OR ???
Blitz3D Forums/Blitz3D Programming/OR ???
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| what is this 'OR' stand for in the middle of the flags... like this? loadtexture("bla.bmp",1 Or 2) ??? |
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loadtexture("bla.bmp",1 Or 2)is the same as loadtexture("bla.bmp",3)Another example: loadtexture("bla.bmp",1 Or 2 Or 8)is the same as loadtexture("bla.bmp",11)Its basically combining the three separate flags into one value. You can also do it like this:loadtexture("bla.bmp",1+2+8) etc... |
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| just that??? geez, why coder bother using OR actually? that is the really question... |
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| If Im using bit flags then I use Or. When I look at the code its more obvious that you are dealing with bit flags. Its just the way Im used to. Plus the advantage is in cases where you want to combine sets of bit flags. SOMETHING = 1 Or 2 Or 8 SOMETHING_ELSE = 1 Or 4 I can still quite happily use Or if I wanted to combine the two sets of flags. NEW_FLAG_SET = SOMETHING Or SOMETHING_ELSE whereas if I added them the result would be wrong. |
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| Isn't Or faster than an addition. This wouldn't matter unless you were doing thousands of them. It's just coder preference you see Or used a lot in windows code for flags using the pipe symbol |. |
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| Probably a very common place you see Or is in an If...EndIf statement. It's all just based on bit maths, but reads nicely. If variable=1 or variable=2 Then first Blitz calculates this -equation- result=variable And (1 or 2) translates to result=variable And 3 And is another thing similar to Or ;] in that it's a bit based dowhacky command. If result returns a value, the 'Then' part is executed, if result=0 after that equation, the 'Else' part is executed, if there is one. |
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| Here is an explanation about how boolean logic gates work: http://www.howstuffworks.com/boolean1.htm |
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| what about this?... if variable$="bla" or variable$="bli" then how blitz work??? is it change the string to a value first? |
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| Nah it performs a byte-per-byte check on the contents of each string, so basically it breaks it down into If (((Chr$(Mid$(variable$,1,1))=Chr$("b")) And (Chr$(Mid$(variable$,2,1))=Chr$("l")) And (Chr$(Mid$(variable$,3,1))=Chr$("a"))) Or ((Chr$(Mid$(variable$,1,1))=Chr$("b")) And (Chr$(Mid$(variable$,2,1))=Chr$("l")) And (Chr$(Mid$(variable$,3,1))=Chr$("i")))) Then =] Meaning strings take a long time to be processed in an If...EndIf statement... |
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| i think he's actually wondering why that comparison actually works in the first place. well this is how the program does it when running: it starts out with this. if variable$="bla" or variable$="bli" it then sees if variable$ is in fact "bla." if it is, that whole comparison turns into ("evaluates as") 1. if 1 or variable$="bli" same thing again. if 1 or 0 (variable$ can't be both bla and bli at the same time now can it?) it finally performs the OR on these two numbers. 1 OR 0 = 1. if 1 IF then checks to see if its test is 1, and it is, so the code is run. this is boolean logic. and come on akat, you've used DB and you never understood what OR was in IF statements? ;) |
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| Hotcakes - I would presume Len is checked as well/first? |
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| It could be. But that would slow down every instance where the two strings are actually the same length. Who knows =] To add to DrakeX's explanation, the If statement actually only checks if the value it's left with after all those calculations is not 0. |
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| i know how or work... but really confusing me is how it can be turn into + operator in flag bit... |
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| It doesn't turn into a + operator. I don't think you've been paying attention to how OR works at all. |
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| Anthony is right. OR works by setting the bits corresponding to its arguments. It is NOT an addition type command. For ex., 3+1 = 4, is not the same as 3 OR 1 (which is 3) it sets the bit you tell it to set. EX. 3 + 1 = 4 00000011 (3) + 00000001 (1) ============== 00000100 (4) in "OR" it's: 3 OR 1 = 3 00000011 (3) OR 00000001 (1) =============== 00000011 (3) since the 0 bit was already set, no different outcome... here's the diff: 2 OR 1 = 3 00000010 (2) OR 00000001 (1) =============== 00000011 (3) The 0 bit get's set and the resulting number is 3. That's why it is so convenient to use it to set bits (flags, in most cases ordered within a byte as powers of 2). If you use XOR, it toggles the bits in question. (Good for figuring out if an item is in use, or something's status has been changed in a game) The above is the bitwise OR, the logical OR usually evaluates down to some values being tested and the condition being true if any of the values evaluate to true. (like in an IF a or b then...) It comes down to a boolean and if any of the booleans are true the condition is met and execution branches accordingly. (See DrakeX's example) |
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| There is no flag with number 3 in Blitz, just because all the flags must be multiple of 2 to work, so, when we are talking about flags, + and Or does the same work. The advantage of using Or is that you cannot accidentally add the same flag two times. Imagine that, instead of passing the flags directly into the function, you store them into a variable. It is easy to accidentally add the same flag two times when you're using +, if the code which sets the flags is too complex. If you use Or, if the flag has been added once, it won't add the number again to the flags. |
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| try ORing 3 and 7. you'll get a rather different result than adding them. |
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| 3 And 7 are NOT flags, because they're not multiple of 2. I've stated that in my previous post. I said that Or and + are the same FOR FLAGS. |
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| not multiple of 2 I guess you meant "not power of 2" :)Apart of that you're right it's better to use Or with flags rather than a '+': it's safer, faster (you won't ever notice it though) and more explicit (it says "I'm working at the bit level" unlike an adition). |
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| Hi, 'Or' is a 'bitwise operator' - it does a bitwise 'Or' of 2 integers. See Antony's link for details on how it works. Basically, it 'merges' the bits in 2 integers, resulting in '1' bits whereever there was a '1' bit in *either* operand. 'And' is similar, only the result has a '1' where there are '1's in *both* operands. Stuff like '<', '>', '>=' etc are themselves just 'operators', like '+', '-', '/' etc. What they do is return a '1' if the comparison is true, or a '0' if its false, eg: Print 5<10 ;prints 1 Print 5<3 ;prints 0 You can use bitwise 'Or' and 'And' with these results to achieve logic like: a=15 If a<10 Or a>20 ;do something. Endif ...is the same as... If 0 Or 0 Endif ...and you can even go... a=10<20 ;=1 |
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| Wow, thanks for the post Mark! For the ones that come froms C/C++ by example, it may be worth mentioning that because And & Or are bitwise operators (there is no logical And & Or in Blitz), you can't safely do like in C: if (i && j) {...// do something if i & j are not 0because in Blitz doing If i And j Then ...will do something if the bitwise and of i & j is not null, which is not the same. it's like using the '&' operator in C rather than '&&'. So always be explicit and do If i<>0 And j<>0 Then.. Also is worth mentioning that Not, unlike And & Or, is a logical operator and not a bitwise operator: (Not 1) is 0 rather than $fffffffe. To do a bitwise Not, use Xor: (1 Xor $ffffffff) = 1 "bitwise noted" = $fffffffe. |
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| I guess you meant "not power of 2" :) Sorry, I am spanish and simply took a look at the dictionary to know how the spanish word "múltiplo" is said in english :) |
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| Yes, but if I understand well, 'multiplo de 2' means (just like 'multiple of 2') a number in the form 2*n, where what you probably meant is 2^n (6 is a multiple of 2 but does not make a flag, while 8 does). Or correct my poor level of spanish ;) simply took a look at the dictionary you stole my secret method! ;) (I'm french) |
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| and u said faster... that's why i will stick with OR style in bit flag... no matter how it works now... |
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| then you're right, it's "potencia de 2" (power of 2) what i should have said... i've never been very good with maths :P |
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