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macOS && ||

L

larswik

macrumors 68000
Original poster
Sep 8, 2006
1,552
11
These AND & OR are driving me nuts! They are always backwards from what I think.

I have this code in my dice program
Code: 
do {
        dieRolls[0] = arc4random() % 100 + 1;
    } while ((dieRolls[0] > [numberNoMoreThen intValue]) || (dieRolls[0] < [numberNoLessThen intValue]));

I read in a book once that said think of these to operators like this

1) && - You can go to the movies if your room is clean AND dishes are done. Both have to be true to return a true.

2) || - You can go to the movies if your room is clean OR dishes are done. One or the other needs to be true to return a true.

This code works, but I first used the && operator. I scrapped the popUpButton idea and went with another slider to set the min low and max high values of the roll.

It would seem that out of a total max range of 1 to 100. I set the low to 20 and the high to 80 that if the roll was above 80 AND bellow 20 it would re-roll the dice until it was in-range. But && does not work, but the || works fine. I could set min to 49 and max to 50 and I would get one or the other value when I press the roll dice button.

Why, it seems so backwards? This logic is not logical.
 
cmaier

cmaier

Suspended
Jul 25, 2007
25,405
33,471
California
larswik said:
These AND & OR are driving me nuts! They are always backwards from what I think.

I have this code in my dice program
Code: 
do {
        dieRolls[0] = arc4random() % 100 + 1;
    } while ((dieRolls[0] > [numberNoMoreThen intValue]) || (dieRolls[0] < [numberNoLessThen intValue]));

I read in a book once that said think of these to operators like this

1) && - You can go to the movies if your room is clean AND dishes are done. Both have to be true to return a true.

2) || - You can go to the movies if your room is clean OR dishes are done. One or the other needs to be true to return a true.

This code works, but I first used the && operator. I scrapped the popUpButton idea and went with another slider to set the min low and max high values of the roll.

It would seem that out of a total max range of 1 to 100. I set the low to 20 and the high to 80 that if the roll was above 80 AND bellow 20 it would re-roll the dice until it was in-range. But && does not work, but the || works fine. I could set min to 49 and max to 50 and I would get one or the other value when I press the roll dice button.

Why, it seems so backwards? This logic is not logical.
Click to expand...

Think of it this way - a number cannot be both above 80 AND below 20. Try to think of a number which is both above 80 AND below 20. It can't be done.

So if a number is above 80 OR the number is below 20, it is out of range.
 
AustinZ

AustinZ

macrumors member
Aug 6, 2008
73
0
It's logically impossible for the value of a number to be both above 80 and below 20 at the same time.

Think of it this way: I decided how to spend my night. Either I did homework, I went to see a movie, or I went to the bar with friends. (Only one can be true.)

I can say that 'if I went to see a movie' || 'if I went to the bar with friends' then 'I failed my test'.

But I can't say that 'if I went to see a movie' && 'if I went to the bar with friends', because then I'd be in two places at the same time and it would be logically impossible.

Likewise, the number is either below 20, between 20 and 80, or greater than 80, and only one of those three options can be true.
 
jiminaus

jiminaus

macrumors 65816
Dec 16, 2010
1,449
1
Sydney
It's not really && vs.|| that are giving you the grief here. It's really while vs. until. While and until are opposite to each other.

You're thinking "loop until so-and-so is true or so-and-so is true". But what you need to code is "loop while so-and-so is false and so-and-so is false". (Boolean algebra says !(A || B) == !A && !B).

Objective-C doesn't have until. So always think in terms of while. "I want this code to keep repeating while ...."
 
L

larswik

macrumors 68000
Original poster
Sep 8, 2006
1,552
11
ahhhh. Now I see the logic. That 1 number can not be 2 things, above and bellow at the same time. I see that now.

I managed to crash my dice program when I made the low number higher then the high number an vice versa. opps. I think it went into an infinite rolling loop.:eek:

-Lars
 
D

dpcontardi

macrumors newbie
Aug 13, 2007
25
1
larswik said:
ahhhh. Now I see the logic. That 1 number can not be 2 things, above and bellow at the same time. I see that now.

I managed to crash my dice program when I made the low number higher then the high number an vice versa. opps. I think it went into an infinite rolling loop.:eek:

-Lars
Click to expand...

Yep, that would do it - if you're testing for a number that is (more than 20) OR (less than 80) then your while test will always evaluate to TRUE, because all numbers are always either more than 20, or less than 80.

So with your while clause always returning TRUE, the DO loop becomes an infinite loop.
 
balamw

balamw

Moderator emeritus
Aug 16, 2005
19,366
979
New England
I would suggest that at this point, you stuff the loop condition into a temporary variable (or more if needed) so that you can watch and debug it separately.

IMHO this would have saved you a lot if grief in both this thread and your last one.

B
 
L

larswik

macrumors 68000
Original poster
Sep 8, 2006
1,552
11
Yep. Last night I saw that and changed the whole thing to an IF statement to avoid the infinite loop that dis happen. This is all a learning process. Thanks.
 
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