question mark operator in expressions

Question

K&R Second Edition (page 71)-- I must have missed the explanation:

sign = (s[i] == '-') ? -1 : 1;

The context of this is a function that converts a string to a double. This part in particular comes after the function skips white space. I infer it is checking for positive or negative value, and saving it as either -1 or +1 for sign conversion at the end of the function... return sign * val /power;

I would like to do better than infer... I'm particularly unsure of what the ? and : 1 are doing here (or anywhere, for that matter).

It kind of seems like an abstract if statement. Where ? checks for truth and : is else... is that so? Is it limited to if/else?

I am a beginner and I haven't come across this expression syntax before, so I am wondering if there is a particular reason it seems to often be replaced by a formal if/else--besides, perhaps, readability?

Solution 1

It kind of seems like an abstract if statement.

That's correct. This is called a "ternary conditional operator".

The normal if works on statements, while the conditional operator works on expressions.

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I am wondering if there is a particular reason it seems to often be replaced by a formal if/else--besides, perhaps, readability?

There are cases where branching on statements is not enough, and you need to work on the expression level.

For instance, consider initialization:

const int foo = bar ? 5 : 3;

This could not be written using a normal if/else.

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Anyway, people who are saying it's equivalent to the if/else are being imprecise. While the generated assembly is usually the same, they are not equivalent and it should not be seen as a shorthand version of if. Simply put, use if whenever possible, and only use the conditional operator when you need to branch on expressions.

Solution 2

?: is the conditional operator in C.

In your example it would produce the same result as this if statement:

if (s[i] == '-')
{
    sign = -1;
}
else
{
    sign = 1;
}

Comments

[d0rmLife]

Chosen as answer because you addressed the why, as well as the what.