# 5.6. If-statement Pitfalls¶

## 5.6.1. Dangerous Semicolon¶

Regular statements must end with a semicolon. It turns out that the semicolon is all you need to have a legal statement:

;


We will see places that it is useful, but meanwhile it can cause errors: You may be hard pressed to remember to put semicolons at the end of all your statements, and in response you may get compulsive about adding them at the end of statement lines. Be careful NOT to put one at the end of a method heading or an if condition:

if ( x < 0); // WRONG PROBABLY!
Console.WriteLine(x);


This code is deadly, since it compiles and is almost surely not what you mean.

Remember indentation and newlines are only significant for humans. The two lines above are equivalent to:

if ( x < 0)
;  // Do nothing as statement when the condition is true
Console.WriteLine(x); // past if statement - do it always


(Whenever you do need an empty statement, you are encouraged to put the semicolon all by itself on a line, as above.)

If you always put an open brace directly after the condition in an if statement, you will not make this error:

if ( x < 0) {
Console.WriteLine(x);
}


Then even if you were to add a semicolon:

if ( x < 0) { ;
Console.WriteLine(x);
}


it would be a waste of a keystroke, but it would just be the first (empty) statement inside the block, and the writing would still follow: The extra semicolon would have no effect.

The corresponding error at the end of a method heading will at least generate a compiler error, though it may appear cryptic:

static void badSemicolon(int x);
{
x = x + 2;
// ...


This is another easy one to make and miss - just one innocent semicolon.

## 5.6.2. Match Wrong if With else¶

If you do not consistently put the substatements for the true and false choices inside braces, you can run into problems from the fact that the else part of an if statement is optional. Even if you use braces consistently, you may well need to read code that does not place braces around single statements. If C# understood indentation as in the recommended formatting style (or as required in Python), the following would be OK:

if (x > 0)
if (y > 0)
Console.WriteLine("positive x and y");
else
Console.WriteLine("x not positive, untested y");


Unfortunately placing the else under the first if is not enough to make them go together (remember the C# compiler ignores extra whitespace). The following is equivalent to the compiler, with the else apparently going with the second if:

if (x > 0)
if (y > 0)
Console.WriteLine("positive x and y");
else
Console.WriteLine("x not positive, untested y");


The compiler is consistent with the latter visual pattern: an else goes with the most recent if that could still take an else. Hence if x is 3 and y is -2, the else part is executed and the statement printed is incorrect: in this code the else clause is only executed when x is positive and y (is tested and) is not positive.

If you put braces everywhere to reinforce your indentation, as we suggest, or if you only add the following one set of braces around the inner if statement:

if (x > 0) {
if (y > 0)
Console.WriteLine("positive x and y");
}
else
Console.WriteLine("x not positive, untested y");


then the braces enclosing the inner if statement make it impossible for the inner if to continue on to an optional else part. The else must go with the first if. Now when the else part is reached, the statement printed will be true: x is not positive, and the test of y was skipped.

## 5.6.3. Missing Braces¶

Another place you can fool yourself with nice indenting style is something like this. Suppose we start with a perfectly reasonable

if (x > 0)
Console.WriteLine("x is: positive");


We may decide to avoid the braces, since there is just one statement that we want as the if-true part, but if we later decide that we want this on two lines and change it to

if (x > 0)
Console.WriteLine("x is:");
Console.WriteLine("  positive");


We are not going to get the behavior we want. The word “positive” will always be printed.

If we had first taken a bit more effort originally to write

if (x > 0) {
Console.WriteLine("x is: positive");
}


then we could have split successfully into

if (x > 0) {
Console.WriteLine("x is:");
Console.WriteLine("  positive");
}


This way we do not have to keep worrying about this question when we revise: “Have I switched to multiple lines after the if and need to introduce braces?”

The last two of the pitfalls mentioned in this section are fixed by consistent use of braces in the sub-statements of if statements. They fix the ; after if-condition problem only if the open brace comes right after the condition, but you still get a nasty error if you put in a semicolon between the condition and opening brace.