C Exercises: Convert a string to uppercase or lowercase

C Callback function: Exercise-5 with Solution

Write a program in C program to convert a string to uppercase or lowercase using a callback function to modify each character.

Sample Solution:

C Code:

#include <stdio.h>

#include <ctype.h>


void modify_string(char * str, int( * modifier)(int)) {
  while ( * str != '\0') {
    * str = modifier( * str);
    str++;
  }
}

int main() {
  char str[100];
  printf("Input a string: ");
  fgets(str, sizeof(str), stdin);

  printf("Select an option:\n");
  printf("1. Convert to uppercase\n");
  printf("2. Convert to lowercase\n");
  int option;
  scanf("%d", & option);

  switch (option) {
  case 1:
    modify_string(str, toupper);
    printf("Uppercase string: %s", str);
    break;
  case 2:
    modify_string(str, tolower);
    printf("Lowercase string: %s", str);
    break;
  default:
    printf("Invalid option");
    break;
  }

  return 0;
}

Sample Output:

Input a string: w3resource
Select an option:
1. Convert to uppercase
2. Convert to lowercase
1
Uppercase string: W3RESOURCE
-------------------------------  
Input a string: JavaScript
Select an option:
1. Convert to uppercase
2. Convert to lowercase
2
Lowercase string: javascript

Explanation:

In the above program modify_string() function takes a string and a function pointer ‘modifier’ as arguments. The ‘modifier’ function is used to modify each character in the string. Then loop through the string and apply the modifier function to each character using pointer arithmetic.

In the main() function, first take a string input from the user using fgets() and then ask the user to select an option to either convert the string to uppercase or lowercase. Depending on the user's choice, we call the modify_string() function with the appropriate modifier function (toupper() or tolower()). Finally, printf() function print the modified string to the console.

Flowchart:

C Programming Code Editor:

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