C language provides toLower() and toUpper() functions in ctype.h to convert a single character to its lowercase or uppercase equivalent respectively.

We can use these functions in a loop over the characters in the string to convert it to lowercase or uppercase.

• tolower(ch) : Converts the character to lowercase
• toupper(ch) : Converts the character to uppercase

Example :

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

void convertToLowercase(char *);
void convertToUppercase(char *);

int main(void) {
  convertToLowercase("Hello");

  convertToUppercase("World");
  return EXIT_SUCCESS;
}
void convertToLowercase(char* str) {
  int i;
  for (i = 0; i <= strlen(str); i++) {
    str[i] = tolower(str[i]);
  }
  printf("String Hello converted to Lowercase is %s ", str);
}

void convertToUppercase(char* str) {
  int i;
  for (i = 0; i <= strlen(str); i++) {
    str[i] = toupper(str[i]);
  }
  printf("\nString World converted to Uppercase is %s ", str);
}

Output :

String Hello converted to Lowercase is hello 
String World converted to Uppercase is WORLD 

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