A function that transforms strings into a double. Supports E notation.

atod.c

#include <ctype.h>
#include <stdio.h>
#include <math.h>

double atod(char * s)
{
	int i;
	for(i=0;isspace(s[i]);i++);

		int sign = 1;
	for(;s[i] == '-' || s[i] == '+';i++)
	{
		if(s[i] == '-')
		{
			sign = -sign;
		}
	}

	double result = 0.0;
	for(;isdigit(s[i]);i++)
	{
		result = 10.0 * result + (s[i] - '0');
	}

	int pow10=0;
	if(s[i] == '.')
	{
		i++;
		for(;isdigit(s[i]);i++)
		{
			result = 10.0 * result + (s[i] - '0');
			pow10--;
		}
	}

	int exp_sign = 1;
	int ex = 0;
	if(s[i] == 'e' || s[i] == 'E')
	{
		i++;
		for(;s[i] == '-' || s[i] == '+';i++)
		{
			if(s[i] == '-')
			{
				exp_sign = -exp_sign;
			}
		}

		for(;isdigit(s[i]);i++)
		{
			ex = 10.0 * ex + (s[i] - '0');
		}

		int j;
		if(exp_sign > 0)
		{
			for(j=0;j<ex;j++)
			{
				pow10++;
			}
		}
		else
		{
			for(j=0;j<ex;j++)
			{
				pow10--;
			}
		}
	}

	double divisor = 1.0;
	for(i=0;i<abs(pow10);i++)
	{
		divisor *= 10.0;
	}

	printf("pow10: %d\n", pow10);//debug
	printf("exp_sign: %d\n", exp_sign);//debug
	printf("divisor: %f\n", divisor);//debug

	if(pow10 > 0)
	{
		result = result * divisor;
	}
	else
	{
		result = result / divisor;
	}

	return result;
}//atod

int main(int argc, char const *argv[])
{
	
	printf("%f\n", atod((char *)argv[1]));

	return 0;
}