How do you prevent a dangling else?

Answer 1

Answer
A "dangling else" is basically a bit of ambiguity as to how an if-then-else statement will be parsed - specifically, when the else clause should end.

A dangling else can look something like this:

void doStuff(int a) {

if(a == 10)
printf("TEN");

else
printf("NOT TEN");
a = 100;

}

The above code looks perfectly normal. If a is 10, then print "TEN" otherwise print "NOT TEN" and set a to 100.

With certain languages (C/C++ and Java included), that code would not quite work as described. The way the code is indented can make someone believe that it is doing something else.

According to the way C/C++ and Java would interpret the above code, it should look more like this:


void doStuff(int a) {

if(a == 10) {
printf("TEN");

}else {
printf("NOT TEN");

}

a = 100;

}

Which means that no matter what, a would be set to 100 at the end of the function call, which is not how the indentation made it look in the original example.

Keep in mind that both examples are identical in function if interpreted by a C/C++ or Java style compiler.

Answer
This problem arises with nested if-s:

if (cond1) if (cond2) stmt1; else stmt2;

It can be either

if (cond1) { if (cond2) stmt1; else stmt2; }

or

if (cond1) { if (cond2) stmt1; } else stmt2;

It is important to rembember that the compiler does not know which of those you meant, so it choose the first interpretation: the dangling else belongs to the inner if.

Answer 2

If you always use braces to explicitly define the scope of the if and the esle statement then the code becomes clearer to other programmers.

if (something) {

do_something;

} else {

do_something_else;

}

done;

Java and C/C++ allow short form constructs such as

if (something)

do_something;

else

do_someing_else;

but this is not always clear as in

if (something)

do_something;

else

do_something_else;

done;

Using a style checking tool like PMD allows developers to double check their code for these types of problems.

Answer 3

The ambiguity that arises in a nested if statement due to more number of if then the number of else clauses is also as dangling else problem. a confusion is created with which if does the additional else clause match up.