Ex_4.2_atof_scientific.c

/* Program demonstrating atof(char s[]).The function which converts the string to a floating 
   point value */
#include<stdio.h>
#include<ctype.h>
#define MAXLINE 100

double myatof(char s[]);
int mgetline(char line[],int maxline);

int main(void)
{
	char str[MAXLINE];
	double num;
	mgetline(str,MAXLINE);

	num=myatof(str);
	printf("%f",num);

	return 0;
}

double myatof(char s[])
{
	double val,pow;
	int sign,i,esign,exp;	
	int power(int base,int exp);	
		
	for(i=0;isspace(s[i]);i++)
		;

	sign=(s[i]=='-')?-1:1;

	if(s[i]=='+' || s[i] == '-')
		i++;

	for(val=0.0;isdigit(s[i]);i++)
		val = 10.0 * val + (s[i] - '0');
	
	if(s[i]=='.')
		i++;

	for(pow=1.0;isdigit(s[i]);i++)
	{
		val = 10.0 * val + (s[i] - '0');
		pow *= 10.0;
	}

	if(s[i]=='e' || s[i] =='E')
		i++;
	if(s[i]=='+' || s[i] =='-')
	{
		esign=s[i];
		i++;
	}

	for(exp=0;isdigit(s[i]);i++)
		exp=10.0 * exp + (s[i] - '0');
	
	if( esign == '-')
		return sign * (val / pow) / power(10,exp);
	else
	
		return sign * (val / pow) * power(10,exp);
}

int mgetline(char s[],int lim)
{
	int i,c;

	for(i=0;i<lim-1 && (c=getchar())!=EOF && c!='\n';++i)
		s[i] = c;
	
	if(c=='\n')
		s[i++]=c;

	s[i]='\0';
}
	
int power(int base,int exp)
{
	int power;
	power=1;
	while(exp-- > 0)
		power *=base;

	return power;
}