AppleScript: The Definitive Guide, 2nd Edition

List Access

When you access an attribute of a list, it is much faster to target a reference to the list, or the list as a script property, than to target the list directly. It's not entirely clear why this is; it seems like a bug. But it's a venerable and acknowledged bug, because even Apple's earliest documentation on AppleScript contains an example illustrating this point.

In this code (based on Apple's example) we total the numbers in a long list:

set L to {}
set total to 0
set bignum to 5000
repeat with i from 1 to bignum
    set end of L to i
end repeat
repeat with i from 1 to bignum
    set total to total + (item i of L)
end repeat
total -- 12502500, and it takes about 22 seconds to run on my machine

The big slowdown here is the second repeat block, accessing items of the list. If we access these items by way of a reference to the list, things speed up dramatically:

set L to {}
set refL to a reference to L
set total to 0
set bignum to 5000
repeat with i from 1 to bignum
    set end of L to i
end repeat
repeat with i from 1 to bignum
    set total to total + (item i of refL)
end repeat
total -- 12502500, and it took less than a second

Instead of a reference, you can get the same extraordinary speed bump by referring to the list as a script property:

set L to {}
set total to 0
set bignum to 5000
repeat with i from 1 to bignum
    set end of L to i
end repeat
repeat with i from 1 to bignum
    set total to total + (item i of my L)
end repeat
total -- 12502500, and it took less than a second

The magic word ...