Select
Blitz3D Forums/Blitz3D Beginners Area/Select
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| Can somone please explain the point of Select/Case? I don't mean that the way it sounds, only I come from a background of earlier Basic programming and there doesn't appear to be a real advantage of select/case over If statements. As an example, here is the code from the online docs, and beneath is how I would approach the same thing. ONLINE CODE: mission=Rnd(1,10) ; Start the selection process based on the value of 'mission' variable Select mission ; Is mission = 1? Case 1 Print "Your mission is to get the plutonium and get out alive!" ; Is mission = 2? Case 2 Print "Your mission is to destroy all enemies!" ; Is mission = 3? Case 3 Print "Your mission is to steal the enemy building plans!" ; What do do if none of the cases match the value of mission Default Print "Missions 4-10 are not available yet!" ; End the selection process End Select My Basic Code: mission=Rnd(1,10) If mission=1 Print "Your mission is to get the plutonium and get out alive!" endif If mission=2 Print "Your mission is to destroy all enemies!" endif if mission=3 Print "Your mission is to steal the enemy building plans!" endif if mission>3 Print "Missions 4-10 are not available yet!" endif |
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| Select is a lot neater, for starters, especially when you have many 'cases'. Also, in your If version, ALL the If statements are executed every time, even if mission=1 and the first block is executed the program continues to check if mission=2 or 3 or >3. If a Case is matched in a Select statement, the prog executes the Case code then drops out to after the End Select - i.e. doesn't bother checking any other Cases. You can also group Cases together that need a common piece of code, e.g. Case 1,2,4,6,8. You can also define a 'Default' case to be executed if none of the other cases are matched. Hope that made sense! |
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A little re-write of your code...mission=Rnd(1,10) If mission=1 Print "Your mission is to get the plutonium and get out alive!" ElseIf mission=2 Print "Your mission is to destroy all enemies!" ElseIf mission=3 Print "Your mission is to steal the enemy building plans!" Else Print "Missions 4-10 are not available yet!" EndIf ...is effectively equal to the sample case code. Yet its main advantage is being neater. You can do everything with IFs, ELSEs, amd ELSEIFs that you can with SELECTs and CASEs, but I suppose in the cases where you've got lots of different options, a Select statement would make more sense. |
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| big10P, My question would be this. If the select case is at the bottom of the cases most of the time, it would still look through all of them like if statements though? mission=Rnd(1,10); MISSION RANDOMS 4-10 EVERY TIME ; Start the selection process based on the value of 'mission' variable Select mission ; Is mission = 1? Case 1 Print "Your mission is to get the plutonium and get out alive!" ; Is mission = 2? Case 2 Print "Your mission is to destroy all enemies!" ; Is mission = 3? Case 3 Print "Your mission is to steal the enemy building plans!" ; What do do if none of the cases match the value of mission Default Print "Missions 4-10 are not available yet!" ; End the selection process End Select |
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| Correct - which is why you should try to code it in the order in which you think it will be most efficient. The first case should be the most frequent occurance, the second the second most, etc... At worst it will be no worse then multiple if..then statements. |
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| Interesting. Thanks - Does this really have a significant impact on speed, though? Considering 1GHz and above processor speeds. I have also just noticed the Default command which would be a little longer to code in If/Thens |
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| Hi Malice. I decided to write a program to test the two of these. Now if this is in error, please let me know since im not an expert. However, it seems that IF statements ARE in fact way faster than select case. Here is my code checking for 5000000 if's against select case :) Graphics 800, 600 number = Input ("Press Enter to check speed of IF statements ") begin# = MilliSecs() Print "Start Time: " + begin# For x = 1 To 5000000 If number = x Then Print "if check" Next If number = 0 Then endCheck# = MilliSecs() result# = (endCheck# - begin#) Print "End Time: " + endCheck# Print "Result: " + result# number = Input ("Press Enter to check speed of Select case statements ") begin# = MilliSecs() Print "Start Time: " + begin# For x = 1 To 5000000 Select number Case x Print "select check" Default endCheck# = MilliSecs() End Select Next result# = (endCheck# - begin#) Print "End Time: " + endCheck# Print "Result: " + result# WaitKey |
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| Wow interesting. Found a problem with my own code. The default takes time to default everytime for 5million selects, so I changed the default to nothing and they are pretty close, but IF statements still win by a tiny tiny fraction - takes 0.059 seconds longer hehe. Ok here is the correct version. By the way its good to note that defaulting with a loop is taking a lot longer hence the example above. Here is the correct version: Graphics 800, 600 number = Input ("Press Enter to check speed of IF statements ") begin# = MilliSecs() Print "Start Time: " + begin# For x = 1 To 5000000 If number = x Then Print "if check" Next If number = 0 Then endCheck# = MilliSecs() result# = (endCheck# - begin#) Print "End Time: " + endCheck# Print "Result: " + result# number = Input ("Press Enter to check speed of Select case statements ") begin# = MilliSecs() Print "Start Time: " + begin# For x = 1 To 5000000 Select number Case x Print "select check" Default End Select Next endCheck# = MilliSecs() result# = (endCheck# - begin#) Print "End Time: " + endCheck# Print "Result: " + result# WaitKey |
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| Ok i think i understand now :) I should have thouroughly tested before posting. Strange enough, if you do the tests over and over, sometimes the select is a little faster, sometimes they are equal, but it seems if statements are .003-.010 milliseconds faster. I would conclude that they are both the same since it depends on what your cpu is up to at the time :) So choosing IF's vs. Select Case, is purely a personal choice :) Hope this has clarified. |
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| Cheers, Bombone. I posted some code in the Archives precisely for checking the speed of functions in a similar way. Interesting that 'If' is faster, despite all that has been said! Thanks! |
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| > Interesting that 'If' is faster, despite all > that has been said! Well not really. Think of it like this If blah then else ; do blah endif In this example only two checks aximum are made. Only first if the first is true. Now look at this. Select blah Case 1 Case 2 Case 3 Case 4 Case 5 Case 6 Case 7 End select If blah=7 then six seperate checks have be made before 7 is reached. SO if you had up to case 100. There are 99 checks before it. To get around this you would simply order then in the most common order and also split it up. If blah<25 Select blah Case 0 Case 1 . . Case 24 End select elseif blah<50 Select blah Case 25 Case 26 . . Case 74 End select elseif blah<75 ;etc else ;etc endif Then you only have a max of 25 checks. However I wouldn't worry about it too much. It's not as essential these days to maximum searches in games. |
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| I would say useage if select/case against If/else/elseif is purely down to personal preference. I would be surprised if there was any difference in execution speed between the two as internally all the same comparisions etc. have to be made. Any disparity in your testing prog will probably be down to OS interference. |
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Select/Case seems more useful, at least to me, because it can also do this:Select number Case 1,2,3 ;Do Something Case 4,5,6 ;Do Somethine else End Select where if you were trying that with if statements then you would have to use a bunch of ORs. |
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| Good points if/endifs/else etc would slow it down alot. During the test your only comparing it to nothing or 0. the then is not needed either. I wonder what would happen if you ran ifs checking for types/arrays etc. |
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