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网易面试，问atoi实现的效率改进问题

silver6 | 10 十一月, 2005 08:58

今天去一面，面试官云风要我把笔试时做的atoi实现改进一下，我想了半天，终于想出一点，即确定某字符所在的万、千、百、十位后，本来是循环乘10，我说可以做一个数组里面就放上10000,1000,100,10,1，找到位数后直接索引再乘那个字符，这样多次乘法就减为一次了。可是云风说还可以改进，然后提示除了在数组里放上10000，1000，100，10，1之外还可以放别的数，甚至多位一起乘，当时真是紧张，完全没明白！
哪位高手指点一下？要是还有二面，我就好答了。我先去debug一下tc库里atoi的实现吧，等看懂了再来帖～

这是我刚开始学C++的时候，不知道C++里有可以把字符串转换成数字的函数是什么时，自己写的一个函数：
int strtoint(string s)
{
if (s[0] != '-')
{
s = "+" + s;
}
int n = 0;
for(int i = 1, l = s.size();i < l;i++)
{
if ((s[i] > 57)||(s[i] < 48))
{
cerr << "Please enter integrity!" << 'n';
system("pause");
exit(1);
}
n = n * 10 + (s[i] - 48);
}
if (s[0] == '-')
n = -1 * n;
return n;
}

最简单的是调试跟进去，前提是你已经安装了CRT的源代码，我VC7看到的代码是这样的：
long __cdecl _tstol(
const _TCHAR *nptr
)
{
int c; /* current char */
long total; /* current total */
int sign; /* if '-', then negative, otherwise positive */
#if defined (_MT) && !defined (_UNICODE)
pthreadlocinfo ptloci = _getptd()->ptlocinfo;

if ( ptloci != __ptlocinfo )
ptloci = __updatetlocinfo();

/* skip whitespace */
while ( __isspace_mt(ptloci, (int)(_TUCHAR)*nptr) )
#else /* defined (_MT) && !defined (_UNICODE) */
while ( _istspace((int)(_TUCHAR)*nptr) )
#endif /* defined (_MT) && !defined (_UNICODE) */
++nptr;

c = (int)(_TUCHAR)*nptr++;
sign = c; /* save sign indication */
if (c == _T('-') || c == _T('+'))
c = (int)(_TUCHAR)*nptr++; /* skip sign */

total = 0;

while ( (c = _tchartodigit(c)) != -1 ) {
total = 10 * total + c; /* accumulate digit */
c = (_TUCHAR)*nptr++; /* get next char */
}

if (sign == '-')
return -total;
else
return total; /* return result, negated if necessary */
}

#define _tchartodigit(c) ((c) >= '0' && (c) <= '9' ? (c) - '0' : -1)

int StrToInt(char* str)
{
int total=0;
while(*str != 0)
{
total = total * 10 + (int)(*str - '0');
}
return total;
}

long __cdecl atol(
const char *nptr
)
{
int c; /* current char */
long total; /* current total */
int sign; /* if '-', then negative, otherwise positive */

/* skip whitespace */
while ( isspace((int)(unsigned char)*nptr) )
++nptr;

c = (int)(unsigned char)*nptr++;
sign = c; /* save sign indication */
if (c == '-' || c == '+')
c = (int)(unsigned char)*nptr++; /* skip sign */

total = 0;

while (isdigit(c)) {
total = 10 * total + (c - '0'); /* accumulate digit */
c = (int)(unsigned char)*nptr++; /* get next char */
}

if (sign == '-')
return -total;
else
return total; /* return result, negated if necessary */
}

vc6下的源码。 atoi 也是调用 atol


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