てきとーに

replace.c

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

void replace(char *dest, char *str)
{
  while (*str != '\0') {
    if (64 < *str && *str < 91) {
      *dest = *str + 32;
    } else if (96 < *str && *str < 123) {
      *dest = *str - 32;
    } else {
      *dest = *str;
    }
    str++;
    dest++;
  }
  *dest = '\0';
}

void print_data(char *dest, char *str)
{
  printf("str : %s\n", str);
  printf("dest: %s\n", dest);
}

int main(int argc, char *argv[])
{
  if (argc == 1) {
    fprintf(stderr, "Usage: ./a.out [string] [string] ...\n");
    exit(1);
  }

  int wordCount;
  int i;

  for (i = 1; argv[i] != NULL ; i++) {
    wordCount = strlen(argv[i]);

    char str[wordCount+1];
    char dest[wordCount+1];
    strcpy(str, argv[i]);

    replace(dest, str);
    print_data(dest, str);
  }

  return 0;
}