C program to change case of a string

#include <stdio.h>
#include <string.h>
 
int main()
{
   char string[1000];
 
   printf("Input a string to convert to lower case\n");
   gets(string);
 
   printf("Input string in lower case: \"%s\"\n",strlwr(string));
 
   return  0;
}

#include <stdio.h>
#include <string.h>
 
int main()
{
   char string[1000];
 
   printf("Input a string to convert to upper case\n");
   gets(string);
 
   printf("Input string in upper case: \"%s\"\n",strupr(string));
 
   return  0;
}

#include <stdio.h>
 
void upper_string(char []);
 
int main()
{
   char string[100];
 
   printf("Enter a string to convert it into upper case\n");
   gets(string);
 
   upper_string(string);
 
   printf("Entered string in upper case is \"%s\"\n", string);
 
   return 0;
}
 
void upper_string(char s[]) {
   int c = 0;
 
   while (s[c] != '\0') {
      if (s[c] >= 'a' && s[c] <= 'z') {
         s[c] = s[c] - 32;
      }
      c++;
   }
}

#include <stdio.h>
 
void lower_string(char []);
 
int main()
{
   char string[100];
 
   printf("Enter a string to convert it into lower case\n");
   gets(string);
 
   lower_string(string);
 
   printf("Entered string in lower case is \"%s\"\n", string);
 
   return 0;
}
 
void lower_string(char s[]) {
   int c = 0;
 
   while (s[c] != '\0') {
      if (s[c] >= 'A' && s[c] <= 'Z') {
         s[c] = s[c] + 32;
      }
      c++;
   }
}

#include <stdio.h>
 
int main () 
{
   int c = 0;
   char ch, s[1000];
 
   printf("Input a string\n");
   gets(s);
 
   while (s[c] != '\0') {
      ch = s[c];
      if (ch >= 'A' && ch <= 'Z')
         s[c] = s[c] + 32;
      else if (ch >= 'a' && ch <= 'z')
         s[c] = s[c] - 32;   
      c++;   
   }
 
   printf("%s\n", s);
 
   return 0;
}