Strain

Instructions

Implement the keep and discard operation on collections. Given a collection and a predicate on the collection's elements, keep returns a new collection containing those elements where the predicate is true, while discard returns a new collection containing those elements where the predicate is false.

For example, given the collection of numbers:

• 1, 2, 3, 4, 5

And the predicate:

• is the number even?

Then your keep operation should produce:

• 2, 4

While your discard operation should produce:

• 1, 3, 5

Note that the union of keep and discard is all the elements.

The functions may be called keep and discard, or they may need different names in order to not clash with existing functions or concepts in your language.

Restrictions

Keep your hands off that filter/reject/whatchamacallit functionality provided by your standard library! Solve this one yourself using other basic tools instead.

Tcl-specific instructions

This exercise gets you to write your own control command (like foreach or while). You will need the upvar and uplevel commands.

The uplevel man page contains an example do ... while ... control construct which should help you solve this exercise.

This discussion on the Tcl wiki, New Control Structures, may also be helpful.

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Source

Conversation with James Edward Gray II