/* Chapter 13 - Program 1 - UPLOW.C */
#include <stdio.h>
#include <ctype.h>       /* Note - your compiler may not need this */

void mix_up_the_chars(char line[]);


int 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);
   
   return 0;
}


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



/* Result of execution

  (The selected file is displayed on the monitor with all of
  the upper case characters converted to lower case, and all
  of the lower case characters converted to upper case.)

*/