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How does Position work?

  • To: mathgroup at smc.vnet.net
  • Subject: [mg85313] How does Position work?
  • From: P_ter <peter_van_summeren at yahoo.co.uk>
  • Date: Tue, 5 Feb 2008 19:45:44 -0500 (EST)

Hello,
János wrote some time ago a solution which involved
lstIn = {2, 3, 4, 4, 2, 1, 1, 5, 4}
Flatten[Position[lstIn, #1]] & /@ lstIn
This last line gives as result:
{{1, 5}, {2}, {3, 4, 9}, {3, 4, 9}, {1, 5}, {6, 7}, {6, 7}, {8}, {3, 
  4, 9}}
One can observe that the last 4 in lstIn has the same result as the first and second 4. Which should be.
But! Did Position make a trie (for example) first and then give the results? The document about Position only tells that expressions are searched for in depth-first order. But that does not tell anything about the data structure.
I used:
lstIn1 = Riffle[ConstantArray[0, 10^4], ConstantArray[1, 10^4]];
One should expect that this would work very fast, but it takes a long time.
Does someone know about this?
with friendly greetings,
P_ter


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