Further Examples: Union

Here is the set­theoretic union of a list of elements.

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Here is another way to write the same expression.

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This works since both ff[a, b, c] and ff[b, c, d] have the same head.

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Union with one argument drops duplicates from a list and sorts the remaining elements.

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We define the function UnsortedUnion that drops duplicates from a list but, unlike Union, does not sort the remaining elements. (See the Further Examples for Reap for another implementation.)

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We define gg to explain how UnsortedUnion works.

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Evaluate gg[5]. The compound expression (gg[5] = Sequence[ ]; 5) is evaluated, giving as the output.

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Since the expression gg[5] = Sequence[ ] was evaluated the first time that gg[5] was called, subsequent evaluations of gg[5] give Sequence[ ].

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Only the first occurrences of gg[a], gg[b], gg[c] and gg[d] appear; later occurrences do not, by the definition of Sequence[ ].

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This is the same thing.

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