Search

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.

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.

Quiz answers

1) Show Solution

2.7 -- Chars
Index
2.5 -- Floating point numbers

363 comments to 2.6 — Boolean values and an introduction to if statements

  • Okay…I am confused. I don’t know whether it is a good question or not. A boolean variable reserves 1 byte in memory but can only hold two possible values, true(1) or false(0). A single binary digit can also hold two values 1 or 0. My question is why a boolean variables takes 8 bit to represent.

    • Alex

      It’s a good question.

      Booleans really only need one bit to represent a true or false state, however as you note, bool variables use a whole byte. Why? Because the smallest unit of memory that a computer architecture can get or set is 1 byte. If booleans were only 1 bit, all of the booleans within the same byte would have the same memory address, and there’s be no way to differentiate them!

      There are ways to pack 8 booleans into a single byte. I cover these some of these techniques in future lessons.

  • Tree Stan

    Hi, I recently attempted the quiz of this section and wanted to avoid writing 4 else if statements. I’m not sure where I went wrong:

    • Alex

      C++ doesn’t parse this expression in the way you’re intending.

      You could use the logical OR operator to do something similar, like this:

      • Tree Stan

        Oh, thank you very much.

      • Daniel

        Hi, I have just completed the quiz, and the program works but it differs from the solution you provided. I’m just wondering if there is something I should do differently:

        • Alex

          Nope, this is great. It’s a better solution than mine because you use logical OR (which we haven’t covered yet).

          • Hey Alex.
            Is this code correct or the quiz solution is the better way to get around it?

            • Alex

              You’re on the right track, but your program doesn’t produce the correct results.

              • Hmmmm,
                I am kind of new in to programming so please excuse me any strange sound questions.
                So the program as far I understand is taking the value from the user and if it’s not 1,4,6 then true value is returned to the function caller which executed the first statement???
                Thanks for any inside on this.
                I am really enjoying this tutorial so Thanks again.

                • Alex

                  Yes, but:
                  1) You don’t handle 8 or 9 properly.
                  2) It’s odd to define the not-prime numbers instead of the prime numbers.

  • shakey2

    I came up with a different solution to the one you listed:

    Aside from having less to type is there a reason your solution is better than this?

    • Alex

      Your solution works but has a lot of redundancy. If you ever wanted to update the string “The digit is prime” you’d have to update it in 4 different places.

      For a simple program like this that’s not that big of a deal, but as programs get more complex, the more you can reduce redundancy the better.

  • Vishal

    Hello Alex,

    First of all, thanks for designing such a great tutorial! It makes learning a lot of fun since the approach is very systematic; not to mention the fact that it uses language that new programmers are comfortable with.

    I have a doubt regarding the following program (one of the examples in this lesson)-

    Why is it ‘if (equal)’ ?
    Why not ‘if (equal == 1)’ or ‘if (equal == true)’ ?
    I checked all three variations and they all work perfectly, so what’s the difference?

    Besides, couldn’t we just write the program as-

    It makes it shorter, and perhaps there’s a lower chance of errors.

    Please enlighten me! 😛
    Thanks!

    • Alex

      As you note, we use “if (equal)” because it evaluates identically to “if (equal == true)”. You can see why this is true by making a table:

      equal equal == true
      true true
      false false

      Therefore the “== true” part is redundant.

  • Brian

    Hi,
    I’m using Visual studio 2010.I believe my code is similar to the solution above(in the quiz)However,my program seems to recognize all inputs from user as being prime including numbers that are not prime. How do I solve this?

    • Alex

      Reply to this message and post your code so we can see what’s wrong.

      • Brian

        • shakey2

          line 12:

          should be

          You want Prime to default to being false because you are only making it true if it matches one of the conditional statements(if (x == 2) for example). If it doesn’t match one of those statements then those statements get ignored and Prime stays false so when the program gets to

          then it will skip to the else statement(telling the user their number is not prime) because its not true.

  • Suggestion for this part, another common prefix for bool functions is has. And since you do a lot to ensure good coding style (love that you do that btw.) I would add that it’s not only common but in most cases considered good style, since it for most boolean purposes makes the purpose of the function clearer.

  • 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.

  • 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.

    • cpplx

      seems like some people have a doubt about operators represented by using common symbols.
      maybe if you extend "1.5 — A first look at operators" with that = and == (for example) should be interpreted as "assign" and "compare" NOT as "equals" and "equals equals".
      also that == or <= should be interpreted as a single symbol, not 2 keys pressed one after the other, but as if it is 1 key pressed.
      make a remark that due the lack of symbols available in the keyboards, the language uses a combination of what is available to represent… stuff.
      some educators like to present programmers as being lazy(which might be true for the majority of them) hence the reason to write "==" instead of "compare".
      you get the idea. use better words 🙂

  • Snowman

    I think I took the quiz too seriously =P

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

  • 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?

  • Tito

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

    • Matthew

      Thank you for this!  I was trying to figure out how to use bool prime(false) or bool prime(int x) as a forward declaration and could not figure out what I was doing wrong.  Now my compiler errors make sense.

  • 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.

  • 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.

  • 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.

  • 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…

  • gurbux

    hey nice explanation alex very helpfull………..

  • 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..

  • 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.

    • Abhishek

      Hi there, I was just wondering if you just wanted to find if the numbers are equal why have you included the code from line 20 to line 32.

  • 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;
      }

  • 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!!!

  • 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.

  • 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)

  • 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).

  • 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!

  • 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.

  • 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.

  • 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,

  • 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>

  • 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.

      • Shanz

        Hi Alex,

        I was trying to make the same type of program.  Basically a yes or no question. I’m able to cin a bool easily by having the person enter a 1 for yes, 0 for no.  However  I’m trying to have them input ‘y’ and ‘n’.   You said earlier:

        "* Tell the user to enter ‘y’ or ‘n’ and then cin a character. "

        I’m confused on how this program is supposed to look.
        I’ve tried many ways, and none work.  This is the last part I’ve tried to make to get the correct answer.  Can you bool a char?  Could you tell me what I’m doing wrong?

        Its ugly but I’m trying haha. I’ve tried using bool equal(n) and other styles but none work.

        • Devashish

          These portions of your code are wrong (First of all, if the user is sleeping, he/she isn’t going to enter ‘y’ 🙂 :
          You can’t define a name twice in the same scope. Giving them different data types won’t help. Consequently, defining char ch and bool ch inside same function is not valid. remove the line

          from your code. In the same line, you are also assigning a char to a bool. Programmers never do that.

          here you are assigning a boolean true to a char. Boolean true evaluates to integer 1 and chars in C++ are treated as integers. If you assign an integer to a char, then the given integer will be treated as an ASCII code and program will print the equivalent character by looking at ASCII table. Integer 1 is equivalent to somewhat called “start of heading” in ASCII table, and is considered unprintable. So, the syntax may be valid, but has no meaning at all. Remove this too.

          , Due to the previous changes in your code, this won’t work. In fact, you don’t even need to define a char n variable. Secondly, this will not do what’s supposed. Pretend, because chars are treated as integers, and boolean true or false means integer 1 or 0 respectively, this line of code is a valid if expression. But what would be evaluated? I am not sure but I think true always.

          I think you need to go back and read about chars, if-else, data types, ASCII table etc. Your code is a superb mess. After reading those articles again, you may fix all of the errors (all errors are not mentioned) yourself.

          • Shanz

            Hi Devashish,

            Yeah, if the user is actually sleeping they can’t press ‘y’, but thats kind of the bad joke :).

            Oh I see, so theres no way to bool a char.

        • Alex

          You could do this:

          Alternatively, you could avoid using a boolean variable altogether:

          • Shanz

            Hi Alex,

            Thanks so much, that makes a lot of sense. I was wondering if I didn’t even need to use the bool, just not sure what I could use since I’ve only gotten up to the chapter for chars so far.

          • himanshu

            HEY ALEX!

            IT IS RIGHT TO ADD THIS OPERATOR ">>"
            TO COUT…..

            HERE WHY AM ASKING THIS
            YOU ADDED ">>" IN COUT IN ABOVE

            ANSWER THAT YOU HAD REPLIED..
                           ‘"
                           ‘"
            AND ONE MORE QUESTION

            IS IT POSSIBLE TO CREATE OR DESIGN
            SOMETHING LIKE FACEBOOK AND WHATSAPP …

            AND ALSO IN MY CODEBLOCKS 16.01 WHEN I COMPILE YOUR ABOVE ANSWER WITHOUT INT MAIN THERE A
            ERRORS COME SAYING "cout does not name a type" "expected unqualified id before if and else".

            • Alex

              No, that was a typo on my part. I’ve changed “cout” to “cin” as intended.

              > IS IT POSSIBLE TO CREATE OR DESIGN SOMETHING LIKE FACEBOOK AND WHATSAPP …

              Given that these things exist, I’d say yes, it’s possible. But if I were going to develop the next Facebook, I’d probably use something other than C++.

              My code above is a snippet, not a full program. You’ll need to #include iostream if you want to compile it, so the compiler knows what cout and cin are.

  • 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.

Leave a Comment

Put C++ code inside [code][/code] tags to use the syntax highlighter