How to select rows in a list that have e.g. their nth element meeting a criterion?

Question

Let a be a list of lists like this:

a= {{"apples", "pears", 1, 17},
    {"pineapples", "apples", 5, 7},
    {"yes", "building", 4, 7}}

How can I make a list b containing only elements of a for which the nth element meets an arbitrary criterion?

Let us say for example that n is 3 and the criterion is "must be greater than 2". Then b would be

{{"pineapples", "apples", 5, 7},
    {"yes", "building", 4, 7}}.

tell · Accepted Answer · 2023-10-19 01:03:12Z

2

No sooner do I post the question than I think I've found the answer!

 b = Select[a, #[[3]] > 2 &]

answered Oct 19, 2023 at 1:03

tell

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$\begingroup$ Also Cases[a, {_, _, n_, ___} /; n > 2] $\endgroup$
– MelaGo
Commented Oct 19, 2023 at 5:25
$\begingroup$ @MelaGo - Thanks for this. Your version is more widely useful because Cases can match strings and numbers, whereas Select matches numbers only. $\endgroup$
– tell
Commented Oct 23, 2023 at 17:34
$\begingroup$ You're welcome. You can use Select with any expression that evaluates to True, for example Select[a, #[[2]] == "apples" &] or Select[a, StringMatchQ[#[[2]], "apples"] &] $\endgroup$
– MelaGo
Commented Oct 23, 2023 at 21:56

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