htoi的实现

 1 /*************************************************************************
 2     > File Name: htoi.c
 3     > Author: ChenPeng
 4     > Mail:479103815@qq.com 
 5     > Blog: http://www.cnblogs.com/cpsmile/
 6     > Created Time: Fri 03 Apr 2015 07:22:15 PM CST
 7  ************************************************************************/
 8 #include<stdio.h>
 9 #include<stdlib.h>
10 
11 #define YES 1
12 #define NO 0
13 
14 /*将十六进制数字组成的字符串转换为与之等价的整型值*/
15 int htoi(char *str)
16 {
17     int hexdigit;//记录每一个十六进制数对应的十进制数
18     int i;//工作指针
19     int ishex;//是否是有效的十六进制数
20     int n;//返回的十进制数
21 
22     i= 0;
23     if(str[i] == ‘0‘)
24     {
25         ++i;
26         if(str[i] == ‘x‘ || str[i] == ‘X‘)
27             ++i;
28     }
29     n = 0;
30     ishex = YES;
31     for(; ishex == YES; ++i)
32     {
33         if(str[i] >= ‘0‘ && str[i] <= ‘9‘)
34             hexdigit = str[i] - ‘0‘;
35         else if(str[i] >= ‘a‘ && str[i] <= ‘f‘)
36             hexdigit = str[i] - ‘a‘ + 10;
37         else if(str[i] >= ‘A‘ && str[i] <= ‘F‘)
38             hexdigit = str[i] - ‘A‘ + 10;
39         else
40             ishex = NO;
41         if(ishex == YES)
42             n = 16 * n + hexdigit;
43     }
44     return n;
45 }
46 
47 int main()
48 {
49     char str[30] = "";
50     int val;
51     while(printf("Please enter a hex string:\n"),scanf("%s",str) == 1)
52     {
53         val = htoi(str);
54         printf("整数是：%d\n",val);
55     }
56     return 0;
57 }