完成对输入的字符串中C关键词的查找统计。

#include <stdio.h>
#include <ctype.h>
#include <string.h>
 
#define MAXWORD 80
 
#define NKEYS (sizeof(keytab) / sizeof(struct key))
 
struct key
{       
    char *word;
    int count;
}        keytab[] =
{       
    "auto", 0,
    "break", 0,
    "case", 0,
    "char", 0,
    "const", 0,
    "continue", 0,
    "default", 0,
    "do", 0,
    "double", 0,
    "else", 0,
    "enum", 0,
    "extern", 0,
    "float", 0,
    "for", 0,
    "goto", 0,
    "if", 0,
    "int", 0,
    "long", 0,
    "register", 0,
    "return", 0,
    "short", 0,
    "singed", 0,
    "sizeof", 0,
    "static", 0,
    "struct", 0,
    "switch", 0,
    "typedef", 0,
    "union", 0,
    "unsigned", 0,
    "void", 0,
    "volatile", 0,
    "while", 0
}       ;
 
void getword(char *, int);
int binsearch(char *, struct key *, int);
 
int main(int argc, char *argv[])
{       
    int n;
    char word[MAXWORD];
 
    printf("本程序将为您统计C语言的关键字的个数，请输入，输入end结束输入：\n");
    do
    {       
        getword(word, MAXWORD);
        if (isalpha(word[0]))
        {       
            if ((n = binsearch(word, keytab, NKEYS)) >= 0)
            {       
                keytab[n].count++; //找到则对应次数+1
            }
        }
 
    }
    while (strcmp(word, "end") != 0);
 
    printf("您的输入中C语言关键字出现的次数统计如下：\n");
    for (n = 0; n < NKEYS; n++)
    {       
        if (keytab[n].count > 0)
        {       
            printf("%-10s: %6d\n", keytab[n].word, keytab[n].count);
        }
    }
 
    return 0;
}       
 
/* 折半查找：在tab[0]到tab[n-1]中查找word */
int binsearch(char *word, struct key tab[], int n)
{       
    int result;
    int low, high, mid;
 
    low = 0;
    high = n - 1;
    while (low <= high)
    {       
        mid = (low + high) / 2;
        if ((result = strcmp(word, tab[mid].word)) < 0)
        {       
            high = mid - 1;
        }
        else if (result > 0)
        {       
            low = mid + 1;
        }
        else
        {       
            return mid;
        }
    }
    return -1;
}       
 
/* getword：从输入中获取某个单词 */
void getword(char *word, int lim)
{       
    int c;
    void ungetch(int);
    char *w = word;
 
    while (isspace(c = getchar()))
    {       
    }
    if (c != EOF)
    {       
        *w = c;
        w++;
    }
    if (!isalpha(c))
    {       
        *w = '\0';
    }
    for ( ; --lim > 0; w++)
    {       
        if (!isalnum(*w = getchar()))
        {       
            //读入的某个字符不是字母，则将它退还给输入缓冲区
            ungetch(*w);
            break;
        }
    }
    *w = '\0';
}    

ctype.h是C标准函数库中的头文件，定义了一批C语言字符分类函数（C character classification functions），用于测试字符是否属于特定的字符类别，如字母字符、控制字符等等。既支持单字节（Byte)字符，也支持宽字符。