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

C Programming Mathematics: Exercise-31 with Solution

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

int strlen(char *str)
{
   int l =0;
   while(str[l]!='\0')
    {
        l++;
    }
    return l;
}

int test(int num){
    char str_num[1000];
    char len;
    int result;
        sprintf(str_num, "%d", num);
    len = strlen(str_num);
    result = num;    
    for (int i = 0; i < len; i++) {
        for (int j = i+1; j < len; j++) {
            swap(str_num+i, str_num+j);
            if (atoi(str_num) > result) {
                result = atoi(str_num);
            }
            swap(str_num+i, str_num+j);
        }
    }
    return result;
}
int main(void) {
  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

C Programming Code Editor:

Improve this sample solution and post your code through Disqus.

Previous: Count all the numbers with unique digits in a range.
Next: Number sums and their reverses.

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C Programming: Tips of the Day

What's the point of const pointers?

const is a tool which you should use in pursuit of a very important C++ concept:

Find bugs at compile-time, rather than run-time, by getting the compiler to enforce what you mean.

Even though it does not change the functionality, adding const generates a compiler error when you're doing things you didn't mean to do. Imagine the following typo:

void foo(int* ptr)
{
    ptr = 0;// oops, I meant *ptr = 0
}

If you use int* const, this would generate a compiler error because you're changing the value to ptr. Adding restrictions via syntax is a good thing in general. Just don't take it too far -- the example you gave is a case where most people don't bother using const.

Ref : https://bit.ly/33Cdn3Q