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exists vs. member vs. memq

Yaosan

When checking if certain item can be found in a list, we can use either exists or member:

exists( i '("a" "b" "c") i == "b" )

member( "b" '("a" "b" "c") )

I'm wondering if one of them is faster than the other.Since SKILL Lint always suggest to use memq instead of member,using exists will avoid those suggestion from showing up.

Now that we have a list of string, memq can't be used. Does it make sense to convert our list of string to list of symbol first then use memq to optimize this process? (assume the list can be short or very long i.e. more than 500 elements)

For example:

myList = '("a" "b" "c")
memq( 'b foreach( mapcar i myList concat(i) ) )

As mentioned in the beginning, all we care is if an item can be found in the list i.e. whether the return is nil or non-nil. Thanks for any feedback.

  • member is likely to be faster than exists, because it is more specific. The comparison is built-in, whereas the comparison has to be explicitly stated with exists(). But you could profile it to be sure.

    The main benefit of memq would be if you are searching for a list within the list - and you know for certain that the list exists (as exactly the same list, not just a list with the same values) within the list you're searching. Converting a lot of strings to symbols is likely to be expensive, pollute the global symbol table, and be slower than justusing member.

     

    Regards,

     

    Andrew.

  • If performance is most important, then use a table.

    myTbl = makeTable("myTbl" nil)
    myTbl["a"]=t
    myTbl["b"]=t
    myTbl["c"]=t
    or
    foreach(item myList
        myTbl[item]=t
    )
    then check for existence in the table:
    if(myTbl[x] then ...

    The table will be MUCH faster than using member/memq/exists for lists.

    Derek

  • Thanks for the reply Andrew, it's very insightful and based on what you said, it seems like using member() will be the most intuitive and optimized way to do things.

    And I would love to try what Derek suggested i.e. using table instead of list to do this type of operations. Thanks for the suggestion and I will try to post back once I gave this a try. It will be nice to have some actual numbers that show the speed differences.

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