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I need to select some lists in the nested list of form of {{1,2,3},{2,3,4}}. I have used Select to complete my work. But when I tried Position, it doesn't work. For example,

 Flatten@@Position[{{1, 2, 3}, {2, 3, 4}}, _?(# == {1, 2, 3} &)]

gives me the correct answer of $\{1\}$,but when I use

Flatten@@Position[{{1, 2, 3}, {2, 3, 4}}, _?(#[[1;;2]] == {1, 2} &)]

returns the error

Part::take: Cannot take positions 1 through 2 in List. >>
Part::take: Cannot take positions 1 through 2 in List. >>
Part::take: Cannot take positions 1 through 3 in 1 >>

It seems that it is not working with lists, and it is also trying to apply to higher level. Why is it and how to fix that?

vapor
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  • I did not realize this was a duplicate until after I answered. I am converting my answer to Community Wiki so that I do not gain additional "reputation" from it, and so that others can add anything that would be helpful. – Mr.Wizard Feb 05 '15 at 10:54

1 Answers1

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Position by default looks at all parts of the expression, including those that do not have sub-parts to extract. You can use levelspec and the Option Heads to restrict what it tests:

Position[{{1, 2, 3}, {2, 3, 4}}, _?(#[[1 ;; 2]] == {1, 2} &), {1}, Heads -> False]

{{1}}

More generally you can simply use a pattern like this:

Position[{{1, 2, 3}, {2, 3, 4}}, _[1, 2, ___]]

{{1}}

This pattern matches all heads:

Position[{foo[1, 2, 3], bar[2, 3, 4]}, _[1, 2, ___]]

{{1}}
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Mr.Wizard
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