for loop without {} does not compile (does with {})

[Thorton]

Why does this not compile?
for (int i = 0; i < 5; i++)
int j = i;

But this does:
for (int i = 0; i < 5; i++) {
int j = i;
}

Never mind that it doesn't really make sense to do either.

[Supamagier]

It indeed doesn't. Interesting question :) I don't know :P

[Lil_Aziz1]

Because of scope. When you do { }, int j is existence inside the brackets. Without, you're trying to overload a variable name.

int j;
for (int i=0; i<5; i++)
j = i;

will work w/out brackets.

[Tolls]

It's not scope.
It's to do with what you're allowed to have as the single line of an if statement without {}. A declaration is not allowed. It has to be a statement.

I'd quote the specifics, but I haven't got the JLS to hand.

[JosAH]

I'd quote the specifics, but I haven't got the JLS to hand.

I have: the syntax of a for-loop statement is basically: "for ( ... ) Statement" and a "Statement" is syntactically explained in paragraph "14.5 Statements"; a declaration is not a "Statement", it is part of a block which in turn is syntactically equivalent to a "Statement".

kind regards,

Jos

[Thorton]

Yes, after some research:

Variable declarations are not allowed for short statements:
Blocks and Statements
Blocks and Statements

This is apparently to prevent dangling else statements. In a for statement, there wouldn't be a dangling else statement, but I can see why the same check is performed for short if statements and short for/while statements.

[JosAH]

When you think of it if a declaration would've been a statement if strongly complicates the scoping rules, e.g.

for (int i= 0; i < n; i++)
   int x;
x= 42; // does x exist?

The body of the for-loop is never executed if n < 0. Should it be determinable at runtime? That is not how lexical scoping works, so Gosling wisely decided that a declaration cannot be a statement.

kind regards,

Jos