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Reap

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Reap[expr]

gives the result of evaluating expr, together with all values sown during this evaluation. Values sown with different tags are given in different lists.

Reap[expr, pattern]

only yields values sown with a tag matching pattern.
Reap[expr] is equivalent to Reap[expr, _].

Reap[expr, {$pattern_1$, $pattern_2$, ...}]

uses multiple patterns.

Reap[expr, pattern, f]

applies f on each tag and the corresponding values sown in the form f[tag, {$e_1$, $e_2$, ...}].

• • Reap[Sow[3]; Sow[1]]

• • Reap[Sow[2, {x, x, x}]; Sow[3, x]; Sow[4, y]; Sow[4, 1], {_Symbol, _Integer, x}, f]

Find the unique elements of a list, keeping their order:

• • Reap[Sow[Null, {a, a, b, d, c, a}], _, # &][[2]]

Sown values are reaped by the innermost matching Reap:

• • Reap[Reap[Sow[a, x]; Sow[b, 1], _Symbol, Print["Inner: ", #1]&];, _, f]

When no value is sown, an empty list is returned:

• • Reap[x]