Apologies - I clicked the wrong button in my email client and managed to
send a partially composed message. Here is the correct version!
On 2016-11-08 12:48, Ben Rubinstein wrote:
The point is that in my first pattern, I have outside the loop
assigned column (item) indices to named variables (based on the items
of the first, header, row). In the loop LC then has to locate the
indexed items in an individual data row.
In the first pattern:
repeat for each line tRec in tTSVdata
doSomething item viUserID of tRec, item viUserName of tRec
...
end repeat
The 'item <constant> of tRec' expressions cause the engine to iterate
through tRect until it has found the relevant item. This means that this
single line will be looking through the tRec string twice from the start
- the first time up until the viUserID'd item, the second time up to the
viUserName'd item. The speed of this will largely depend on how large
the item indicies are, and how large tRec is (and where the items fall
in tRec).
If the item indices are small, close and near to the start, and tRec is
small, and you don't use 'item ... of tRec' anywhere else in the loop,
then it will likely be faster than anything else.
In the second pattern, the code which happens to be in a function for
neatness has to create a new empty array, and chunk both the data row
and the header row in order to get column names and values to put into
the array. You can loop over one set of items, but not both, so LC
still has to locate indexed items in at least one case.
put line 1 of tTSVdata into tColumnNames
delete line 1 of tTSVdata
repeat for each line tRec in tTSVdata
put explodeRow(tRec, tColumnNames) into aData
doSomething aData["User ID"], aData["User Name"]
...
end repeat
The performance will largely depend on the implementation of explodeRow
and (as you said subsequently) how many columns you want from the row.
If you only want 2 then unless each tRec is very long and you are
fetching two items near the end then the non-array version will likely
be faster. If, however, the two items are near the end of the row or you
are wanting to access lots of items then this will be faster than
either:
repeat for each line tRec in tTSVdata
split tRec by tab
doSomething tRec[viUserID], tRec[viUserName]
...
end repeat
The difference here is that with the 'item' approach the speed will
reduce quadratically with the length of tRec and the max(viUserId,
viUserName); with the 'split' approach the speed will reduce linearly
with the length of tRec.
Depending on the average lengths of tRec and values of viUserId /
viUserName, at somepoint the 'item' approach will start to be
significantly slower than the 'split' version.
The explodeRow approach sounds like it has lots of overhead. A fair
amount of the overhead could probably be eliminated by doing 'split
tColumnNames by tab', and then using array access in explodeRow to form
the aData array (also making sure explodeRow is private will help too).
Just my two pence.
Mark.
--
Mark Waddingham ~ m...@livecode.com ~ http://www.livecode.com/
LiveCode: Everyone can create apps
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