string to float conversion? - c++

Question

I'm wondering what sort of algorithm could be used to take something like "4.72" into a float data type, equal to
float x = 4.72;

Answer 1

scanf, operator>> for istreams, and strtof would be the obvious choices.
There is also atof, but, like atoi, it lacks a way to tell you there was an error in the input, so it's generally best to avoid both.

Answer 2

For C++ you can use boost::lexical_cast:
std::string str( "4.72" );
float x = boost::lexical_cast< float >( str );
For C you can use sscanf:
char str[]= "4.72";
float x;
sscanf( str, "%f", &x );

Answer 3

For C++ This is the algorithm I use:
bool FromString(const string& str, double& number) {
std::istringstream i(str);
if (!(i >> number)) {
// Number conversion failed
return false;
}
return true;
}
I used atof() in the past for the conversion, but I found this problematic because if no valid conversion can be made, it will return (0.0). So, you would not know if it failed and returned zero, or if the string actually had "0" in it.

Answer 4

For C strtod() and C99 friends strtof() and strtold() (description on same link) already have that algorithm implemented.
If you are having problems writing your own, post your code and specific questions about it.

Answer 5

As you've asked for an algorithm, not a method, here is my explanation for a simple algorithm (and an implementation in C):
Initialize 4 integer variables, one for the value before dot, one for the after part, one for the power of the mantissa, one for the sign. Let's say, f, m, d, sign = 1.
First look for + or - sign at the beginning. If there are no sign characters or + sign then continue. If the first character is -, then sign = -1.
Then, read an integer value into f until a . or NULL character.
If you end up with a dot character, then start reading the mantissa part as in the previous step into m. But this time also multiply d by 10 with each digit.
In the end, return sign*(f + (float) m/d). The casting makes sure the division is done in floating-points and the type of the expression is float.
I guess, reading the code might be easier. So here is the code:
float atof(char *s)
{
int f, m, sign, d=1;
f = m = 0;
sign = (s[0] == '-') ? -1 : 1;
if (s[0] == '-' || s[0] == '+') s++;
for (; *s != '.' && *s; s++) {
f = (*s-'0') + f*10;
}
if (*s == '.')
for (++s; *s; s++) {
m = (*s-'0') + m*10;
d *= 10;
}
return sign*(f + (float)m/d);
}

Answer 6

You can use boost:lexical_cast
http://www.boost.org/doc/libs/1_44_0/libs/conversion/lexical_cast.htm

Answer 7

I assume you want an actual algorithm, not a library function that already does it. I don't have time to write and test actual code, but here is what I would do:
Initialize a float that will be used as an accumulator to 0.
Figure out where the decimal place is in the string, this will let you know what "column" each of the digits is (i.e. 100s, 10s, 1s, 1/10ths, etc).
Start at the beginning of the string.
Take that digit, convert to an int (trivially done by subtracting 0x30 from the ASCII value)
Multiply the value by the place column (for the first digit in your example, that would be 4 * 1 == 4, for the next digit 7 * 0.1 == 0.7).
Add the result to the accumulator
Repeat from step 4 for each remaining digit.
The accumulator now contains your result.
Due to the roundoff of converting between base 10 and base 2 in every iteration of this loop, the result you get from this algorithm may not be the closest possible binary representation to the original value. I don't really know of a good way to improve it though... perhaps someone else can chime in with that.

Answer 8

The atof() function can be helpful.
http://www.cplusplus.com/reference/clibrary/cstdlib/atof/

Answer 9

From cplusplus.com: "stringstream provides an interface to manipulate strings as if they were input/output streams."
You can initialize a stringstream with your string then read a float from the stringstream using operator>> just like you would with cin.
Here is an example:
#include<iostream>
#include<string>
#include<sstream>
using namespace std;
int main() {
string s = "4.72";
stringstream sstrm(s);
float x;
sstrm >> x;
cout << x << endl;
}