KeepIf Command

KeepIf[ <Condition>, <List> ]

Creates a new list that only contains those elements of the initial list that fulfil the condition.

Example: KeepIf[x<3, {1, 2, 3, 4, 1, 5, 6}] returns the new list {1, 2, 1}.

Note: For list of numbers arbitrary condition may be used. For list of other objects one can use only conditions of the form x==constant or x!=constant.

KeepIf[ <Condition>, <Variable>, <List> ]

This syntax allows a more flexible condition, eg for Points P, Q, R KeepIf[x(A) < 3, A, {P, Q, R}] will filter the points whose x-coordinate is greater than 3 out of the list. The Variable A is replaced in turn with P then Q then R for the check.