#include<stdio.h>
#include<string.h>
int main()
{    int alphabetic(char);
    int longest(char[]);
    int i;
    char line[100];
    printf("input the line:\n");
    gets(line);
    printf("The longest word is:");
    for(i=longest(line);alphabetic(line[i]);i++)
        { printf("%c",line[i]);}
    printf("\n");
    return 0;
}
    int alphabetic(char c)
    {
        if((c>='a'&&c<='z')||(c>='A'&&c<='Z'))
        {return 1;}
        else
        {return 0;}
    }
    int longest(char string[])
    {    int len=0,i,length=0,flag=1,place=0,point;
        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;
    }

#include<stdio.h>
#include<string.h>
int main()
{  
    int alphabetic(char);        //判断是否是字母的函数
    int longest(char[]);
    int i;
    char line[100];
    printf("input the line:\n");//提示输入line
    gets(line);                    //输入字符串line
    printf("The longest word is:");
    for(i=longest(line);alphabetic(line[i]);i++)//i等于最长单词的起始下标，当line[i]不是字母时跳出循环
    {
        printf("%c",line[i]);
    }
    printf("\n");
    return 0;
}
int alphabetic(char c)
{
    if((c>='a'&&c<='z')||(c>='A'&&c<='Z'))
    {
        return 1;
    }
    else
    {
        return 0;
    }
}
int longest(char string[])
{  
    int len=0,i,length=0,flag=1,place=0,point;//len，length记录单词长度，flag标志作用，place,point记录单词起始的下标
    for(i=0;i<=strlen(string);i++)
    {  
        if(alphabetic(string[i]))//string[i]为字母时，执行
        {
            if(flag)
            {
                point=i;
                flag=0;
            }
            else
            {
                len++;
            }
        }
        else
        {
            flag=1;
            if(len>=length)
            {
                length=len;//
                place=point;
                len=0;//置0，计算后面字符中的最长单词长度
            }
        }
       
    }
    return  place;
} 

虽然还不是很懂 但是谢谢你的解答

哪里不懂，发短信告诉我。我再帮你看看。你回复，我看不到的。