Write a function that returns 2 for input 1 and returns 1 for 2 in C programming

A function that returns 2 for input 1 and 1 for input 2 can be implemented using various approaches. This is a common programming exercise that demonstrates different logical and mathematical techniques for value swapping between two specific numbers.

Syntax

int functionName(int x);

Method 1: Using Conditional Statement

The simplest approach uses an if-else statement to check the input value and return the corresponding output −

#include <stdio.h>

int reverseif(int x) {
    if (x == 1) 
        return 2;
    else 
        return 1;
}

int main() {
    printf("Input 1, Output: %d<br>", reverseif(1));
    printf("Input 2, Output: %d<br>", reverseif(2));
    return 0;
}

Input 1, Output: 2
Input 2, Output: 1

Method 2: Using Mathematical Operation

This method uses subtraction from the sum of both numbers (1 + 2 = 3). Subtracting the input from 3 gives the desired result −

#include <stdio.h>

int reversesub(int x) {
    return (3 - x);
}

int main() {
    printf("Input 1, Output: %d<br>", reversesub(1));
    printf("Input 2, Output: %d<br>", reversesub(2));
    return 0;
}

Input 1, Output: 2
Input 2, Output: 1

Method 3: Using XOR Operation

The XOR operation provides an elegant bitwise solution. Since 1 XOR 3 = 2 and 2 XOR 3 = 1, we can use 3 as the XOR key −

#include <stdio.h>

int reversexor(int x) {
    return (x ^ 3);
}

int main() {
    printf("Input 1, Output: %d<br>", reversexor(1));
    printf("Input 2, Output: %d<br>", reversexor(2));
    return 0;
}

Input 1, Output: 2
Input 2, Output: 1

Comparison

Conclusion

All three methods effectively swap values between 1 and 2. The mathematical approach (3-x) is the most concise, while the conditional method is the most readable. The XOR method demonstrates bitwise manipulation techniques.