C Programming: Largest number, swapping two digits in a number

Last update on August 07 2025 12:49:45 (UTC/GMT +8 hours)

31. Largest Number from One Swap Variants

Write a C programming to calculate the largest number that can be generated by swapping just two digits at most once

Test Data:
(89) -> 98 [Swapping 8 and 9]
(568) -> 865 [Swapping 8 and 5]
(4499) -> 9494 [Swapping 9 and 4]
(12345) -> 52341 [Swapping 1 and 5]
(7743) -> 52341 [No Swap]

Sample Solution:

C Code:

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

// Function to swap two characters
void swap(char *x, char *y) {
    char temp = *y; // Store the value of *y in temp
    *y = *x; // Assign the value of *x to *y
    *x = temp; // Assign the value of temp to *x
}

// Function to calculate the length of a string
int string_length(char *str)
{
   int l = 0; // Declare and initialize variable l to 0
   while(str[l] != '\0') // While loop to iterate through the string until the null terminator is encountered
    {
        l++; // Increment l
    }
    return l; // Return the length of the string
}

// Function to find the largest number by swapping two digits
int test(int num){
    char str_num[1000]; // Declare an array to store the number as a string
    char len; // Declare a variable to store the length of the string
    int result; // Declare a variable to store the result
    
    // Convert the integer to a string
    sprintf(str_num, "%d", num);
    
    // Get the length of the string
    len = string_length(str_num);
    
    // Initialize result to the input number
    result = num;
    
    // Nested loops to swap each pair of digits and find the maximum number
    for (int i = 0; i < len; i++) {
        for (int j = i+1; j < len; j++) {
            swap(str_num+i, str_num+j); // Swap the digits at positions i and j in the string
            if (atoi(str_num) > result) { // Convert the modified string to an integer and compare with the result
                result = atoi(str_num); // Update the result if the modified number is greater
            }
            swap(str_num+i, str_num+j); // Restore the original order of digits
        }
    }
    return result; // Return the largest number
}

int main(void) {
    // Test cases
    int n = 89;
    printf("n = %d",n);
    printf("\nLargest number from the said number swapping two digits = %d", test(n));
    n = 568;
    printf("\n\nn = %d",n);
    printf("\nLargest number from the said number swapping two digits = %d", test(n));
    n = 4499;
    printf("\n\nn = %d",n);
    printf("\nLargest number from the said number swapping two digits = %d", test(n));
    n = 12345;
    printf("\n\nn = %d",n);
    printf("\nLargest number from the said number swapping two digits = %d", test(n));
    n = 7743;
    printf("\n\nn = %d",n);
    printf("\nLargest number from the said number swapping two digits = %d", test(n));
}

Sample Output:

n = 89
Largest number from the said number swapping two digits = 98

n = 568
Largest number from the said number swapping two digits = 865

n = 4499
Largest number from the said number swapping two digits = 9494

n = 12345
Largest number from the said number swapping two digits = 52341

n = 7743
Largest number from the said number swapping two digits = 7743

Flowchart:

$Flowchart: Count all the numbers with unique digits in a range$

For more Practice: Solve these Related Problems:

Write a C program to generate the largest number possible by swapping two digits at most once using brute-force.
Write a C program to optimize the largest number generation by identifying the first pair that increases the number when swapped.
Write a C program to determine the maximum number obtainable from one swap and validate with multiple test cases.
Write a C program to simulate digit swapping and backtracking to ensure the optimal swap is performed.

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