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Quiz: String performance optimization

  • Article

This code takes .03 seconds to run on my computer Try running it on yours.

ns=SECONDS()

x=""

FOR i = 1 TO 30000

      x=x+"OneTwoThree"

ENDFOR

?SECONDS()-ns

?LEN(x)

Modified slightly, it takes 15 seconds. That's 500 times slower!

ns=SECONDS()

x=""

FOR i = 1 TO 30000

      x="OneTwoThree"+x

ENDFOR

?SECONDS()-ns

?LEN(x)

I

Why the difference?

Comments

  • Anonymous
    September 28, 2005
    I would guess that the first method could expose an optimisation that uses an efficient memory reallocation scheme, as C's realloc or C++'s std::vector could, thus growing the string as it needs to, with only handful of memory allocations (assuming the allocated memory doubles in size when it needs to grow).

    In the second case, a temporary string object is probably created that has the appropriate size, this is then placed in x, with the original x being discarded. This would result in 30000 allocations and 30000 deallocations.
  • Anonymous
    September 28, 2005
    darn somebody beat me to it!!
  • Anonymous
    September 28, 2005
    Even if the second method is doing a realloc also (which it ought to be, if it's smart enough to do it in the first case), the second method has to be O(n^2) because the original string has to be shifted to the right, which means copying n*len("OneTwoThree") chars of data n times.

    The first case can leave the existing data in place and stick the new data on the end.
  • Anonymous
    September 28, 2005
    Great little micro optimization post, I would have never thought of this. I'll have to try this out in C# and see if the results are similar.
  • Anonymous
    September 28, 2005
    Hi Travis,

    Markus Egger and Rod Paddock wrote an article about the string performances in VFP and C#:

    http://www.eps-cs.com/VFPConversion/Article.aspx?quickid=030054
  • Anonymous
    September 29, 2005
    however the optimization
    should be removed when
    there is a passage for reference

    CLEAR

    x=REPLICATE('1',100)

    X= x + CSTUFF(@x)

    ? x && 'null'

    x=REPLICATE('1',101)

    X= x + CSTUFF(@x)

    ? x && '4444'


    PROCEDURE cstuff(sss)
    SSS = '4444'
    RETURN NULL
  • Anonymous
    September 27, 2007
    In this post: Quiz: String performance optimization , I showed some very similar code with drastically