Entering If Statement Despite Not Meeting Condition

Question

This is a specific problem, but I can't seem to figure out what is wrong.

else if (X == 2)
        //move left
    {
        if (Level[X-1][Y] == 0);
        {

            cout << Level[X-1][Y] << "\n";
            cout << "\n MOVING LEFT from RIGHT\n";  //PROBLEM IS HERE

            Level[X][Y] = 1; // block it
            X = X - 1;
            moved = 1;
        }
    }

What I am doing is I am checking if Level[X-1][Y] is 1, indicating a column, so I can not move my player there. However for some reason, despite it being 1 and not 0 (as indicated by the output), the IF statement is still accessed. Any help will be appreciated.

I think compilers can warn about this ([*and I was right*](http://coliru.stacked-crooked.com/a/b27d75968d98eb0b)). — chris, Oct 22 '13 at 00:51
I'm not sure I understand. Why would it enter if the statement is false? My G++ compiler isnt giving any warnings either. — msmf14, Oct 22 '13 at 00:55
First, look at the warning message in the link. It's pretty clear. Secondly, add the warning options shown in the link's command line and I'm sure you will. — chris, Oct 22 '13 at 01:00

Shafik Yaghmour · Answer 1 · 2013-10-23T00:58:23.933

Your problem is here:

if (Level[X-1][Y] == 0);
                       ^

the ; ends the if statement. What follows the if statement is just a compound statement like this:

{
  //Code
}

and are completely valid on their own and have many uses, one use of which is creating a block scope.

For completeness sake if we go to the draft C++ standard section 6.2 Expression Statements we see that the ; just terminates a null statement. The grammar is as follows:

expression-statement:
  expressionopt ;
                ^

and it also says:

[...]An expression statement with the expression missing is called a null statement.[...]

score 6 · Accepted Answer · answered Oct 22 '13 at 00:56

6

Semi-colon!!!

   if (Level[X-1][Y] == 0);

answered Oct 22 '13 at 00:56

paddy

60,864
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3

Can you explain to the OP what the semicolon means? – pburka Oct 22 '13 at 00:58
wooopsies. Thanks a lot, silly question on my part, spent hours on this!! – msmf14 Oct 22 '13 at 00:58
@msmf14 - you're not the first to get hung up on something like this. Eventually, checking for that accidental semi-colon becomes automatic. Until then it can be very frustrating. – Pete Becker Oct 22 '13 at 11:57

score 4 · Answer 3 · answered Oct 22 '13 at 00:58

if (Level[X-1][Y] == 0);
//                     ^

Get rid of this semicolon.

It would make the logic like this: if Level[X-1][Y] is zero, do nothing, then run the following code(the compound statement you thought was belong to the if). It's equivalent to:

if (Level[X-1][Y] == 0)
{
    ;
}
{
    cout << Level[X-1][Y] << "\n";
    cout << "\n MOVING LEFT from RIGHT\n";
    Level[X][Y] = 1;
    X = X - 1;
    moved = 1;
}

Entering If Statement Despite Not Meeting Condition

3 Answers3

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