Beginning question about While loops

This is a discussion on Beginning question about While loops within the C Programming forums, part of the General Programming Boards category; Hello! So I'm currently trying to teach myself C via the Internet, and I'm stuck on the While loop. Here's ...

  1. #1
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    Beginning question about While loops

    Hello! So I'm currently trying to teach myself C via the Internet, and I'm stuck on the While loop. Here's what I'm trying to do: the user should input a non-negative number no greater than 23; if the number is greater than 23, then the prompt should let the user try again and then re-evaluate the new number to see if it's a non-negative number.

    Here's what I have:

    Code:
    #include <cs50.h>
    #include <stdio.h>
    
    int main(void)
    {
        printf("Insert a non-negative integer no greater than 23: ");
        
        int x = GetInt();
        while (x > 23)
            printf("Insert a non-negative integer LESS than 24!: ");
            x = GetInt();
    
         while (x < 0)
            printf("Insert a NON-NEGATIVE integer less than 24!: ");
            x = GetInt();
    
         return 0;
    }
    However, when I run the program and type in 28 for example, it's an infinite loop and does NOT ask for another integer (no GetInt(); activated), which I don't understand because in my while loop, I state that the prompt should allow the user to type in another number and then evaluate to see if it should get out of the while loop.

    What am I doing wrong? Thank you!
    Last edited by Rubik; 02-16-2013 at 07:50 PM. Reason: Wrong code

  2. #2
    Cat
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    1. Your first problem is that you need braces around your loop. Omitting braces means the next single statement is looped over.

    What happens right now is equivalent to this:

    Code:
    int main(void)
    {
        printf("Insert a non-negative integer no greater than 23: ");
         
        int x = GetInt();
        while (x > 23)
        {
            printf("Insert a non-negative integer LESS than 24!: ");
        }
        x = GetInt();
     
        while (x < 0)
        {
            printf("Insert a NON-NEGATIVE integer less than 24!: ");
        }
        x = GetInt();
     
        return 0;
    }
    In the actual body of the loop, GetInt() is never called - x never changes throughout the while loop, so it infinitely loops because it never gets a new x to evaluate.

    2. You also have a second problem - you want to check both conditions in a single loop, not in two separate loops. Consider the || operator, which takes two boolean (conditional) statements and combines them such that the result is true if either of the conditions is true.
    You ever try a pink golf ball, Wally? Why, the wind shear on a pink ball alone can take the head clean off a 90 pound midget at 300 yards.

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    Also, I forgot to include this, but: I know you can easily execute this code using the do-while loop, but I want to do it using the While loop. Thank you.

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    Got it! Thank you so much, Cat! That makes so much more sense. You're freaking awesome.

  5. #5
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    C does not care about indentation, so you need to add braces to do what you want

    Code:
    while (...) {
       statement1;
       statement2;
       ...
       statement_n;
    }

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