Evaluating the expression 1 <= month <= 12

Question

I'm trying to evaluate expression (1 <= month <= 12) in a if condition.

This statement seems valid in javascript, but not in Java.

In Java,

int month = 0;
boolean flag = (1 <= month <= 12);

It throws following error:

The operator <= is undefined for the argument type(s) boolean, int

In Javascript,

var month = 0;
console.log('Expression evaluates to: ', (1 <= month <= 12));

It always returns true no matter what the value of month is.

Can someone please explain:

• If it is a valid expression or not?
• Why does it always yield to true in javascript?
• Why does java consider it as an invalid expression?

Also I know I can get it to work it this way (1 <= month && month <= 12). So, not looking for a solution but an explanation.

Thanks. Also let me know if my questions are not clear.

Answer 1

<= is non-associative, so you can't use it by repetition. You can specify it with:

1 <= month && month <= 12

The reason is that the JavaScript parser parses 1 <= month <= 12 as:

(1 <= month) <= 12

It's a consequence of the grammar of JavaScript, they could have defined it otherwise, but it would complicate the matter a bit. Most grammars define the expressions as:

expr -> [0-9]+
expr -> identifier
expr -> expr '<=' expr

(with an LALR) parser.

And Java uses the following (approximate) grammar:

expr -> numExpr '<=' numExpr
expr -> numExpr
numExpr -> identifier
numExpr -> [0-9]+
(...and so on...)

In Java it is thus even impossible to parse such expression (unless you perform a cast which makes it a numExp again).

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For the JavaScript part, why does it always return true?

Now (1 <= month) is a boolean (true/1 or false/0), and that value cannot be compared (reasonable) with 12 (0 and 1 are always less than or equal to 12). Only very limited programming languages support such feature.

Answer 2

Regarding the subquestion

Why does java consider it as an invalid expression?

It's because Java evaluates it the following way:

(1 <= month) <= 12
 boolean     <= int

Booleans and ints cannot be compared because of the type safety.