Thread: How do you pass variables between functions?

  1. #1
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    How do you pass variables between functions?

    Hi.

    This is what I have so far

    Code:
    #include <stdio.h>
    
    int num1;
    int num2;
    int sum;
          
    int main()
          
    {
          
              scanf("%d", &num1);
              scanf("%d", &num2);
          
              printf("Number 1=%d\n", num1);
              printf("Number 2=%d\n", num2);
          
              addnumbers(int &num1, int &num2);
          
              return 0;
           
    }
          
    int addnumbers()
           
    {
          
              sum = num1 + num2;
          
              printf("sum is %d", sum);
          
              return 0;
           
    }
    I tried to compile it using "gcc calc.c -o calc"

    These are the errors I receive. I really need help understanding what they mean and how to fix the issues.
    Code:
    calc.c: In function ‘main’:
    calc.c:21:5: warning: implicit declaration of function ‘addnumbers’ [-Wimplicit-function-declaration]
         addnumbers(&num1,&num2);
         ^~~~~~~~~~
    Thank you for your time.
    Last edited by tman904; 4 Weeks Ago at 06:11 AM.

  2. #2
    C++ Witch laserlight's Avatar
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    The warning is because you didn't declare the function addnumbers before calling it in the main function. What you need to do is to either forward declare the function (i.e., declare it before main without the function body), or more simply in this case, move the function definition (implementation) to before the main function.

    The first error is simply bad syntax. You need to refer to your learning material as to how to declare and call functions.

    The second error is because you have an extra semi-colon on the line where you start the definition of addnumbers.
    Quote Originally Posted by Bjarne Stroustrup (2000-10-14)
    I get maybe two dozen requests for help with some sort of programming or design problem every day. Most have more sense than to send me hundreds of lines of code. If they do, I ask them to find the smallest example that exhibits the problem and send me that. Mostly, they then find the error themselves. "Finding the smallest program that demonstrates the error" is a powerful debugging tool.
    Look up a C++ Reference and learn How To Ask Questions The Smart Way

  3. #3
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    Thank you. I saw the semicolon after I first posted. As for the syntax I'll keep working at it. I'm coming from programming mostly in python and shell programming and I'm not used to the huge differences is syntax etc.

  4. #4
    C++ Witch laserlight's Avatar
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    It might be helpful to avoid global variables: only declare variables within functions. Then you find out how you can provide addnumbers with two int parameters such that the main function can call it by passing two int arguments.
    Quote Originally Posted by Bjarne Stroustrup (2000-10-14)
    I get maybe two dozen requests for help with some sort of programming or design problem every day. Most have more sense than to send me hundreds of lines of code. If they do, I ask them to find the smallest example that exhibits the problem and send me that. Mostly, they then find the error themselves. "Finding the smallest program that demonstrates the error" is a powerful debugging tool.
    Look up a C++ Reference and learn How To Ask Questions The Smart Way

  5. #5
    and the hat of int overfl Salem's Avatar
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    > How do you pass variables between functions?
    Erm, how did you manage to call printf and scanf then?
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
    If at first you don't succeed, try writing your phone number on the exam paper.

  6. #6
    null pointer Structure's Avatar
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    uh...

    Code:
    #include <stdio.h>
    
    int add( int number1, int number2 ) {
      return ( number1 + number2 );
    };
    
    int main( int args, char *argv[] ) {
      printf( "%i" , add( 1, 2 ) );
      return 0;
    };
    "without goto we would be wtf'd"

  7. #7
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    Code:
    #include <stdio.h>
    
    int addnumbers(int num1, int num2)
    
    {
     
        int sum;
    
        sum = num1 + num2;
    
        printf("sum is %d\n", sum);
    
        return 0;
    
    }
    
    int main() 
    
    {
    
        int num1;
        int num2;
    
        scanf("%d", &num1);
        scanf("%d", &num2);
    
        addnumbers(num1, num2); 
    
        return 0;
    
    }
    Is this not suppose to work? Or is there many different ways to code this in order to add the two numbers? I ask since it compiles with no errors/warnings and runs correctly.

    I have a question about
    int addnumbers(int num1, int num2)

    Does this mean the function addnumbers is expecting two variables num1 and num2?

    Also when addnumbers completes what does the return 0 do? Does it return execution to main() or is my program exiting in a way that's bad?
    Last edited by tman904; 4 Weeks Ago at 06:47 AM.

  8. #8
    null pointer Structure's Avatar
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    Cool

    compiles with no errors/warnings and runs correctly.




    This might help...
    C - Functions - Tutorialspoint
    Last edited by Structure; 4 Weeks Ago at 08:37 AM.
    "without goto we would be wtf'd"

  9. #9
    and the hat of int overfl Salem's Avatar
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    > Does this mean the function addnumbers is expecting two variables num1 and num2?
    As far as the function is concerned these are what they're called.

    The names of your variables where you call the function is immaterial. So long as they're int's, everything is good.
    For example, addnumbers(3,4); is valid, and there are no named variables at all.

    > Also when addnumbers completes what does the return 0 do?
    In this case, nothing special, because you don't pay attention to the return result in main anyway.
    This is bad form if nothing else.

    Consider:
    Code:
    #include <stdio.h>
    
    // do one thing, and do it well.
    int addnumbers(int num1, int num2)
    {
        int sum;
        sum = num1 + num2;
        return sum;
    }
    
    int main() 
    {
        int num1;
        int num2;
    
        scanf("%d", &num1);
        scanf("%d", &num2);
    
        int sum = addnumbers(num1, num2); 
        printf("sum is %d\n", sum);
    
        return 0;
    }
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
    If at first you don't succeed, try writing your phone number on the exam paper.

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