Thread: Problems interpreting simple syntax construct and explanation from K&R

This is excerpted from K&R pages. 42-43

The explanation reads that this function coverts a string of digits into its numeric equivalent.

Do they explicitly mean numbers by saying digits? If so, what does it's numeric equivalent mean if digits are already numbers?

Code:

int atoi(char s[])
{
	int i, n;
	
	n = 0;
	for (i = 0; s[i] >= '0' && s[i] <= '9'; ++i)
		n = 10 * n + (s[i] - '0');

	return n;
	
}

I'm having problems understanding what is happening here:

Code:

(s[i] - '0');

It says that this gives the numeric value of the character stored in s[i], because the values of '0', '1', etc, form a contiguous increasing sequence.

I really don't understand that construct or what this is saying.

Help would be greatly appreciated.

What is the point of including - '0', if I'm just looking for the value of whatever is stored in s[i]?

Woah. I was really confused. I didn't realize that '0' was an ASCII character that had an internal value, so it read as just zero to me. Laser cleared that up for me.

Thanks to all of you guys though, and the speedy replies.

Originally Posted by billybob2020

Woah. I was really confused. I didn't realize that '0' was an ASCII character that had an internal value, so it read as just zero to me. Laser cleared that up for me.

Thanks to all of you guys though, and the speedy replies.

Ascii Table - ASCII character codes and html, octal, hex and decimal chart conversion