remove, select, selectremove

pagan — Thu, 13 Oct 2011 19:54:39 Z

Use `remove` for this kind of thing, since doing it in a loop is unnecessarily inefficient. You want to have maple walk the list just the once, and to have it internally construct just the single result (instead of many temporary lists as collectible garbage).

> f:=rand(-10..10):
> A:=[seq(f(),i=1..17)];

       [-7, 8, -7, -5, -7, -7, -1, -3, -9, 5, -9, 0, 10, 4, 0, -8, -6]

> remove(t -> t < 0 and t > -7, A); # just an example

               [-7, 8, -7, -7, -7, -9, 5, -9, 0, 10, 4, 0, -8]

You could also use evalf inside the predicate (the first argument of the remove command). Eg,

   t -> evalf(t) < 0 and evalf(t) > -7

[edit] Forgot to use your actual example.

> A := [[0, 1], [2, 2], [4, 5], [-1, -2]]:

> remove( x -> 2*evalf(x[1]) > evalf(x[2]), A );

                             [[0, 1], [-1, -2]]

Both `remove` and `map` are efficient for this kind of thing, relative to iterating (with a loop).

It was asked and answered

Markiyan Hirnyk — Thu, 13 Oct 2011 20:19:44 Z

A similar question was asked and answered here: http://www.mapleprimes.com/questions/87697-Select-Parts-Of-A-List

This can be done as follows:
> A := [[0, 1], [2, 2], [4, 5], [-1, -2]];

[[0, 1], [2, 2], [4, 5], [-1, -2]]

> L := map(x->`if`(2*evalf(x[1]) <= evalf(x[2]), x, NULL), A);

[[0, 1], [-1, -2]]
> M := map(x-> `if`(0 <=evalf( x[2]), x, NULL), A);

[[0, 1], [2, 2], [4, 5]]

MaplePrimes - answers and comments on Question, Removing elements from a list.

remove, select, selectremove

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