how to print output of a subroutine in C?

Question

I am using a very simple code for subroutine but it is not printing right values. Please help me in this. My code is:

#include<stdlib.h>
#include<stdio.h>

main(){
  int i, a, b=0.0, c=0.0;
  void sum(int a , int b);
  for ( i = 0; i < 2; i++ ) {   
    sum (a, b); c = c+b;
  }
  printf("%d\n", c);
}
void sum(int a , int b){
  int i;
  for ( i = 0; i <6; i++ )  {a = i*i;b = b+a;}
}

Also, stop using a 25+ year-old book that declares `main` with no return type, encourages the declaration of functions *within* functions, etc., etc... — The Paramagnetic Croissant, Aug 12 '14 at 10:27
here i am calling a function sum (a, b)that is the subroutine, therefore it should work. — user3132983, Aug 12 '14 at 10:29

score 2 · Answer 1 · answered Aug 12 '14 at 10:27

The effects of sum(int,int) are not visible to the caller, because a and b are passed by value. You need to use pointers to achieve the results that you want:

void sum(int *a , int *b){
    for (int i = 0; i <6; i++ ) {
        *a = i*i;
        *b += *a;
    }
}

Change the forward declaration to use pointers as well, and call sum (&a, &b) to pass the pointers.

score 1 · Answer 2 · answered Aug 12 '14 at 10:30

sum(a, b) is a NOP, you need to pass a pointer:

#include <stdio.h>

void sum(int *a, int *b)
{
    int i;

    for (i = 0; i < 6; i++) {
        *a = i * i;
        *b = *b + *a;
    }
}

int main(void)
{
    int i, a, b = 0.0, c = 0.0;

    for (i = 0; i < 2; i++) {   
        sum(&a, &b);
        c = c + b;
    }
    printf("%d\n", c);
    return 0;
}

score 1 · Answer 3 · answered Aug 12 '14 at 10:47

1

You are calling the function by values. You should call the function by reference that is using pointers or reference.

answered Aug 12 '14 at 10:47

Karan Jawalkar

21
2

score 0 · Answer 4 · answered Aug 12 '14 at 10:29

For adding in subroutines you need to use Call by reference Method. Then only the values are properly assigned to variables. But you are using Call by Value Method. so it will pass only the copy of the values to subroutine function. What are the changes made in subroutine that does not affect in main function. Try the following changes

#include<stdlib.h>
#include<stdio.h>

main(){
int i, a, b=0, c=0; // fix 1
void sum(int *a , int *b); // fix 2
  for ( i = 0; i < 2; i++ ) {
  sum (&a, &b); c = c+b; // fix 3
  }
  printf("%d\n", c);
}
void sum(int *a , int *b){ // fix 4
  int i;
  for ( i = 0; i <6; i++ )  {*a = i*i;*b = *b+*a;}
}

score 0 · Answer 5 · answered Aug 12 '14 at 10:29

#include<stdio.h>


void sum(int *a , int *b);
int main() {
  int i, a, b=0.0, c=0.0;
  for ( i = 0; i < 2; i++ )
  {
        sum (&a, &b);
        c = c+b;
  }
  printf("%d\n", c);
}

void sum(int *a , int *b){
  int i;
  for ( i = 0; i <6; i++ )
  {
        *a = i*i;
        *b = (*b) + (*a);
  }
  return;
}

Output:

I have done required formating of your code and changes function arguments to pointer type.

how to print output of a subroutine in C?

5 Answers5