/* Upper and lower case letter functions */

#include <stdio.h>
#include <ctype.h>       /* Note - your compiler may not need this */

void mix_up_the_chars(char line[]);

void main()
{
FILE *fp;
char line[80], filename[24];
char *c;

   printf("Enter filename -> ");
   scanf("%s",filename);
   fp = fopen(filename,"r");

   do {
      c = fgets(line,80,fp);   /* get a line of text */
      if (c != NULL) {
         mix_up_the_chars(line);
      }
   } while (c != NULL);

   fclose(fp);
}

void mix_up_the_chars(char line[])
			       /* This function turns all upper case
                            characters into lower case, and all
                            lower case to upper case. It ignores
                            all other characters.               */
{
int index;

   for (index = 0;line[index] != 0;index++) {
      if (isupper(line[index]))     /* 1 if upper case */
         line[index] = tolower(line[index]);
      else {
         if (islower(line[index]))  /* 1 if lower case */
            line[index] = toupper(line[index]);
      }
   }
   printf("%s",line);
}
UPLOW.C
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