I've made, which results in wholeDivisions always being equal to 0.. What am I doing wrong?

• Riux

Hey Alex!
I managed to fix it myself 😀
My mistakes were:
1. In my for loop I wrote:

but it should be:

2. I started

at 0, which resulted in dividing by zero.
3. And I don't quite get this one: I made 'input' and 'i' integers which for some reason made my program treat division1 like and integer (5/2 = 2, for example). I fixed that by making 'input' and 'i' floats. But could you explain why it was treating division1 like an integer?
So yea now it works 🙂

• nascardriver

Hi Riux!

When you divide an int by and int the result will be an int. As you figured, this can be fixed changed by having the dividend or divisor being a float.
I don't know how the result type is determined so I wrote a test program.

This produces the output

It appears the result will be of the type of the dividend or divisor with the highest precision. (int < float < double)

• Riux

Thanks nascardriver :D!
So if the result will be of the type of the dividend/ divisor with the highest precision, then that means that I could only change 'i' into a float and leave 'input' as an integer and the result would still be a float right?

And I'm curious does deleting and rewriting your comments cause problems? I mean I will obviously stop deleting them now but I'd just like to know.

• nascardriver

Correct, but don't loop floating point numbers unless you have a good reason to do so. Instead, cast one of the variables to a float while doing the calculation:

You'll learn about casting in lesson 4.4a, if you think this is overkill for now you can keep doing it with floating point numbers in your loop but you shouldn't get used to it.

Here's the comment problem:
- You post
- UserX presses reply and starts writing their comment
- You delete and repost
- The comment entered UserX cannot be added as a reply, because the original post doesn't exist. The reply will be published as a regular comment and you won't receive a notification.

• Riux

Great thanks a lot!
I think I'll keep my method for now.. maybe I'll change it after 4.4a.
And thanks for explaining the comment problem 🙂

• Alex

> I don’t know how the result type is determined so I wrote a test program.

I cover this in lesson 4.4 under the subsection, "Evaluating arithmetic expressions".

You're basically correct, but the list in that lesson is a little more comprehensive.

Dear Alex,

I’ve got a question about the following little program:

When I enter 1 it says "The input is true.", and when I enter 0 it says "The input is false.", as expected. What I wonder about is that the program takes every number, even negative numbers and fractions as true, except those that begin with 0. And every letter is considered false. I thought that if I don’t enter 0 or 1, "input" would be any random number because it wasn’t initialized. But why is then every letter false and every number true except 0?

• Alex

In C++, bool is an integral type, meaning your input is treated as an integer. That means integer 0 is treated as false, and other integers (including negative ones) are treated as true.

Letters are invalid inputs in this context, and cause no input to be assigned to your variable. Thus, input remains uninitialized. This will cause undefined behavior when you use it later (to compare against false or true).

• Weckersduffer

Teacher,

Thanks for your excellent work with this great site.

• Alex

Honestly, it's not that great (but that's okay, that's how we learn). Here's why:
1) calcu and calcu2 have horrible names. I don't know what they do without looking at the code. They could also be combined since they essentially exist to serve the same purpose.
2) The logic you're using in main() isn't obvious. It really needs a good comment.
3) Your program produces the wrong result when the user enters 2.
4) You don't need to test for user == 9 since this should be covered by calcu2().

• Weckersduffer

When I run this code:

And I write a decimal number, it runs infinite times.

Thanks from Spain,

teacher

• nascardriver

Hi Weckersduffer!
A quick recap of for-loops:
A for-loop consits of three parts: for (p1;p2;p3)
p1: This will be executed only one time before the loop starts.
p2: This will be executed every time before the loop cycles. If this is true, the loop will run, if it's false the loop will stop.
p3: This will be executed after every loop cycle.

You are setting y to 5 right before the loop starts.
Then you are checking if y is bigger than 4, which it is, because it's 5.
Now your code inside the loop is executed, but y remains at 5.
After your code has been executed, y will be increased by 1, now y is 6, which still is bigger than 4.
What I'm trying to say is, you're increasing y but never decreasing it. All you're checking is if y is bigger than a certain value. This is what's causing the loop to run for an infinite amount of cycles.
(It's not actually infinite it will stop at some point, you shouldn't worry about it just yet)

I'd like to tell you what you need to change to make your code work, but I don't know what you are trying to do.

• Weckersduffer

What I was trying to do was the code to run infinite times. So I write an integer, I obtain the answer, and then the code starts again, so I can repeat the process. This way, if I want to enter more than one number, I don´t have to press "F9" each time.

Merry Christmas

• nascardriver

In that case you can use a while loop

Output:

More about loops in chapter 5.5 onward.

• Alex

There are two issues here.
1) Your program runs infinite times no matter what.
2) When you write a decimal number, that's invalid input for an integer. std::cin goes into "failure mode". I talk about how to address such issues in lesson 5.10.

• merocom

• Abdulrahim Rana

main.cpp

user_cred.cpp

verify.cpp

verify.h

Sir, Why this is not printing "Authenticated" or "Unauthenticated"...?

• Tim K.

(Scope.)  verifyAuth() cannot access the values assigned from main() because you didn't pass any arguments to the function - like: "verifyAuth(acc, pin)".

These are where your problem areas are:
line 12 in main.cpp
line 5 of verify.h
line 3 of verify.cpp
line 5 of verify.cpp

Good luck!

Regards.

• Alex

Looks fine to me.

• Lucian Cati