自己实现字符串转数字

A. atoi，字符串转整数

使用assert强制规定字符串一定是合法的，剩下的就是正负号了。

#include <stdio.h>
#include <assert.h>

int atoi(const char* str) {
if (str == NULL)
return 0;

int num = 0, i = 0;
bool neg = false;
if (str[0] == '-') {
        neg = true;
        ++i;
    }

while (str[i] != '\0') {
assert (str[i] >= '0' && str[i] <= '9');
        num *= 10;
        num += int(str[i]-'0');
        ++i;
    }

if (neg)
return -num;
return num;
}


int main() {
    printf("%d\n", atoi("233"));
    printf("%d\n", atoi("980"));
    printf("%d\n", atoi("066"));
    printf("%d\n", atoi("-123"));

return 0;
}

B. stof，字符串转float

这里只实现了简单的浮点数形式，没有实现像1e-10这样子的转化。
一开始输出的时候，以为是精度的问题，然后发现，转化的精度方面不会差太多，主要是float表示不了那么精确的浮点数，所以导致结果输出与原始数字有点不同：
233.0000000000, real=233.0000000000

3.1415927410, real=3.1415927410

123.4567871094, real=123.4567871094

-123.4567871094, real=-123.4567871094

#include <stdio.h>
#include <assert.h>


/*
// version 1
float stof(const char* str) {
 float num = 0;
 int i = 0;
 for (i = 0; str[i] != '\0' && str[i] != '.'; ++i) {
 assert(str[i] >= '0' && str[i] <= '9');
 num *= 10;
 num += int(str[i] - '0');
 }

 if (str[i] == '\0')
 return num;

 float digit = 0.0, div = 1.0;
 for (++i; str[i] != '\0' && str[i] != '.'; ++i) {
 digit *= 10.0f;
 digit += int(str[i] - '0');
 div *= 10.0f;
 }

 return digit/div + num;
}*/

// version 2: maybe more precise
float stof(const char* str) {
if (str == NULL)
return 0.0f;

float num = 0;
int i = 0;
bool neg = false;
if (str[0] == '-') {
        neg = true;
        ++i;
    }

while(str[i] != '\0' && str[i] != '.') {
assert(str[i] >= '0' && str[i] <= '9');
        num *= 10;
        num += int(str[i] - '0');
        ++i;
    }

if (str[i] == '\0')
return (neg) ? -num : num;

int start = i + 1, end = start;
while (str[end] != '\0' && str[end] != '.')
        ++end;

float digit = 0.0f;
for (int j = end-1; j >= start; --j) {
assert(str[j] >= '0' && str[j] <= '9');
        digit += int(str[j] - '0');
        digit *= 0.1f;
    }

return (neg) ? -(digit + num) : (digit + num);
}

int main() {
float a = 233.0f, b = 3.1415928f, c = 123.456789f;
    printf("%.10f,\t real=%.10f\n\n", stof("233"), a);
    printf("%.10f,\t real=%.10f\n\n", stof("3.1415928"), b);
    printf("%.10f,\t real=%.10f\n\n", stof("123.456789"), c);
    printf("%.10f,\t real=%.10f\n\n", stof("-123.456789"), -c);

return 0;
}