てきとーに

replace.c

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

#define DIFF_UPPER_LOWER ('a'-'A');

void replace(char *dest, char *str)
{
  int length = strlen(str);
  for (int i=0; i < length; i++) {
    if ( 'A'<=str[i] && str[i]<='Z' ) {
      dest[i] = str[i] + DIFF_UPPER_LOWER;
    } else if ( 'a'<=str[i] && str[i]<='z' ) {
      dest[i] = str[i] - DIFF_UPPER_LOWER;
    } else {
      dest[i] = str[i];
    }
  }
}

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 length;
  char *str;
  char *dest;

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

    str  = (char *)calloc(length, sizeof(char));
    dest = (char *)calloc(length, sizeof(char));

    strncpy(str, argv[i], length);

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

    free(str);
    free(dest);
  }

  return 0;
}