函数fun的功能是进行字母转换。若形参ch中是小写英文字母，则转换成对应的大写英文字母；若ch中是大写英文字母，则转换成对应的小写英文字母；若是其他字符则保持不变；并转换后的结果作为函数值返回。#include#includechar fun(char ch){if ((ch>='a')___1___(ch<='z'))return ch -'a' + 'A';if ( isupper(ch) )return ch +'a'-___2___ ;return ___3___;}main(){ char c1, c2;printf("\nThe result :\n");c1='w'; c2 = fun(c1);printf("c1=%cc2=%c\n", c1, c2);c1='W'; c2 = fun(c1);printf("c1=%cc2=%c\n", c1, c2);c1='8'; c2 = fun(c1);printf("c1=%cc2=%c\n", c1, c2);}