Re: selecting from lists

• To: mathgroup at smc.vnet.net
• Subject: [mg6132] Re: [mg6120] selecting from lists
• From: Allan Hayes <hay at haystack.demon.co.uk>
• Date: Sun, 23 Feb 1997 00:10:45 -0500
• Sender: owner-wri-mathgroup at wolfram.com

```murray at math.umass.edu (Murray Eisenberg)
[mg6120] selecting from lists

writes
(1)
> given an arbitrary list v and a list b of the same length, whose
> entries are all Boolean (True or False), construct a function
> s[v, b] whose result is a list consisting only of those entries
of > v for which the corresponding entries in b are True.
> For example,
>   v = {3, 82, 7, -12, 5};
>   b = {False, True, True, False};
>   s[v, b]
>	{82, 7}

(2)
> construct a function firstnonzero that returns the index of the
> first nonzero entry in a list (or, more generally, the first
entry > in a list satisfying a given property).

Murray:
Here are some ideas:
Both functions , s and p, allow the first n qualifying entries to
be chosen. We can easily go further, in particular by allowing s and
p to pass level specifications.

(1)

s[v,b]
{82, 7}

s[v,b,1]
{82}
(2)

p[v_, pattern_, n_:Infinity] := Position[v, pattern, 1, n]

p[{1,2,1,2,2,1}, 1]
{{1}, {3}, {6}}

p[{1,2,1,2,2,1}, 1, 2]
{{1}, {3}}

Allan Hayes
hay at haystack.demon.co.uk
http://www.haystack.demon.co.uk

```

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