#include<stdio.h> void changeS_letter(char *tmp, const char *str) { while (*tmp == *str) { if ('a' <= *str && *str <= 'z') { *tmp = *str - ('a' - 'A'); str++; tmp++; } else { *tmp = *str + ('a' - 'A'); str++; tmp++; } } } int main(void) { char str[30]; char tmp[30]; printf("Please input a string\n"); scanf("%s", str); printf("result\n"); changeS_letter(tmp, str); printf("%s\n", tmp); } **こちらを参考にしていただきました**--------------------------------------------- #include <stdio.h> void upperstring(char*, const char*); void lowerstring(char*, const char*); char upper(char); char lower(char); void upperstring(char *out, const char *in){ int i; i = 0; while(in[i] != '\0'){ out[i] = upper(in[i]); i++; } } void lowerstring(char *out, const char *in){ int i; i = 0; while(in[i] != '\0'){ out[i] = lower(in[i]); i++; } } char upper(char c){ if('a' <= c && c <= 'z'){ c = c - ('a' - 'A'); } return c; } char lower(char c){ if('A' <= c && c <= 'Z'){ c = c + ('a' - 'A'); } return c; } int main(void){ char s[20]; upperstring(s, "I am a CAT"); printf("%s\n", s); lowerstring(s, "I am a CAT"); printf("%s\n", s); return 0; }