Shorthand for assignment if zero, like c = str[i] || 'x'

Question

(This is a very basic question, but I can't seem to find an answer because it involves ||, which is a problematic search term.)

Since a char can be interpreted as a boolean in a conditional like this:

if (str[i]) ...  
if (c) ...

where '\0' is considered falsey and any other character is truthy, I was expecting this to work too:

char c = str[i] || 'x';

but it's always giving a value of 1. Is there a simple shorthand in C for this? I've become used to it in JavaScript, and I find it clear and concise.

About the shortest you're going to get is `char c = str[i] ? str[i] : 'x';`. — Oliver Charlesworth, Aug 14 '16 at 22:37
@OliverCharlesworth I was hoping to avoid repeating str[i], but I guess the overhead is only visual, and therefor unimportant. — m69's been on strike for years, Aug 14 '16 at 22:38
The problem is that JS handles boolean expressions fundamentally different. It only works because in JS, any expression can be coerced into a boolean. In C, this is simply not the case. — Ingo Bürk, Aug 14 '16 at 22:42
@IngoBürk - Depends what you mean. In C, anything that equates to 0 is effectively equivalent to `FALSE` in relevant situations (i.e. `if`, operands to boolean operators, conditional operator). That applies to all types. — Oliver Charlesworth, Aug 14 '16 at 22:49
the `||` returns either 1 (if either operand is true) or 0 (if both are false), and it's that returned result you're assigning to `c`. You *could* do `char c; (c = str[i]) || (c = 'x');`, but that would do `c = str[i]` regardless of its value and follow with `c = 'x'` only if it was false. — Dmitri, Aug 14 '16 at 22:52
See also http://stackoverflow.com/q/16925429/583456 for a link to GCC extension which allows for `char c = str[i++] ?: 'x';`. — mike.dld, Aug 14 '16 at 22:54
What was the problem getting a list of C operators and learning what the `||` operator means? Best would be, of course to get a C book. Seraching for `||` is definitively not the only way! Any reason you expect two very different languages behave the same? — too honest for this site, Aug 14 '16 at 22:59
@Olaf Well, I read "C Primer Plus" in 1996, but that's a long time ago, and I haven't used C much since. — m69's been on strike for years, Aug 14 '16 at 23:04
@m69: The meaning of that operator has noit changed much. Though you should read a more recent book. Anyway, you apparently did no research on your own. — too honest for this site, Aug 14 '16 at 23:09
Well here I was thinking I'd do someone a favour by adding a code example to an answer; I won't be doing that again anytime soon. — m69's been on strike for years, Aug 14 '16 at 23:44

score 2 · Answer 1 · answered Aug 14 '16 at 22:38

2

The ternary operator is what you are looking for:

char c = str[i] ? str[i] : 'x';

Exaplanation: if str[i] is truthy, assign value of str[i] to variable c, other case assign letter 'x'.

answered Aug 14 '16 at 22:38

jedrzejginter

402
3
10

Thanks; any idea what to do with `c = str[i++] || 'x'`? – m69's been on strike for years Aug 14 '16 at 22:48
I assume that it works like in PHP, where `'x'` would be treated as truthy value, so `[whatever] OR true -> true`. That's why you get `1` (true in C). – jedrzejginter Aug 14 '16 at 22:51
I meant what to do with the index increment; both `char c = str[i++] ? str[i] : 'x'` and `char c = str[i] ? str[i++] : 'x'` do something different than `c = str[i++] || 'x'`. (But I guess that's another question.) – m69's been on strike for years Aug 14 '16 at 22:54
So, you want to always increment the value of `i`. Maybe something like this (it looks a little bit stupid, but if you want to do it in one line...): `var c= str[i++] ? str[i-1] : 'x';` – jedrzejginter Aug 14 '16 at 22:59
2

Please don't actually write that :( – Oliver Charlesworth Aug 14 '16 at 23:00
1

Yeah, I know :) just wanted to show, that's possible – jedrzejginter Aug 14 '16 at 23:01
@dzej.5: That has undefined behavior of `i++` overflows. – Keith Thompson Aug 14 '16 at 23:14
Yup, I regret that I wrote that line :) – jedrzejginter Aug 14 '16 at 23:17

score 1 · Accepted Answer · answered Aug 14 '16 at 22:56

You should handle it like any other abstraction, and write a function for it:

char non_zero_or_default(const char c, const char d)
{
    return c ? c : d;
}

then use it as:

char c = non_zero_or_default(str[i++], 'x');

You can inline it if you're worried about overhead.

Shorthand for assignment if zero, like c = str[i] || 'x'

2 Answers2