Question

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What does '?' do in C++?

xo = ((temp.npieces%2)==1)? 1 : 2;

I can’t figure out what this means in it’s entirety.

xo is an integer (I think simply determining the difference between a naught and a cross)

temp is an instance of position which stores information about a naughts and crosses board (positions on the board of the naughts and crosses, the state of the game (i.e. whether it has been won, lost, or is undecided), and the number of pieces placed so far upon the board

npieces stores the number of pieces placed so far

%2 is obviously determining the remainder of npieces divided by 2, in order to determine whether this is odd or even, and thus whether naughts or crosses plays next

It’s the piece at the end that confuses me, the ? 1 : 2 bit, I’m sure I’ve seen this or things like this before, but I’m afraid I’m not sure what this does. I know this is likely pretty basic stuff, but I was quite ill in my first year of University (I’m in my second year now) and missed odd fragments so I have irritatingly random, often embarassing, gaps in my knowledge.

(This is part of a mass of code for an assignment I’m working on – trying to make sense of a few hundred lines of code, this included. As this is an assignment, I am more than happy not to be told exactly what this line means – and indeed just to be pointed in the direction of a resource that explains ? 1 : 2 so that I can work it out for myself. Either an online resource, or a larger chapter in any textbook available in Safari Books Online would be very helpful, or, of course, a very brief explanation here.)

Thanks.

And my apologies for the un-specific question title, without knowing what the unfamiliar code does or is called, I couldn’t think of a better way to phrase it.

Answer 1

xo = ((temp.npieces%2)==1)? 1 : 2;

if temp.npieces is Even (therefore division remainder is 0) xo is now equal to 2
other wise xo is now equal to 1;

Edit : It does not check if it is Odd, it checks whether the remainder is positive 1.

X ? A : B 

is another way of writing

if(X)
{
    A
}else
{
    B
}

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Edit2 :

The purpose of the conditional operator is not to be faster, or more consise. The conditional operator returns a value. if does not. That’s the main difference. Therefore x ? A : B isn't exactly the same as if( x ) { A; } else { B; }

Credit @ John Dibling

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in C Something is true if it is not equal to 0. (even if it is -145)