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comp.lang.ruby

How to collect over two arrays then look back to one?

Peter Booth

9/13/2006 4:12:00 PM

I am reading Ruby for Rails and Ruby Cookbock and delighted by the
Array#find_all and Enumerable#collect methods. I'm wondering what the
idiomatic way to do the following is?

Two parallel arrays A, B . For each element n of A for which f(An) is true,
is g(B[n-3:n+3]) true? Where B[n-3:n+3] refers to seven elements of B whose
position is centered over An' position.

Can the block inside Enumerable#collect know it's position inside the
Enumerable? Can one collect two parallel arrays?

Peter
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4 Answers

Jason Nordwick

9/13/2006 5:13:00 PM

0

I think...

a.values_at(*(0...a.size).to_a.select {|i| f(a[i]) && g([i-3,0].max..[i+3,a.size-1].min)})

Ruby seems a little verbose on this one.

-j



Peter Booth wrote:
> I am reading Ruby for Rails and Ruby Cookbock and delighted by the
> Array#find_all and Enumerable#collect methods. I'm wondering what the
> idiomatic way to do the following is?
>
> Two parallel arrays A, B . For each element n of A for which f(An) is true,
> is g(B[n-3:n+3]) true? Where B[n-3:n+3] refers to seven elements of B whose
> position is centered over An' position.
>
> Can the block inside Enumerable#collect know it's position inside the
> Enumerable? Can one collect two parallel arrays?
>
> Peter
> ----------------------------------------------------------------------------
> ----------------------------------------
>
> The information contained in and accompanying this communication is strictly
> confidential and intended solely for the use of the intended recipient(s).
> If you have received it by mistake please let us know by reply and then
> delete it from your system; you should not copy the message or disclose its
> content to anyone.
> MarketAxess reserves the right to monitor the content of emails sent to or
> from its systems.
> Any comments or statements made are not necessarily those of MarketAxess.
> For more information, please visit www.marketaxess.com. MarketAxess Europe
> Limited is regulated in the UK by the FSA, registered in England no.
> 4017610, registered office at 71 Fenchurch Street, London, EC3M 4BS.
> Telephone (020) 7709 3100.
> MarketAxess Corporation is regulated in the USA by the SEC and the NASD,
> incorporated in Delaware, executive offices at 140 Broadway, New York, NY
> 10005. Telephone (1) 212 813 6000.
>
>


William James

9/13/2006 9:16:00 PM

0


Peter Booth wrote:
> I am reading Ruby for Rails and Ruby Cookbock and delighted by the
> Array#find_all and Enumerable#collect methods. I'm wondering what the
> idiomatic way to do the following is?
>
> Two parallel arrays A, B . For each element n of A for which f(An) is true,
> is g(B[n-3:n+3]) true? Where B[n-3:n+3] refers to seven elements of B whose
> position is centered over An' position.
>
> Can the block inside Enumerable#collect know it's position inside the
> Enumerable? Can one collect two parallel arrays?

a = Array.new(12) {|i| i*i}
b = Array.new(12) {|i| i*3}
def f(n)
1 == n % 2
end
def g(a)
return false if a.size != 7
0 == a.inject{|p,n| p*n}/a.inject{|s,n| s+n} % 4
end
result = []
a.each_with_index{ |e,i|
result << (f(e) && g( b[i-3 .. i+3] ))
}

dblack

9/15/2006 8:44:00 AM

0

Logan Capaldo

9/15/2006 1:16:00 PM

0

On Fri, Sep 15, 2006 at 06:15:59PM +0900, Marshall T. Vandegrift wrote:
> dblack@wobblini.net writes:
>
> > To answer the first question: no. You are free to join Citizens for
> > map_with_index, of which I am the founder and president :-)
>
> What about Citizens Who Are Happy With enum_with_index.map ? I think they
> have a thing-or-two to say on this issue...
>
> -Marshall
>
Here, here! Down with the giant collection of methods! Up with
higher order methods!