Convert a double variable to a string

DoubleToString.c

#include <windows.h>
#include <stdio.h>
#include <tchar.h>

TCHAR * DoubleToString (TCHAR * buffer, double value, unsigned int precision)
{
	//	'i' is going to be the current index into the buffer
	unsigned int i = 0, j = 0, nDigits = 0;
	unsigned long integer;
	
	//	First check the sign of the number
	if (value < 0.0) {
		value *= -1.0;
		buffer[i++] = '-';
	}
	
	//	Then take care of the integer part of the number
	integer = (unsigned long)value;
	//	Count how many digits the integer part has
	do 
	{
		nDigits++;
		integer /= 10;
	} while (integer);
	
	//	Get back the integer part because we just messed with it when we got the number of digits above
	integer = (unsigned long)value;
	//	Substract the integer part from the number so that we'll be left with the decimal part
	value -= (double)integer;	
	j = nDigits;
	do {
		//	Now copy each digit from the integer part into the string buffer
		buffer[i+nDigits-1] = '0' + (integer%10);
		integer /=10;
		nDigits--;
	} while(integer);
	//	Keep track of where the current index in the string buffer is after adding the digits
	i += j;
	//	Add the decimal point
	buffer[i++] = '.';
	
	//	Then take care of the real part
	//	Basically we're going to multiply the real part by 10, which gives us a number like x.yzw (because the real part was 0.xyzw)
	//	We then cast that number to an integer, which gives us 'x', and we then append that to the buffer.
	//	After that we substract the 'x' from x.yzw and we're left with 0.yzw
	//	We then repeat, decrementing the precision variable so that we can stop after 'precision' decimals.
	while (precision)
	{
		value *= 10.0;
		buffer[i++] = '0' + (unsigned int)value;
		value -= (double)(unsigned long)value;
		precision--;
	}
	//	Null terminate the buffer and return it
	buffer[i] = 0;
	return buffer;
}

int main ()
{
	double value = 23.4119;
	TCHAR buffer[32];
	unsigned int precision = 5;
	
	_tprintf (TEXT("sizeof(TCHAR) is %d\n"), sizeof(TCHAR));
	
	MessageBox (0, DoubleToString (buffer, value, precision), TEXT("Value"), MB_ICONINFORMATION);
	return 0;
}