Convert String user input to a double - c

Question

I need to know how to convert a user input, which is a string, to a double. like if he writes in the string "23.45", it converts into double 23.45
(without any library functions).
I already got this code for integer, but don't know how to continue with double:
#include <stdio.h>
void main()
{
char input[100];
printf("Type a String which will be converted to an Integer: ");
scanf("%s", input);
int number = 0;
int i = 0;
if (input[i] >= 48 && input[i] <= 57)
{
while (input[i] >= '0' && input[i] <= '9')
{
number = number * 10;
number = number + input[i] - '0';
i++;
}
printf("string %s -> number %d \n", input, number);
}
else
{
printf("Enter a number! \n");
}
}

Answer 1

There's probably no reason why you'd roll out your own version of this, as strtod in stdlib.h already covers all manner of formats.
Here's a version which covers signed numbers as input and has some hints of where more suitable error handling could be placed:
#include <stdbool.h>
static void halt_and_catch_fire (void);
double strtod_homebrewn (const char* str)
{
double result = 0;
// handle signs:
bool is_negative = false;
if(*str == '-')
{
is_negative = true;
str++;
}
else if(*str == '+')
{
str++;
}
// handle the dot position:
bool is_dot_found = false;
double multiplier = 0.1;
// the actual conversion:
for(const char* s=str; *s!='\0'; s++)
{
if(*s >= '0' && *s <= '9') // ctype.h isdigit() would be preferred here
{
if(is_dot_found)
{
result += (*s - '0') * multiplier;
multiplier /= 10;
}
else
{
result *= 10;
result += *s - '0';
}
}
else if(*s == '.')
{
if(is_dot_found) // two dots?
{
halt_and_catch_fire(); // replace this with error handling
}
is_dot_found = true;
}
else if(*s != '\0') // all cases tested, some weird unknown character found
{
halt_and_catch_fire(); // replace this with error handling
}
}
if(is_negative)
{
result = -result;
}
return result;
}
static void halt_and_catch_fire (void)
{
halt_and_catch_fire();
}

Answer 2

#include <stdio.h>
void main()
{
char input[100];
printf("Type a String which will be converted to a double: ");
scanf("%s", input);
double number = 0.0;
double divider = 1.0;
int inFraction = 0;
int i = 0;
if (input[i] >= 48 && input[i] <= 57)
{
inFraction = 0;
while ((input[i] >= '0' && input[i] <= '9') || input[i] == '.')
{
if (input[i] == '.')
{
i++;
inFraction = 1;
continue;
}
number = number * 10.0;
number = number + input[i] - '0';
i++;
if (inFraction) divider *= 10.0;
}
number /= divider;
printf("string %s -> number %g \n", input, number);
}
else
{
printf("Enter a number! \n");
}
}

Answer 3

Edit: As clux pointed out, this fails when the fraction starts with zeroes. Bummer. Anyway, perhaps someone conceives a simple fix? I can only think of adding a "readzeroes()" function and let that run after the dot.
You already have a function to read an int. Simply use that. Pseudo code:
float read_float()
{
float f = read_int()
if(next is dot) skipdot else return f;
float frac = read_int()
while (frac>1) frac /= 10
return f+frac;
}

Answer 4

Edit: only use this approach for small number of digits after the decimal point.
Read the comments to know why it would fail for a large number of digits.
Since you mentioned without using any library functions, you could do something like this.
float number;
int decimal = 0;
int decimal_found =10;
while(input[i]!='\0')
{
if((input[i] <='0' || input[i] >='9')&&input[i]!='.' )
break;
if(input[i] == '.')
decimal = 1;
if(decimal == 1)
{
number = number + (input[i] - '0')/decimal_found;
decimal_found = decimal_found*10;
}
else
{
number = number *10;
number = number + input[i] - '0';
}
i++;
}
Simply check a decimal variable to know when decimal has been reached, then use and if else to have separate conditions for the number variable