How to parse float number

[amarendrajoshi]

Hi, my name is Amarendra..
I am building a parser for CAD translator.. My parser right now can parse integer numbers easily. But it has a problem while reading float number. Could anybode please help me.
The code is as follows.

if (isdigit(ch))
            {
                int num=0;
                do
                {
                    //no checking for overflow
                    num=10 * num + ch - '0';
                }while((ch = read_ch())!=EOF && isalnum (ch));
                put_back(ch);
                return num;
            }
}
 

[horace1]

It depends how sophisticated you want to be. You could
1.loop reading the whole number part – exit on a .
2.loop reading the fractional part exit on a non digit

For example the following assumes the first non digit is a . so you would want to add error checking, etc

#include <stdio.h>
 
int main()
{
    float num=0, decimal, divisor=10;;
    char ch;
    while(1)
         if(isdigit(ch=getche())) num=10 * num + ch - '0';
         else break;                   // assume a . here
    while(1)
          if(isdigit(ch=getche())) {num=num + (ch - '0')/divisor; divisor *= 10; }
          else break;                   // non digit
    printf("num = %f\n", num);
}
 

[horace1]

Simpler version of previous code would be

#include <stdio.h>
#include <ctype.h>
int main()
{
    float num=0, decimal, divisor=10;;
    char ch;
    while(isdigit(ch=getchar())) num=10 * num + ch - '0';
    // assume a . here
    while(isdigit(ch=getchar())) {num=num + (ch - '0')/divisor; divisor *= 10; }
    // non digit
    printf("num = %f\n", num);
}
 

or in C you could use sscanf(), e.g.

// parse a decimal number
#include <stdio.h>
#include <ctype.h>
int main()
{
    char data[]=" 3.14159";
    float fnum=0;
    // attempt to read a float from data[]
    if(sscanf(data, "%f", &fnum)) printf("float = %f\n",fnum);
    else                          printf("input fail\n");
}
 

program output would be
float = 3.141590


or in C++ you could use strstream, e.g.

// parse a decimal number
#include <iostream>
#include <strstream>
using namespace std; 
 
int main()
{
    char data[20]=" 3.1415926   2.718";
    float fnum=0;
    // attempt to read a float from data[]
    istrstream str(data,20);
    str >> fnum;
    if(str.fail())  { cout << "input fail\n"; exit(1); } 
    cout << "float = " << fnum << endl;
    str >> fnum;
    if(str.fail())  { cout << "input fail\n"; exit(1); } 
    cout << "float = " << fnum << endl;
}
 

program output would be
float = 3.14159
float = 2.718