Increasing Script Performance, Part I
All those guys that migrated from good old HyperCard to MC, were delighted by the speed enhancement. Nevertheless, in a number of cases, MC is slow compared to languages like Pascal or C. Scott Raney gives some good advice about how to speed up your scripts. However, there are some of less known tricks that can speed up your scripts amazingly. Suppose you want to create a list consisting of a number of lines with something put into each line. For the sake of simplicity, let us create a list of 10000 random numbers between 1 and 1000. If your script looks like this:
and you are eager to know how to write a script that executes more then ten times as fast, you should read on! But first some questions. Do you think that the same script, but now generating random numbers between 1 and 1000000 (one million) takes more time? If your answer is “yes”, you’re right. It will take about twice as much time. Do you think that’s because generating a random number between 1 and 1000000 takes more time than generating a random number between 1 and 1000? If your answer is “yes”, you’re wrong. It takes about the same amount of time. To write scripts that executes as fast as possible, you should know something about how a computer (or MetaCard) works, how data are stored, etc. I’m going to tell you that in a very, very simplified way. But first, the fast script:repeat with i = 1 to 10000 put random (1000) into line i of randList end repeat
Script B yields exactly the same result as script A, but is more then ten times as fast. If we wanted to generate random numbers between 1 and 1000000, it is even more then 20 times as fast.repeat with i = 1 to 10000 put random (1000) & cr after randList end repeat delete last char of theList
Here is the reason why:
Data are stored in memory. Each piece of data has a particular length. For example, in script A, the 75th call to random (1000) may yield 234, which is three characters long. This piece of data had to be put after the CR (carriage return) at the end of line number 74. But where in memory is that? The computer can only figure that out by counting the number of CR’s from the start of ‘randList’. And that takes time. And each call to random (1000) the computer (the poor thing) starts counting the CR’s again.
Script B does not have this drawback. The computer does not have to bother about counting CR’s, but can just put the result of random (1000) after the last char of ‘randList’. In script B a CR is placed after this line, to warrant that each new result of random (1000) appears on a new line. Finally, the last cr is deleted, to ensure that the results of script A and B are exactly the same. To count the number of CR’s in script A, the computer has to walk through all characters of ‘randList’ and to decide whether or not the character is a CR.
If we generated random numbers between 1 and 1000000, the computer has to walk through about twice (actually, less than twice, because also the CR’s are characters) as much characters than in case of generating random numbers between 1 and 1000. Hence generating random numbers between 1 and 1000000 with script A takes about twice as much time than generating random numbers between 1 and 1000 with script A. In script B there is no difference of course.
If you understand the principle, you can easily imagine that the speed increase will become negligible if the number of characters on a line is very small, but very large if the mean number of characters on each line as large.
Moreover, you will also understand why using array variables is so fast. Array variables
are indexed. If the variable AR is an array variable, AR directly points to
the correct position in memory. Hence, a statement like
get AR  is very fast.
On the other hand, the statement
get line 345 of randList is much slower: again, the
computer has to count 344 CR’s, walking through the data of ‘randlist’.
Similarly, if you want to perfom some action on each number in ‘randList’ using the script:
is much faster thenrepeat for each line randNumber in randList put randNumber into temp -- do something end repeat
Why? In script D the computer has to count CR’s to get the i’s line. In script D, the computer ‘remembers’ where it is, getting lines; it just proceeds through the lines and doesn’t count CR’s. Moreover, of course, the statement ‘put randNumber into temp’ is superfluous.repeat with i = 1 to the number of lines of randList put line i of randList into temp -- do something with temp end repeat
A strategy that can give good results, is:
Posted on 4/1/03 by Will Dijkstra to the MetaCard list (See the complete post/thread)
Another 2 things to consider in your loops:
set cursor to busywill slow down scripts considerably.
So remember to avoid screen updates...repeat with x = 1 to 100000 if char -3 of x is not oldx then set cursor to busy showstatus put char -3 of x into oldx end if
Another consideration is
But to avoid mistakes or substring-matching errors,if x is in "picasso" or x is in "miro" then... is slower than if x is in "picassomiro" then
Enjoy more speed! Xavierif x is among the items of "picasso,miro" then...
Posted on 3/31/03 by Xavier Bury to the MetaCard list (See the complete post/thread)