C assignments in an 'if' statement

Question

I came across this line of code written in C that confuses me coming from a JavaScript background.

short s;
if ((s = data[q])) 
    return s;

Is this assigning s to data[q], and if it equals true/1, return s?

Yes, you are right. This has been discussed [here](http://stackoverflow.com/questions/151850/why-would-you-use-an-assignment-in-a-condition) — byxor, Sep 02 '16 at 10:14
basically it returns s as long as data[q] is != 0. In C, everything except 0 is true. (of course s is assigned data[q]). — Nidhoegger, Sep 02 '16 at 10:15

Serve Laurijssen · Accepted Answer · 2016-09-02T10:19:49.020

11

Yes, an assignment...well assigns...but it's also an expression. Any value not equalling zero will be evaluated as true and zero as false.

it would be the same as

if ((s = data[q]) != 0) return s;

edited Sep 02 '16 at 10:19

answered Sep 02 '16 at 10:15

Serve Laurijssen

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2

"Evaluated as 1" is really confusing, it's more like "any non-zero value will be considered to be true". – unwind Sep 02 '16 at 10:18

score 7 · Answer 2 · answered Sep 02 '16 at 10:16

7

Your code is assigning data[q] to s and then returns s to the if statement. In the case when s is not equal to 0 your code returns s otherwise it goes to the next instruction.

Or better said it would expand to the following:

short s;
s = data[q];
if (s != 0)
    return s;

answered Sep 02 '16 at 10:16

Adrian Jałoszewski

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Oh that's interesting. So any other value than 0? Because what I learned from JavaScript is if (true) return something. – cocacrave Sep 02 '16 at 15:36
Every value which is not `0` is considered true in C. – Adrian Jałoszewski Sep 02 '16 at 15:37

score 2 · Answer 3 · answered Sep 02 '16 at 10:16

Basically C evaluates expressions. In

s = data[q]

The value of data[q] is the the value of expression here and the condition is evaluated based on that.

The assignment

s <- data[q]

is just a side-effect.

^{Read this [ article ] on sequence points and side-effects}

C assignments in an 'if' statement

3 Answers3