一、运行结果；

二、源代码；

# define _CRT_SECURE_NO_WARNINGS
# include <stdio.h>//声明函数；
int aiphabetic(char);
int longest(char[]);int main()
{//初始化变量值；int i;char line[100] = { 0 };//获取用户输入字符串；printf("请输入一句话：");gets(line);//输出最长单词；printf("最长单词为：");for (i = longest(line); alphabetic(line[i]); i++){//输出；printf("%c", line[i]);}//换行；printf("\n");return 0;
}//实现函数；
int alphabetic(char c)
{//判断；if ((c >= 'a' && c <= 'z') || (c >= 'A' && c <= 'Z')){return 1;}else{return 0;}
}int longest(char string[])
{//初始化变量值；int len = 0, i, length = 0, flag = 1, place = 0, point;//循环判断；for (i = 0; i <= strlen(string); i++)if (alphabetic(string[i]))if (flag){point = i;flag = 0;}else{len++;}else{flag = 1;if (len >= length){length = len;place = point;len = 0;}}return place;
}