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cp4-2.c
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cp4-2.c
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#include<stdio.h>
#include<ctype.h>
#define MAX 100
double atof(char s[]);
main()
{
char s[20];
double f;
printf("input:");
gets(s);
f=atof(s);
printf("%f\n",f);
}
double atof(char s[])
{
int i,sign;
double val,power;
int sign_power;
int e;
for(i=0;isspace(s[i]);i++)
;
sign=(s[i]=='-')?-1:1;
if(s[i]=='+'||s[i]=='-')
i++;
for(val=0.0;isdigit(s[i]);i++)/*进行到小数点的时候循环就跳出了*/
val=10.0*val+(s[i]-'0');
if(s[i]=='.')
i++;
for(power=1.0;isdigit(s[i]);i++)
{
val=10.0*val+(s[i]-'0');
power*=10.0;/*与小数位数相同,123.12=12312/100 */
}
if(s[i]=='e'||s[i]=='E')
{
sign_power=(s[++i]=='-')?-1:1;/*指数的符号*/
if(s[i]=='+'||s[i]=='-')/*跳过符号*/
i++;
for(e=0;isdigit(s[i]);i++)
e=10*e+(s[i]-'0');
if(sign_power==1)/*正指数*/
while(e-- >0)
power/=10;/*缩小power,次数与指数相同,若原来power=100,正指数3,则缩小power到0.1*/
else
while(e-- >0)/*负指数*/
power*=10;/*扩大power*/
}
return sign*val/power;
}
/*
123.12e-6=123.12*10^-6=123.12/1000000
123.12=12312/100
123.12e-2=123.12/100=12312/10000
*/
/*sign_power=(s[i]=='-')?-1:1 如果写成sign_power=s[i],那么必须输入加号,此处则不需要*/