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Reap

Reap[expr]
gives the value of expr together with all expressions to which Sow has been applied during its evaluation. Expressions sown using Sow[e] or Sow[e, tagi] with different tags are given in different lists.
Reap[expr, patt]
reaps only expressions sown with tags that match patt.
Reap[expr, {patt1, patt2, ...}]
puts expressions associated with each of the patti in a separate list.
Reap[expr, patt, f]
returns {expr, {f[tag1, {e11, e12, ...}], ...}}.
  • Sow and Reap provide a convenient way to accumulate a list of intermediate results in a computation.
  • Reap accumulates expressions in the order in which Sow is applied to them.
  • Expressions sown with a particular tag are collected by the innermost Reap whose pattern matches the tag.
  • Reap[expr] is equivalent to Reap[expr, _].
Evaluate a sequence of expressions, "reaping" ones that have been "sown":
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Compute a sum, "reaping" i^2 "sown" at each step:
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Click for copyable input
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