十六进制数字组成的字符串的程序&库函数strtoul()

//编写函数htoi()，把由十六进制数字组成的字符串（包含可选前缀0X或0x）转换为与之等价的整型值，

//字符串中允许包含的数字为：0~9，a~f及A~F

#include <stdio.h>

#include <stdlib.h>

//该函数将字母（a~f，A~F）转换成相应的数字

int hexalpha_to_int(int c)

{

char hexalpha[] = "aAbBcCdDeEfF";

int i;

int answer = 0;

for(i = 0; answer == 0 && hexalpha[i] != '\0'; i++)

{

if(hexalpha[i] == c)

{

answer = 10 + (i / 2);

}

}

return answer;

}

unsigned int htoi(const char s[])

{

unsigned int answer = 0;

int i = 0;

int valid = 1;

int hexit;

if(s[i] == '0')//前缀0x，0X

{

++i;

if(s[i] == 'x' || s[i] == 'X')

{

++i;

}

}

while(valid && s[i] != '\0')

{

answer = answer * 16; //前一步的计算结果乘以16

if(s[i] >= '0' && s[i] <= '9')

{

answer = answer + (s[i] - '0');

}

else

{

hexit = hexalpha_to_int(s[i]);

if(hexit == 0) //遇到无法识别的字符，出错，停止计算

{

valid = 0;

}

else

{

answer = answer + hexit;

}

}

++i;

}

//遇到无法识别的字符，记为0

if(!valid)

{

answer = 0;

}

return answer;

}

/* Solution finished. This bit's just a test driver, so

* I've relaxed the rules on what's allowed.

*/

int main(void)

{

char *endp = NULL;

char *test[] =

{

"F00",

"bar",

"0100",

"0x1",

"0XA",

"0X0C0BE",

"abcdef",

"123456",

"0x123456",

"deadbeef",

"zog_c"

};

unsigned int result;

unsigned int check;

size_t numtests = sizeof test / sizeof test[0];//字符串指针的个数

size_t thistest; //size_t =unsigned int

for(thistest = 0; thistest < numtests; thistest++)

{

result = htoi(test[thistest]);

check = (unsigned int)strtoul(test[thistest], &endp, 16);//字符串转换成无符号整型数

if((*endp != '\0' && result == 0) || result == check)

{

printf("Testing %s. Correct. %u\n", test[thistest], result);

}

else

{

printf("Testing %s. Incorrect. %u\n", test[thistest], result);

}

}

return 0;

}

//strtoul()函数将字符串转换成无符号长整型数

//函数原型

//#include<stdlib.h>

//unsigned long int strtoul(const char *nptr,char **endptr,int base);

//strtoul()会将参数nptr字符串根据参数base来转换成无符号的长整型数。

//参数base范围从2至36，或0。参数base代表采用的进制方式，如base值为10则采用10进制，

//若base值为16则采用16进制数等。当base值为0时则是采用10进制做转换，

//但遇到如'0x'前置字符则会使用16进制做转换。一开始strtoul()会扫描参数nptr字符串，

//跳过前面的空格字符串，直到遇上数字或正负符号才开始做转换，

//再遇到非数字或字符串结束时('\0')结束转换，并将结果返回。

//若参数endptr不为NULL，则会将遇到不合条件而终止的nptr中的字符指针由endptr返回。