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C program to change case of a string

Strlwr function converts a string to lower case, and strupr function converts a string to upper case. Here we will change string case with and without strlwr and strupr functions. These functions convert the case of alphabets and ignore other characters that may be present in a string.

Function strlwr in C

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

int main()
{
   char string[1000];
   
   printf("Input a string to convert to lower case\n");
   gets(string);
   
   printf("The string in lower case: %s\n", strlwr(string));
   
   return  0;
}

Function strupr in C

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

int main()
{
   char string[1000];
   
   printf("Input a string to convert to upper case\n");
   gets(string);
   
   printf("The string in upper case: %s\n", strupr(string));
   
   return  0;
}

Change string to upper case without strupr

#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("The string in upper case: %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++;
   }
}

Change string to lower case without strlwr

#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("The string in lower case: %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++;
   }
}

You can also implement functions using pointers.

C program to change the case from upper to lower and lower to upper

The following program changes the case of alphabets. If a lower/upper case alphabet is present, we convert it to upper/lower case.

#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;
}

An output of the program:

Input a string
abcdefghijklmnopqrstuvwxyz{0123456789}ABCDEFGHIJKLMNOPQRSTUVWXYZ
ABCDEFGHIJKLMNOPQRSTUVWXYZ{0123456789}abcdefghijklmnopqrstuvwxyz

If a digit or special character is present in a string, it's left as it is.

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