Swap Variables with Pointers

CSC-115 · Semester I · C Programming

intermediate20 minAvailable

Cin-browser interpreter · runs locally

#include <stdio.h>

void swap(int *x, int *y) {
  int temp = *x;
  *x = *y;
  *y = temp;
}

int main() {
  int a = 10, b = 20;
  printf("Before: a = %d, b = %d\n", a, b);
  swap(&a, &b);
  printf("After:  a = %d, b = %d\n", a, b);
  return 0;
}

16 lines

Output

Write some code, then press Run. Stdout and errors appear here.

Lab brief

Objective

Swap two variables through a function using pointers, and understand why call by value alone cannot do it.

Theory & concepts

A swap function written with values (void swap(int a, int b)) swaps copies, so the caller sees no change. To modify the caller's variables, the function must receive their addresses and write through the pointers with the dereference operator *.

Rendering diagram…

Swapping through pointers versus call by value

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

The key steps: temp keeps the value at *a before it is overwritten, *a receives the value at *b, and *b takes the saved temp. Every write goes through a pointer, so the caller's variables change.

Prerequisites

Lesson: Pointer operators
Lesson: Parameters and call by value

Instructions

1Declare two int variables x and y and initialize them to different values.
2Declare a function swap(int *a, int *b) that exchanges the values via the pointers.
3Call swap(&x, &y) from main.
4Print x and y after the call and confirm the swap happened.
5Explain in a comment why swap(x, y) (without &) would not work.

Tasks

Implement the pointer swap
Input 5 9 - output must show x = 9, y = 5 after the swap.
Explain call by value
Describe in one sentence why a value-based swap fails.
Swap with a temp vs XOR
Rewrite the swap using a temporary int, then optionally using XOR: *a ^= *b; *b ^= *a; *a ^= *b;
Verify with extremes
Test with 0 and a large number like 2147483647. Does the swap stay correct?

Self-check

x and y exchange values correctly after the call.
The function signature uses pointers, not values.
The student can explain the role of & and *.
The XOR variant (if attempted) swaps correctly without a temp.

Environment

editorstandard inputstdout consoletest cases