Lisp filter out results from list not matching predicate

elispfilterlisplistpredicate

I am trying to learn lisp, using emacs dialect and I have a question.
let us say list has some members, for which predicate evaluates to false. how do I create a new list without those members? something like { A in L: p(A) is true }. in python there is filter function, is there something equivalent in lisp? if not, how do I do it?

Thanks

Best Answer

These functions are in the CL package, you will need to (require 'cl) to use them:

(remove-if-not #'evenp '(1 2 3 4 5))

This will return a new list with all even numbers from the argument.

Also look up delete-if-not, which does the same, but modifies its argument list.

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For cryptographically secure random choices (e.g., for generating a passphrase from a wordlist), use secrets.choice():

import secrets

foo = ['battery', 'correct', 'horse', 'staple']
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secrets is new in Python 3.6. On older versions of Python you can use the random.SystemRandom class:

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Use del and specify the index of the element you want to delete:

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>>> a
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Also supports slices:

>>> del a[2:4]
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[0, 1, 4, 5, 6, 7, 8, 9]

Here is the section from the tutorial.

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