Subject: [xsl] RE: Cheaper to prepend or append an item to a sequence? From: "Costello, Roger L." <costello@xxxxxxxxx> Date: Tue, 22 Feb 2011 08:50:07 -0500 |
Hi Folks, Thank you for your responses. They are surprising. In the functional language Haskell, there is a clear disadvantage of appending an item onto the end of a list, and developers are strongly encouraged to build lists by prepending an item onto a list. For example, this prepending approach is strongly _preferred_: [3] 2:[3] 1:[2,3] This results in creating the list: [1,2,3] This appending approach is strongly _discouraged_: [1] [1] ++ [2] [1,2] ++ [3] This results in creating the same list: [1,2,3] Here is what one Haskell book says: Watch out when repeatedly using the ++ operator on long strings. When you put together two lists (even if you append a singleton list to a list, for instance: [1,2,3] ++ [4]), internally, Haskell has to walk through the whole list on the left side of ++. That's not a problem when dealing with lists that aren't too big. But putting something at the end of a list that's fifty million entries long is going to take a while. However, putting something at the beginning of a list using the : operator (also called the cons operator) is instantaneous. >'A':" SMALL CAT" "A SMALL CAT" > 5:[1,2,3,4,5] [5,1,2,3,4,5] Notice how : takes a number and a list of numbers or a character and a list of characters, whereas ++ takes two lists. Even if you're adding an element to the end of a list with ++, you have to surround it with square brackets so it becomes a list. I vaguely recall that Lisp behaving the same way (as Haskell) and with the same recommendation to build lists from the left not the right. Why is XSLT different from these other functional languages? /Roger
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