Reap
 Usage
Notes
 Further Examples
The first element in the following output is the result of evaluating u;v;w. The second element in the output is the collection of what was sown.
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When one Reap encloses another, sown expressions will go to the first matching Reap.
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This gives the same result.
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Items sown for each Sow tag are collected separately. Here tag comparison is based on SameQ.
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The tag in Reap can be a pattern. Sow[expr, tag] collects expr for each tag that matches the pattern.
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The tag expression in either Reap or Sow can be a list. For Sow[expr, tags], expr is sown for each tag in the list tags. Here the result consists of the lists of items collected successively for u, v, w and x. When v is repeated in the tag list of Sow, an item is sown once for each instance.
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For Reap, the matches for each tag are collected separately and the result is returned as a list.
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Sown items are reaped for any Sow tags which match any of the Reap tags. This expression has the same lists for tags v and w as in the previous example, but the matches with the pattern catch all of those items as well.
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The third argument of Reap is a function that takes two arguments. It allows you to determine which Sow tags are associated with which collection of items. The default is (#2)& which returns only the reaped items. A convenient value for this function is Rule, since it allows you to see the associations naturally.
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If the Reap tags are in a list, the function is applied to the Sow tags for each Reap tag separately.
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This groups elements based on whether or not they are integers.
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This defines the function SelectEquivalents, a kind of generalization of the previous example. It groups together elements from a list x that have the same image under a function f. (See the Further Examples for Split for a similar function: Classify.)
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This groups elements in a list based on their value modulo 10.
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This separates a list into symbols and nonsymbols.
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This defines the function UnsortedUnion that drops duplicates from a list but, unlike Union, does not sort the remaining elements. (See the Further Examples for Union for another implementation.)
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