Hello guys,
I am following K&R to advance my C skills. In the book I found a piece of code which showed how atoi() works. It said like s[i] - '0' will give a numeric value of the character. Can someone please explain it to me. I tried to search around but they only explain it with integers and not with characters. Say you are given s[i] = 'Z'.
Thanks.
Characters (as in the char types in C) are actually represented as a set of integral values.Originally Posted by Sankait Laroiya
The character '0' is a printable character that, when displayed on or written to an output device (a screen, etc) produces a result that looks like a zero. However, the character '0' does NOT have a numeric value of zero (for example, in the ASCII character set, '0' has the numeric value of 48) in any character set - and doesn't necessarily have the same value in different character sets. Similarly for '1' through '9'.
With all standard character sets (ASCII, EBCDIC, etc) that might be used by people who represent characters using arabic digits (0-9) the characters '0'-'9' are sequential (i.e. '0' is immediately before '1' is immediately before '2' ....) and the character '0' (the one rendered so it looks like a zero) does not have a numeric value of zero. So '1' is equal to '0' + 1, '2' is equal to '0' + 2, etc. Or, conversely, subtracting '0' is the way to map the printable digits ('0' through '9') to the numeric values (0 through 9).
This mapping is not useful for characters that represent things other than digits (letters, punctuation, whitespace, control codes, etc). The result of subtracting '0' from 'Z' will give an integral value, but that value is not guaranteed to correspond to any particular character (the meaning does differ between character sets).
when I input Z as an argument, or any other alphabet, atoi gives value 0. So, what is the use of it.
If you were writing a function to convert a string to an integer, what result would you produce from the string "ZZZZ"?
What exactly were you expecting atoi to do with 'Z' as input? It is not a number, and therefore cannot be converted to a number. If you use strtol with a base of 36, 'Z' will likely give you a meaningful value. Under basically any other conditions, 'Z' is meaningless as a numeric digit.
What can this strange device be?
When I touch it, it gives forth a sound
It's got wires that vibrate and give music
What can this thing be that I found?
Well, if it cannot convert character, and only returns integers passed to it then what is the use of it?
It is still a 0.
Its use is for converting strings known to be numeric to integer representation. What is it that you were hoping for?
What can this strange device be?
When I touch it, it gives forth a sound
It's got wires that vibrate and give music
What can this thing be that I found?
I know what atoi() does. You haven't answered my question though.
Okay, guys, I thought atoi() would convert string (including characters) to integers. Now I came to know it just converts the type of the char to int if the char is a string of digits.
You just worked that out, eh? The documentation for atoi() does say that.
