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Select

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Select[{$e_1$, $e_2$, ...}, crit]

returns a list of the elements ei for which crit[ei] is True.

Select[{$e_1$, $e_2$, ...}, crit, n]

returns a list of the first n elements ei for which crit[ei] is True.

Get a list of even numbers up to 10:

• • Select[Range[10], EvenQ]

Find numbers that are greater than zero in a list:

• • Select[{-3, 0, 10, 3, a}, #>0&]

Find the first number that is list greater than zero in a list:

• • Select[{-3, 0, 10, 3, a}, #>0&, 1]

Select works on an expression with any head:

• • Select[f[a, 2, 3], NumberQ]

  • $$f\left[2,3\right]$$