Detect a Dimmension object without .typename

thomthom

I didn't understand that example. But I did a test:


def always_return_false
	puts 'call always_return_false'
	return false
end

def always_return_true
	puts 'call always_return_true'
	return true
end


def test1
	if always_return_false && always_return_false
		#...
	end
end

def test2
	if always_return_true && always_return_true
		#...
	end
end

When I run the code:


>> test1
call always_return_false
nil
>> test2
call always_return_true
call always_return_true
nil

In the first case when the first check returns false it doesn't trigger the second check. I can't understand anything else than .kind_of? would do the same.

tbd

my mistake, i was wrong. if left side is false the right side doesn't get evaluated

Jim

@unknownuser said:

if left side is false the right side doesn't get evaluated

This is my understanding also, but watch because the and operator is not the same as &&. I don't think and will shortcut.

thomthom

@jim said:

but watch because the and operator is not the same as &&. I don't think and will shortcut.

I did two more tests for this:


def test3
	if always_return_false and always_return_false
		#...
	end
end

def test4
	if always_return_true and always_return_true
		#...
	end
end

Results:


>> test3
call always_return_false
nil
>> test4
call always_return_true
call always_return_true
nil

thomthom

Found what the difference between and and && is:

@unknownuser said:

The binary "and" operator will return the logical conjunction of its two operands. It is the same as "&&" but with a lower precedence

AdamB

@thomthom said:

Found what the difference between and and && is:

@unknownuser said:

The binary "and" operator will return the logical conjunction of its two operands. It is the same as "&&" but with a lower precedence

Well that explains something that I've been bitten by a few times..

mask = mask or object.getmask

assigns mask to itself and ors with the results of object.getmask()!!

I end up having to do:

mask = (mask or object.getmask)

What kind of madman would introduce such an operator?

Actually the one that beats all for sheer insanity is ruby.h #define-ing fopen() to be something completely different calling some Ruby thing. What the!, Argggh...

I've got some harsh language for Mr.Ruby when/if I meet him.

Adam

thomthom

I think I've always used && and || so I've avoided such problems. But I have had unexpected behaviour when I used not instead of !. I used not some times simply because I thought it was the same thing - but not would read better.

Looking at the table of Operator Precedence I can see how it all fits together now. http://www.techotopia.com/index.php/Ruby_Operator_Precedence

AdamB

Sure, it lists them out.

But I see absolutely no compelling reason to have "Logical AND" differing from "Logical composition" wrt precedence.

What is the 'use case' for the 2 forms? Does anyone know?

Adam

tbd

@adamb said:

What is the 'use case' for the 2 forms? Does anyone know?

maybe to play jokes like this :


myvar = true and false
=> false
myvar
=> true

RickW

@adamb said:

I end up having to do:

mask = (mask or object.getmask)

But now you know you can use

mask = mask || object.getmask

It saves typing the parentheses... (FWIW)

thomthom

Saves you typing even more if you type mask ||= object.getmask