See the documentation: grep and map. Briefly (not not entirely accurately):

grep expression list runs across a list and returns a list containing the elements of the original list for which the expression is true.

map expression list runs across a list and returns a list containing the value of expression for each of the elements of the original list.

True laziness is hard work

In reply to Re: difference between grep and map by GrandFather
in thread difference between grep and map by boom

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