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2.6 — Boolean values

The next data type we’re going to look at is the boolean data type. Boolean variables only have two possible values: true (1) and false (0).

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

When assigning values to boolean variables, 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:

When boolean values are evaluated, they actually don’t evaluate to true or false. They evaluate to the numbers 0 (false) or 1 (true). Consequently, when we print their values with cout, it prints 0 for false, and 1 for true:

Outputs:

1
0
0
1

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

This prints:

1
0
true
false

A first look at booleans in if-statements

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

if (condition) statement

or

if (condition) statement
else statement2;

In both forms, condition is evaluated. If condition evaluates to true (non-zero), then statement is executed. In the second form, if condition evaluates to false, then statement2 is executed instead.

prints:

b is true

Boolean values are also useful as the return values for functions that check whether something is true or not. Such functions are typically named starting with the word is.

In the following example, we use the equality operator (==) to test if two values are equal. Operator== returns true if the operands are equal, and false if they are not.

Note that in C++, a single equals (=) is the assignment operator, whereas a double equals (==) is a comparison operator to test for equality.

In this case, because we only use equal in one place, there’s really no need to assign it to a variable. We could do this instead:

Boolean variables are quite refreshing in their simplicity!

One additional note: when converting integers to booleans, the integer zero resolves to boolean false, whereas non-zero integers all resolve to true.

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.

Quiz answers

1) Show Solution

2.7 -- Chars
Index
2.5 -- Floating point numbers

40 comments to 2.6 — Boolean values

  • some guy

    I’m confused as to why you make the IsEqual function a bool, wouldn’t making it an int function work just as well? Or is it declared as a bool function so you have an idea of what the return values will be and what the function does? Sorry if I sound like a newbie :p

    • IsEqual function isn’t a bool, it returns a bool. Boolean values are used to represent values that can only be true or false. When we’re talking about whether a value is equal, either it is (true), or it isn’t (false). Consequently, it makes more sense to return a bool than an int. While technically returning an int would work (returning the value 1 if the parameters are equal, and 0 otherwise), returning a bool is more intuitive and leaves less room for errors to be made.

  • programmer

    I have problem actually I don’t know how to use the boolean with ( cin ) ..

    like this example:

    if i want to ask some one about his car ?

    ===========
    bool car;

    =====
    is the above good solution or not ??

    i’m witing for your answer

    thank you so much

    • When trying to cin boolean values, if the user enters the number 0, this will be treated as the value false. If the user enters anything else, it will be treated as the value true.

      There are several ways to proceed:
      * Tell the user to enter 0 for no and 1 for yes and then cin a bool.
      * Tell the user to enter ‘y’ or ‘n’ and then cin a character.

      Once you get to the lesson on strings, you could also do this:
      * Tell the user to enter “yes” or “no” and then cin a string.

  • another_programmer

    Yup, I had trouble with this one quite a while ago, one thing you’d have to remember is that bool is based on being simply on or off, where if something exists, it’s on, and if it doesn’t it’s off. 1 or 0.

    Boolean values are more ideally used if preexisting values exist much like radio buttons, in HTML for example…

    <form>
    Is your car new or not?<br>
    <input name="newCar" type="radio" value="1" /> Yes<br>
    <input name="newCar" type="radio" value="0" /> No<br>
    </form>

  • Kiran

    Please correct me if I am wrong:
    If the Value of TRUE is 1 (00000001), Doesn’t (!TRUE) make the value to be equal to (111111110) internally? If that is the case then evaluating:
    if (!TRUE) will always be TRUE.

    • Ashish Gupta

      Hi,

      Actually what u r doing is not correct. u r performing bit wise not that is ~ operator in c.
      now u r using operator ! which actually ORs all the bits of the value and then compliments it i.e.
      if TRUE is 1(00000001) then ORing will give its value 1(binary not 00000001) this is only a bit so now it will compliment it to 0 and then return the resulting false(000000000).

      For futher query you can ask me..on new_programers@yahoo.com
      Thnks,

  • Spock

    Hey Alex,

    Nice tutorial section! Some comments that might improve readability of this section:

    1. Mention that the test x==y will output a 0 if false and a 1 if true and discuss the use of this equality as a logic ‘test’. It took me a few moments to appreciate that for myself.

    2. Logic commands have not been discussed in the tutorial yet (if, not etc) so someone new to programming might find this confusing.

    You could also mention that if(1) runs the first command in an if statement and if(0) runs the ‘else’ statement. Interestingly if(5), if(11) and all if(!=0) has the same effect as if(1). i.e all non-zero values are true in the context of the if statement.

    :)

    • Thanks for your thoughts. I added a comment about conversion of integers to booleans because that’s not very intuitive to new programmers. Your other comments are valid, but are discussed in future chapters when I tackle those specific topics.

  • Noha

    Hey!
    In which cases do we use the implicit assignment?

    • You can use it in any case where you’re creating a variable and initializing it with a value. It’s neither better nor worse than explicit assignment as far as I know.

  • someone

    Please i need some help…
    Can please anyone review this code? It is supossed to accept the input of 2 numbers, the asks for the answer if they are equal and not equal and says if the answer is either right or not:

    Thanks!

  • joel

    Great Tutorials, well written and structured. many thanks.

    coming from a background in Oracle and using mainly PL/SQL i have gotten used to the idea that booleans are tristate (true, false or null/unknown) and so an if statement against an uninitialsed boolean var would always return null (and thereby only be handled by “else”) without any warnings - dangerous.

    It might be worth noting for people like me that uninitialised boolean variables, aside from raising a warning at compile and a “break” at runtime (in VC2008 at least), will almost always return true (i assume this because the uninitialised memory address will almost always contain a non-zero value as below).

  • Sam

    I don’t get how it can run the function bool IsEqual(int x, int y), if there is no function call in the program later. I would figure you would need,

    bool Isequal(x,y); <---in main function to call the IsEqual function?

    for the program to read

    bool IsEqual(int x, int y)
    07.{
    08. return (x == y); // use equality operator to test if equal
    09.}

    so how does it work without a function call being made?

    • Collin

      the if statement calls the function to check if (x==y)

  • Alex

    Could someone please give me a definitive answer about the difference between explicit and implicit assignments.

    Thanks

    • Daniel

      I’ve never seen that type of assignment used for a built-in, although I do not doubt that it works. I believe that “int i = 1;” uses int’s default constructor THEN uses int’s assignment operator. Whereas “int j(1);” uses a version of int’s constructor which accepts an integer. In theory the second should be more efficient although compilers are most likely smart enough to optimise away the extra step.

  • Stephan

    Hi Alex (Admin)!

    Great site.
    Just wanted to say that I find your site extremely educational, and for me, easy to follow. Good work!

    P.S.
    I get just as much fun/learning reading the comments at the end of the tutorial as I do from the lessons. I can’t finish a section without reading the comments!

    Brilliant!!!

  • Mac

    Hi, i tried to make this program work with three numbers:

    i.e.
    return (x == y == z)

    the program compiled fine and ran, but did only gave the result for “else” (false)

    after i took away “int z” it worked just fine…

    does bool only work for two variables?

    • The reason is when you say a == b == c, first it checks a and b and if those two numbers are equal, you are trying to check if c is equal to 1(true). Try this:

      bool IsEqual(int a, int b, int c)
      {
      if(a == b)
      {
      if(b == c)
      return true;
      }
      else
      return false;
      }

  • Guntis

    Yay, I think I completely understand how bool works now.
    Thanks Alex, this is so much better than even some of those books on cpp.

    P.S. Here’s my little test that succeeded btw.

  • joha

    wouldnt it be the same to put instead of x==y, this: x=y????
    i tried and got the same result so…..what is the difference and when do I use each operator????

    Thanks..

  • gurbux

    hey nice explanation alex very helpfull………..

  • xero

    I want to ask why following code works?

    #include
    int main()
    {
    using namespace std;
    int x;
    cout << "Please feed in the first number" <> x;
    int y;
    cout << "Please feed in the second number" <> y;
    bool z;
    if (x==y)
    z = true;
    else
    z = false;
    if (z)
    cout << "Both digits are equal" << endl;
    else
    cout << "Both digits are different" << endl;
    return 0;
    }

    When the user feeds in two different numbers then the statement which says
    if (z)
    must hold 0 as z and in return should type "Both digits are equal" because this cout statement follows if (z) statement.
    Please comment…

  • John

    Hi!

    So I’m getting the coding right when trying some things, I’m just struggling to understand something. Line 27 of your example to be exact.

    On line 26 we declare the variable beEqual. Then we have a function call to IsEqual in order to initialize beEqual. Am I right in assuming that IsEqual will return to the caller function a 1 if both values are equal, and a 0 if they are not equal?

    Now line 27. We say: if (beEqual)… what exactly does this mean? Because I’m sure beEqual=beEqual, so why dont we always print “x is equal to y” ?

    Or should I read it as: if (1) and if (0)?

    I hope I’m making sense here!

    • Alex

      isEqual() will return true (1) if the values are equal, and 0 otherwise.

      “if (beEqual) …” is the same as writing “if (beEqual==true) …”. So in this case, we’re testing beEqual to see if it’s true, and which statement is executed depends on whether that equality test is true or false. I’ll make this more clear in the lesson text.

  • Carol

    Can any one help check my code? There’s some semantic error, but I can’t figure out why.

    #include <iostream>
    using namespace std;

    bool isPrime(int)
    {
        int x;
        if (x==2)
            return true;
        else if (x==3)
            return true;
        else if (x==5)
            return true;
        else if (x==7)
            return true;
        else
            return false;
    }

    int main()
    {
        cout << "Please enter a 1-digit number." << endl;
        int myNumber;
        cin >> myNumber;
        
        if (isPrime(myNumber))
            cout << "The digit is prime" << endl;
        else
            cout << "The digit is not prime" << endl;
            
        return 0;
    }

    • Alex

      Change:

      to:

      As you wrote it, you didn’t give your function parameter a name, so the value being passed in (myNumber) is being discarded. You defined variable x inside the function, but you never initialized it with a value, so it’s essentially useless.

  • Daniel Ricci

    One additional note: when converting integers to booleans, the integer zero resolves to boolean false, whereas non-zero integers all resolve to true.

    At this point, you should just specify that any numerical value that is not 0 or 0.0 w.r.t its data type will be true.

  • Tito

    I just wanted to share my quiz answer for any future readers.

  • Melissa Arredondo

    Hi I have an assignment at school similar to this but I’m having a hard time with it. Hopefully someone can help! So the input can be from 2-100 and then determine whether it is prime or not. Then if it is not prime, I need to be able to print whether it is divisible by 2, 3, 5, or 7. How do I do that?

  • Snowman

    I think I took the quiz too seriously =P

    PS. No, I didn’t read the hint, it’s cheating =D

  • Rich S.

    I’m not sure if I understand the significance between = and ==. I get that = is assigning a value while == is comparing, but it seems like replacing the == with = in the sample code should either give a syntax error or the correct result. When I try this, it compiles correctly, but entering an 8 gives me the result "This digit is prime." I would appreciate it if someone could help me to understand why this is so.

    • Alex

      It sounds like you’re doing something like this:

      So first, this will assign the value 2 to x, which probably isn’t what you want.

      But the real question here is what does x = 2 evaluate to? The answer, it turns out, is x.

      So after x = 2 evaluates, you’re left with:

      which will be true if x is anything other than 0. So the … part of the code is executing when you aren’t expecting it to, and this is setting the value of isPrime to true.

  • programmer, another one

    in the above solution, couldn’t you have put the

        if (prime)
            cout << "The digit is prime" << endl;
        else
            cout << "The digit is not prime" << endl;

    part inside the other if statements like you would do in most other languages?

    for example:

        if (x == 2)
            prime = true;
            std<<cout::"the digit is true";
        else if (x == 3)
            prime = true;
            std<<cout::"the digit is true";
        else if (x == 5)
            prime = true;
            std<<cout::"the digit is true";
        else if (x == 7)
            prime = true;
            std<<cout::"the digit is true";

    I’ve also noticed that c++ does not automatically edit the indentation in the statements like compilers in most other languages would.

    Thanks for the reply, and for building this amazing website

    • Alex

      You could, but I wouldn’t recommend doing it this way. Why? Because if you want to change what is printed, you have to change 4 statements instead of one. Although it’s not that bad to do with 4 statements, imagine if there were 100, or 1000.

      One of the goals when programming should be to reduce redundancy, and your code adds redundancy.

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