單精準度轉換成十進位

http://zh.wikipedia.org/wiki/%E5%96%AE%E7%B2%BE%E5%BA%A6%E6%B5%AE%E9%BB%9E%E6%95%B8

#include<iostream>
#include<math.h>
#include<string>
using namespace std;
int main()
{
//01000000011000000000000000000000
//10111111100000000000000000000000
//01000001010010000000000000000000
string input;
string a="",b="",c="",b_invert="";
double d_b=0.0,d_c=0.0,ans=0;
cout<<"please input number";
cin>>input;
cout<<"szie="<<input.length()<<endl;
a=input[0];

for(int i=1;i<=8;i++)
b=b+input[i];
for(int j=7;j>=0;j--)
b_invert=b_invert+b[j];

for(int i=0;i<=7;i++)
d_b=d_b+(b_invert[i]-48)*pow(2.0,i);
d_b=d_b-127;

cout<<"exp = "<<d_b<<endl;


for(int k=9;k<=31;k++)
c=c+input[k];

for(int l=0;l<=22;l++)
d_c=d_c+(c[l]-48)*pow(2.0,-1*(l+1));
cout<<"dc = "<<d_c<<endl;
d_c=1+d_c;

cout<<"fraction = "<<d_c<<endl;

ans=d_c*pow(2.0,d_b);
if(a=="0")
cout<<"ans = +" ;
else
cout<<"ans = -" ;
cout<<ans<<endl;

return 0;
}