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 that can be answered with a “true” or “false”: “Apples are a fruit”. 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 (note: Boolean is properly capitalized in the English language because it’s named after its inventor, George Boole).
Boolean variables
Boolean variables are variables that can have only two possible values: true, and false.
To declare a Boolean variable, we use the keyword bool.
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bool b; |
To initialize or assign a true or false value to a Boolean variable, we use the keywords true and false.
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bool b1 { true }; bool b2 { false }; b1 = false; bool b3 {}; // default initialize to 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:
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bool b1 { !true }; // b1 will be initialized with the value false bool b2 { !false }; // b2 will be initialized with the value 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). Because Booleans actually store integers, they are considered an integral type.
Printing Boolean variables
When we print Boolean values with std::cout, std::cout prints 0 for false, and 1 for true:
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#include <iostream> int main() { std::cout << true << '\n'; // true evaluates to 1 std::cout << !true << '\n'; // !true evaluates to 0 bool b{false}; std::cout << b << '\n'; // b is false, which evaluates to 0 std::cout << !b << '\n'; // !b is true, which evaluates to 1 return 0; } |
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. Here’s an example:
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#include <iostream> int main() { std::cout << true << '\n'; std::cout << false << '\n'; std::cout << std::boolalpha; // print bools as true or false std::cout << true << '\n'; std::cout << false << '\n'; return 0; } |
This prints:
1 0 true false
You can use std::noboolalpha to turn it back off.
Integer to Boolean conversion
You can’t initialize a Boolean with an integer using uniform initialization:
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#include <iostream> int main() { bool b{ 4 }; // error: narrowing conversions disallowed std::cout << b; return 0; } |
(note: some versions of g++ don’t enforce this properly)
However, in any context where an integer can be converted to a Boolean , the integer 0 is converted to false, and any other integer is converted to true.
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#include <iostream> int main() { std::cout << std::boolalpha; // print bools as true or false bool b1 = 4 ; // copy initialization allows implicit conversion from int to bool std::cout << b1 << '\n'; bool b2 = 0 ; // copy initialization allows implicit conversion from int to bool std::cout << b2 << '\n'; return 0; } |
This prints:
true false
Inputting Boolean values
Inputting Boolean values using std::cin sometimes trips new programmers up.
Consider the following program:
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#include <iostream> int main() { bool b{}; // default initialize to false std::cout << "Enter a boolean value: "; std::cin >> b; std::cout << "You entered: " << b << '\n'; return 0; } |
Enter a Boolean value: true You entered: 0
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. A failed input will also zero-out the variable, so b also gets assigned value false. Consequently, when std::cout prints a value for b, it prints 0.
To make std::cin accept “false” and “true” as inputs, the std::boolalpha option has to be enabled
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#include <iostream> int main() { bool b{}; std::cout << "Enter a boolean value: "; // Treat "false" and "true" as booleans std::cin >> std::boolalpha; std::cin >> b; std::cout << "You entered: " << b << '\n'; return 0; } |
However, when std::boolalpha is enable, “0” and “1” will no longer be treated as booleans.
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:
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#include <iostream> // returns true if x and y are equal, false otherwise bool isEqual(int x, int y) { return (x == y); // operator== returns true if x equals y, and false otherwise } int main() { std::cout << "Enter an integer: "; int x{}; std::cin >> x; std::cout << "Enter another integer: "; int y{}; std::cin >> y; std::cout << std::boolalpha; // print bools as true or false std::cout << x << " and " << y << " are equal? "; std::cout << isEqual(x, y); // will return true or false return 0; } |
Here’s output from two runs of this program:
Enter an integer: 5 Enter another integer: 5 5 and 5 are equal? true
Enter an integer: 6 Enter another integer: 4 6 and 4 are equal? false
How does this work? First we read in integer values for x and y. Next, the expression “isEqual(x, y)” is evaluated. In the first run, this results in a function call to isEqual(5, 5). Inside that function, 5 == 5 is evaluated, producing the value true. The value true is returned back to the caller to be printed by std::cout. In the second run, the call to isEqual(6, 4) returns the value false.
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! Boolean values are also a huge part of the language -- you’ll end up using them more than all the other fundamental types put together!
We’ll continue our exploration of Boolean values in the next lesson.
 4.10 -- Introduction to if statements |
 Index |
 4.8 -- Floating point numbers |
std::boolalpha is also usable for std::cin according to cppreference. So, I have used the code block:
and get the output:
true
1
Thus, I think editing the sentence "It turns out that std::cin only accepts two inputs for Boolean variables: 0 and 1 (not true or false)." could be better. I apologize if I am missing something.
I've added a paragraph about `std::boolalpha` to the section, thanks for pointing it out!
Isn't
bool b3 {};
uniform initialization?
QUESTION :
std::cout << std::boolalpha; can be turned off
can setprecision be turned off ?
Hi guys, I'm trying to gain familiarity with these concepts and i'm currently hitting a kind of brick wall, the following code compiles fine but it's tiresome to write out the conditions for if statement (and it doesn't run correctly/has a semantic error that I can't find)
Is there a neater/more convenient way to do this? For instance if I had many, many more possible values for these variables, writing out the if statements could take hours upon hours...
additionally, the memberLevel string value gets evaluated as "p" no matter what i plug into it, even integers.. why would this happen?
You have a whole bunch of duplicate comparisons. Nest your if-statements so you don't have to compare things more than once. Also write functions to make things easier.
If you don't need exact prices, you can define multipliers for each category
> memberLevel string value gets evaluated as "p"
I can't reproduce this.
#include <iostream>
using namespace std;
int main()
{
bool isfishgood=true;
if(isfishgood)
{
cout << "Fish is tasty";
}
else
{
cout << "I hate fish";
}
return 0;
}
You can change the outcome of the program just by setting the value of the boolean variable, if set to true, it will return fish is tasty, otherwise it wil return I hate fish
Hi, quick question. You said in earlier chapter that only initialized value to variable if you are going to use it. Then, why initialized it in this case?
It would make more sense to not initialize it? or C++ initialize it to 0 by default?
exp:
std::cout << "n/a";
int i{}; //in this line, leave it blink? why initialize it to 0?
std::cin >> i;
Those should be left blank. Thanks for pointing it out!
Hey I was playing with the different examples and noticed that if I enter a value other than 1 or 0 for the boolean b, causing std::cin to "silently fail" and zeroing the variable, later uses of std::cin also seem to fail. In fact when I later try to ask for user inputted integers, the program fails to ask me at all for any numbers and instead just prints to std::cout with a and c also set to 0. Can you tell me why this is and if there is someway to reset std::cin? Or are scenarios like this dealt with through error/exception handling? Thanks.
When extraction fails, `std::cin` enters a failed state. As long as `std::cin` is in this failed state, it will refuse to do anything. To get out of that failed state call `std::cin.clear()`. Note that if there was any bad input, it will still be there and your next extraction might fail for the same reason as the previous one.
We cover input error handling in more detail later.
a little typo here
"It turns out that std::cin only accepts two inputs for Boolean variables: 0 and 1 (not true or false). "
it should be true or false instead of "not true or false"
It is correct as written, it means that `std::cin` does not accept true or false as input for a boolean.
Design a C++ code that has the same behavior as a XOR gate. Input variable must be
of Bool Data type.
It seems to be that a best practice should be to **not** use cin directly for booleans unless there are performance reasons. It seems to me that the best practice should be to use cin to populate a string, and then populate a boolean after that. If this is correct, you may want to add a best practice as such. Possibly to name an alternative best practice as to ask people to put in a "1" or a "0"
Can't this be done without using boolean integrals?
I think it can be done without boolean integrals.
What do you mean by "boolean integrals"?
i meant boolean values.
You can do everything without types, but that's not the point. Types are there to help you.
BUT I like asparagus :(
well it is good for health.
Inputting Boolean values
Inputting Boolean values using std::cin sometimes trips new programmers up.
Consider the following program:
you forget the #include <iostream>; its a small thing, which might have been forgotten deliberately.
I added it, thanks! Some old snippets don't have includes. If you find more, feel free to point them out.
What exactly is std::boolalpha? From what I gather it's a function. Assuming it is, why can we use it without parentheses?
`std::boolalpha` is both a function and a variable of type `std::ios_base::fmtflags`, which is an implementation-defined type, ie. it's unknown to us what this type is, it can vary between standard libraries. In this example, we're using the variable. When a stream, for example `std::cout`, receives a variable of this type, it changes some of its internal settings rather than printing the variable.
brace initialization shouldn't be allowed on line 5, in fact when I tried it wouldn't compile, complaining about "narrowing conversion of '4' from 'int' to 'bool'". so I changed it to copy initialization, but left the brace initialization on line 8 and it did compile, why is that?
Brace initialization doesn't allow conversions which can cause loss of data. If you convert 4 to a bool, you lost information, because afterwards you only know if the integer was 0 or not.
why my compiler allows implicit conversion from int to bool using uniform initialization?
(code::blocks 20)
this prints true.
shouldn't compile. I tried with clang, gcc, and msvc, all of which rejected the code. Which compiler are you using?
GNU GCC compiler.
gcc falsely accepts
up to version 8. From gcc 9 onward, the code is rejected. Unless you have a good reason to still be using gcc 8, I suggest using a more recent version of gcc (Currently gcc 10 is the most recent release).
Sorry,
How can I check the version of my GCC compiler?
Run `g++ -v` in your terminal if you're on linux or mac. I don't know how to do this on Windows.
Thank you very much for your time.
Hi.In "printing Boolean variables" section, shouldn't it be \n in the code instead of std::endl ?
It should, lesson updated. Thanks for pointing it out!
You're welcome.
Asparagus are great, come on! Fried (not deep fried) with salt and olive oil... :p Bacon is great with aspargus, by the way.
Haha, funny. I knew someone would comment on this. Yes, Alex, if your only experience is with canned (or even frozen store-bought) asparagus... I hear ya. Fresh grilled asparagus with lots of butter is amazing!
If you have to drown it in butter, aren't you just masking its true nature? :)
When you wrote:
"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. In C++11 or newer, this also sets b to 0 (which is the same value it had already been initialized with)."
Did you mean that when a user inputs a value other than 0 and 1 for Boolean variables using std::cin, the variable would be unaffected and would retain the value it had been initialized with?
Because I found that this program does not behave accordingly:
When the user inputs "false" or "true", the program always prints "Boolean value: 0", inconsistent with the initial value of the variable b (true).
Edit: My current guess is that I misunderstood you, in which you meant that such input would result in the variable b getting the value of 0 instead of retaining its initial value.
In C++11, failed inputs via std::cin operator >> cause the variable to be zero'd.
This program on running shows 0.1 with no precision. Explain?
`std::setprecision` doesn't do anything on its own. It returns an object that can be passed to a stream to modify the stream's precision.
Oook! Thanks.