atof函数的实现

#include <stdio.h>


double my_atof(char *s)
{
int flag=1;
double sum=0.0;
if(s==NULL)
return 0;
while(*s==' ')
s++;
if(*s=='-')
{
flag=-1;
s++;
}
else if(*s=='+')
{
s++;
}
while(*s<='9'&&*s>='0')
{
sum=sum*10+*s-'0';
s++;
}
if(*s=='.')
s++;
double cn=1.0;
while(*s<='9'&&*s>='0')
{
sum=sum*10+*s-'0';
cn*=10;
s++;
}
return flag*sum/cn;




}
int main()
{
char t[100];
double a;
printf("Enter a string:");
scanf("%s",t);
a=my_atof(t);
printf("%lf",a);
    return 0;
}