p123.28

#include<stdio.h>int main(){char ch,x;while(1){printf("输入字符串:  ");while((ch=getchar())!='\n'){if(ch>='0'&&ch<='9'){printf("%c",ch);}else if((ch>='a')&&(ch<='z')){x='A'+ch-'a';printf("%c",x);}else{x='a'+ch-'A';printf("%c",x);}}printf("\n");}return 0;}