RETURN

[ARAMDEVASTATOR]

I am a beginner and all help will be fine. I was trying to learn what "return" does and I did 2 func (one of with return, one of without return) but 2 of them works right. Can anyone tell me what return does? Thanks.

  #include <stdio.h>	

int topla(int x, int y)
{
	int toplam=x+y;
	return toplam;
	
}

int carp(int s1, int s2)
{
	int carpim=s1*s2;
	
}

int main()
{
	int a,b;
	printf("2 sayi giriniz : ");
	scanf("%d%d",&a, &b);
	printf("%d\n",topla(a,b));
	printf("%d",carp(a,b));
}

[Peter87]

You need the return statement otherwise you can't be sure what value you will get back from the function.

You are just lucky if you get the correct value. When I compile your program with GCC 4.9.2 I get different outputs. If I compile without optimizations (-O0) it prints the value of b and if I compile with optimizations (-O2) it always prints 0.

[ARAMDEVASTATOR]

Is it check system or something?

[Chervil]

What about if you do something like this:

#include <stdio.h>

int carp(int s1, int s2)
{
	int carpim=s1*s2;
	printf("Hello\n");
	
}

int main()
{
	int a = 7, b = 5;

	printf("%d",carp(a,b));
}

I get different results with different compilers, but don't get 35 at all.
As Peter87 said, you are just lucky if you get the correct value.

[naaissus]

You can also use return to exit early from function.
Something like:

int calc(int x, int y) {
    int z;
    printf("If you don't want to calculate sum enter 0: ");
    scanf("%d", &z);
    if (z == 0) return -1; // "I don't want to execute rest of code, just return -1"
    return x + y;
}

If your function is void you can use return; for the same purpose:

void calc(int x, int y) {
    int z;
    printf("If you don't want to calculate sum enter 0: ");
    scanf("%d", &z);
    if (z == 0) return; // "I don't want to execute rest of code, break here"
    printf("%d", x + y);
}

Note that this is equivalent to:

void calc(int x, int y) {
    int z;
    printf("If you don't want to calculate sum enter 0: ");
    scanf("%d", &z);
    if (z != 0) {
        printf("%d", x + y);
    }
}

[ARAMDEVASTATOR]

I was studying return again and my head confused. How can system know that return 1 and 0 where they go?

#include <stdio.h>

int leap_year(int); /* function's prototype */

int main()
{
  int year;

  printf("Enter a year: ");
  scanf("%d",&year);

  if( leap_year(year) ) 
      printf("%d is a leap year\n",year);
  else
      printf("%d is not a leap year\n",year);
}


int leap_year(int year)
{
  if(  year % 4   == 0 &&
       year % 100 != 0 ||
       year % 400 == 0  ) return 1;
  else return 0;
}

[TarikNeaj]

They go back to where they were called from. If 1 is returned, the if statement will take it as "true" and "is a leap year" will be outputted. If the return is 0, it the if statement will be false.