Moving from Microsoft Office VBA to AppleScript:
MacTech's Guide to Making the Transition

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April, 2007
Page 12

    -- code here

end repeat

for “For Each” loops. This should work fine without problems when eachItem is an application object reference, as it usually will be. But you can often go awry if you forget that eachItem in this construction is a reference, not yet evaluated. The = equality operator will behave unexpectedly rigorously in this block:

repeat with eachItem in {1, 2, 3, 4, 5}

    if eachItem = 2 then

        beep

        display dialog (eachItem as string)

        exit repeat

    end if

end repeat

display dialog (eachItem as string)

No beep is heard, and the dialog displays 5. That's because eachItem never equals 2: it equals 'item 2 of {1,2,3,4,5}'. To avoid dealing with this issue, which you can solve by replacing the second line by

if contents of eachItem = 2 then

(or by other ways of forcing an evaluation of eachItem such as coercion), many people find it simpler just to use

repeat with i from 1 to (count {1, 2, 3, 4, 5})

    set eachItem to item i of {1, 2, 3, 4, 5}

    if eachItem = 2 then

i.e., a For…Next loop, in all cases.

Note: In VBA if you have "For i = 1 to 5 … Next i", the equivalent of this type of repeat loop, when you drop out of the loop having been round 5 times, the value of i is now 6. In AppleScript, i is 5 on exit from the loop 'repeat with i from 1 to 5'. This will surprise VBA coders who often rely on reading the value of n to see whether a loop was exited normally or by means of an Exit For statement.

exit repeat is the equivalent of Exit For, and gets you out of the block.

Check out the last subsection here – Special AppleScript Features – for two very powerful methods that can replace some repeat loops altogether.

On Error Resume Next/Go To : try…on error…end try

While not identical to VBA's On Error, AppleScript has something similar, but you must frame the section of code in which you want to trap the error with a try…end try block. A simple

try

    --code here

end try

is like an On Error Go To line after 'end try' applied to any runtime error that occurs within the block. This code:

try

    --code here

on error

    --alternate code here  

end try

provides an alternative in the event of an error, like On Error Resume Next but which can be located anywhere in your script. A more elaborate

try

    --code here

on error errMsg number errNum

    if errNum = -1708 then
        --alternate code here quoting or parsing the message