將一個字符串轉換成數字

/*
function:將輸入的字符串轉換成數字；默認爲double型
*/
#include<stdio.h>
#include<stdlib.h>
#include<string.h>


double convert(char str[],int n);
int main(void)
{
char str[40];
double s;
scanf_s("%s", str, sizeof(str));
s = convert(str,40);
printf("%.40lf\n", s);
system("pause");
return 0;
}
double convert(char str[],int n)
{
double sum = 0.0;
int sign;
int count;
int scount;
int power = 1;
int p;
double spower = 0.1;
double s;
if (str[0] == '-')
{
sign = -1;
}
else
{
sign = 1;
}
for (int i = 0; i < n; i++)
{
if (str[i] == '\0')
{
p = i;
count = i;
scount = i;
break;
}
}
for (int i = 0; i < n; i++)
{
if (str[i] == '.')
{
count = i;
scount = i;
break;
}
}
while (count - 1 >= 0 && str[count - 1] >= '0' && str[count - 1] <= '9')
{
s = (double)(str[count - 1] - '0')*power;
sum += s;
count--;
power *= 10;
}
while (scount + 1 < p && str[scount + 1] >= '0' && str[scount + 1] <= '9')
{
s = (double)(str[scount + 1] - '0')*spower;
sum += s;
scount++;
spower /= 10;
}
sum *= sign;
return sum;
}