Swapping two values in C

Question

I have written this program to make the value of a in b and the value of b in a.

For this I created temp as a temporary variable and this is my program:

#include <stdio.h>

int changevalue(int a, int b)
{
    int temp;
    temp = a;
    a = b; 
    b = temp;
}

int main()
{
    int a;
    int b;
    a = 10;
    b = 20; 
    printf("the value of a is %d \n and b is %d",a,b);
    changevalue(a,b);
    printf("the value of a is %d \n and b is %d",a,b);
    return 0;
}

But the values of a and b did not change .

Where is the problem?

changevalue change the values of its parameters (copied from main's a and b) but returns nothing, and main's variables are untouched. — L. Scott Johnson, Oct 09 '20 at 13:02
In C language, when you declare a function with `int function(int a, int b)` and invoke it, the values of `a` and `b` inside the function are copies, thus the original values (in `main`) won't change — Hollyol, Oct 09 '20 at 13:05
@GuidoFlohr I have started programming since a week ! looking for pointers on google gives too much information that i can not start studying successfly, can you suggest a good reference to learn with with basic and simple examples ? — wadie el, Oct 09 '20 at 13:12
Pointers are described in every book or course for C. Just follow your current course, and they will come, when it's time. — Guido Flohr, Oct 09 '20 at 13:38
You can also replace your *function* `changevalue` with a *macro*: `#define changevalue(x, y) { x += y; y = x - y; x -= y; }`. The difference is that a macro is like a template. The code `changevalue(a,b)` is expanded by the preprocessor into `{ a += b; b = a - b; a -= b; }`. In fact, that is a simpler and faster solution for your problem. — Guido Flohr, Oct 09 '20 at 13:53

score 3 · Accepted Answer · edited Oct 09 '20 at 13:39

3

You need to use the pointers:

#include <stdio.h>

void changevalue(int *a, int *b)
{
    int temp;
    temp = *a;
    *a   = *b; 
    *b   = temp;
}

int main()
{
    int a=10;
    int b=20;
    printf("the value of a is %d \n and b is %d",a,b);
    changevalue(&a,&b);
    printf("the value of a is %d \n and b is %d",a,b);
    return 0;
}

edited Oct 09 '20 at 13:39

TechGeek49

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answered Oct 09 '20 at 13:04

Robert

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Swapping two values in C

1 Answers1