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VB Forum / General 2 / June 2004fast bit check
occasionally i would like to check if in a Long integer a bit is checked or not i use now: B = CBool(x And 2 ^ j) 'where x and j are integers. i suspect that calculating 2^j is rather time consuming and would like to test other/faster methods. all suggestions/remarks are appreciated. thanks and regards, Martin > . . . . . I suspect that calculating 2^j is rather time consuming > and would like to test other/faster methods . . . . . It depends on what you mean by "time consuming". On my machine (2 ^ j) takes about a quarter of a microsecond, so unless I am using it a very large number of times in some very time critical code then I wouldn't worry about it. If you want a faster method though you could do a lot worse than a simple look up table (which on testing is about a hundred times faster): Option Explicit Private Bit(0 To 15) As Long Private Sub Form_Load() Dim n As Long For n = 0 To 15 Bit(n) = 2 ^ n Next n End Sub Then when you want to check a specific bit you can use: B = CBool(x And Bit(j)) You could even eliminate the CBool and simply test for a non zero result, but in practice that doesn't make much difference. Mike >> . . . . . I suspect that calculating 2^j is rather time consuming >> and would like to test other/faster methods . . . . . [quoted text clipped - 4 lines] >it. If you want a faster method though you could do a lot worse than a >simple look up table (which on testing is about a hundred times faster): Mike is right. And didn't Donald Knuth say premature optimization is the root of all evil? Having said that, (and noting that programmer time is more expensive than computer time), you could take advantage of the fact that 2^n=n*n, because multiplication can be faster than exponentials. But the best thing would be to precalculate it using an array/table - and trade speed for memory space. Have you done the *timing*? Premature optimization is "EVIL". >Option Explicit >Private Bit(0 To 15) As Long [quoted text clipped - 14 lines] > >Mike _______________________ Michael B. Johnson snip | Having said that, (and noting that programmer time is more expensive | than computer time), you could take advantage of the fact that | 2^n=n*n, because multiplication can be faster than exponentials. But | the best thing would be to precalculate it using an array/table - and | trade speed for memory space. 2^n is not n*n, that would be n^2 2^n = 2 * 2 * 2 (with n twos) :) Jon >| Having said that, (and noting that programmer time is more expensive >| than computer time), you could take advantage of the fact that [quoted text clipped - 9 lines] > >Jon Quite right. Thanks, Jon for correcting my oversight. _______________________ Michael B. Johnson _______________________ Michael B. Johnson > occasionally i would like to check if in a Long integer a bit is > checked or not [quoted text clipped - 3 lines] > > i suspect that calculating 2^j is rather time consuming and would like Not necessarily ! 2 ^ j can be done as bitshifting by the processor and is probably extremely fast. Try a test. See it you can get it to do it a 100.000 times a second or more :-) It's probably a hundred times faster than CBool() anyway...  Signature/\ preben nielsen \/\ prel@post.tele.dk Anybody for addition only? Private Bit(0 To 15) As Long Private Sub cmdBits_Click() Dim b As Long Dim BitValue As Long BitValue = 1 For b = 0 To 15 Bit(b) = BitValue Debug.Print "Bit(" & b & ") = " & BitValue BitValue = BitValue + BitValue Next End Sub Eric >> occasionally i would like to check if in a Long integer a bit is >> checked or not [quoted text clipped - 11 lines] >second or more :-) It's probably a hundred times faster than >CBool() anyway... |
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