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(!( && !

Started by Tazel Jan 28, 2003 at 3:43 PM 13 replies 450+ views

Original Post

Jan 28, 2003 at 3:43 PM

Do these things mean the same thing:

if(!(blah))
return(0);

and

if(!blah)
return(0);

BTW, if(blah) is checking whether or not blah is existant? [edited by - Tazel on January 28, 2003 4:46:18 PM]

Sneftel

296

Sneftel

Jan 28, 2003 at 3:45 PM

They mean the same thing. They are each checking whether the value in "blah" is equal to 0.

Don't listen to me. I've had too much coffee.

[edited by - sneftel on January 28, 2003 4:49:40 PM]

Tazel

Author

Tazel

Jan 28, 2003 at 3:51 PM

It won''t compile? Then why do error checking that way?

tcache
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Tazel

Author

Tazel

Jan 28, 2003 at 3:52 PM

Never mind. I just saw Sneftel said that it''s checking if it''s FALSE.

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Tazel

Author

Tazel

Jan 28, 2003 at 4:57 PM

now, "if(!blah)" expands to "if(!foo && myfunc(4))", but "if(!(blah))" expands to "if(!(foo && myfunc(4)))"

Again, wouldn''t that be the same thing?

tcache
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Palidine

199

Palidine

Jan 28, 2003 at 5:02 PM

quote:
Original post by Tazel
Again, wouldn''t that be the same thing?

no.

"if(!foo && myfunc(4))" means if foo is false and myfunc(4) is true then execute the following logic

"if(!(foo && myfunc(4)))" means if BOTH foo and myfunc(4) are false then execute the following logic.

those are quite different

-me

felisandria

143

felisandria

Jan 28, 2003 at 5:04 PM

No. You have to use order of operations.

"if(!blah)" expands to "if(!foo && myfunc(4))"

would resolve to "if foo is not true, and myfunc(4) is true"

"if(!(blah))" expands to "if(!(foo && myfunc(4)))"

would resolve to "the opposite of the result of checking if foo and myfunc(4) are both true".

-fel

Tazel

Author

Tazel

Jan 28, 2003 at 5:45 PM

I see. So that happens when there are more than one things being evaluated.

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aftermath

Jan 28, 2003 at 5:57 PM

the parens tell the operand ( ''!'' in this case ) what context to evaluate.

felisandria

143

felisandria

Jan 28, 2003 at 6:09 PM

You understand Order of Operations, right Tazel? You know that

2 + 1 * 6 is different than
(2 + 1) * 6 ?

We just added some more operations into the order of operations, that you might not be used to. So, when in doubt, use more parenthesis. Or look up "operator precedence" and see what tier it''s on.

-fel

Thunder_Hawk

Jan 28, 2003 at 6:26 PM

This is why, if you feel the need to use macros, you must make liberal use of parentheses in the macro if it starts to gain much any kind of complexity, unless it''s the intended behaviour.

Stoffel

Jan 28, 2003 at 6:42 PM

!(a && b) is equivalent to (!a || !b)

Knowing a little boolean logic can get you pretty far in programming. Example:

(a || (!a && b)) is equivalent to (a || b), and easier to read if using long variable names.

T h i n k L i b e r a l l y ..

Tazel

Author

Tazel

Jan 28, 2003 at 9:17 PM

Fel: Yes. The parentheses and any grouping symbol''s contents are evaluated first.

Then powers.

Then multiplication and division from left to right.

Then add. and sub. from left to right.

tcache
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