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

Ruby
1

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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2

On 9/13/06, Peter B. [email protected] wrote:

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

Peter

Hmm this seems interestingly complicated - or I am just dully stupid;)

Well here is what I came up with:

a = [*1…20]
b = a.map{ |e| “Line #{e}” }

def ranges a, b, n, &block

The following is what you were asking for :wink: 

b.values_at(*(0..a.length-1).zip(a).map{|e| block.call(e.last) ?

e.first: nil}.
compact.map{|i|
[*(i-n)…(i+n)]}.flatten.uniq.reject{|i|i<0}).compact

end

p ranges(a,b,1) { |e| (e%3).zero? }
p ranges(a,b,2) { |e| (e%3).zero? }
p ranges(a,b,1) { |e| (e%5).zero? }
p ranges(a,b,2) { |e| (e%5).zero? }

HTH
Robert

PS:
Golfers cut it down :wink:

R

–
Deux choses sont infinies : l’univers et la bêtise humaine ; en ce qui
concerne l’univers, je n’en ai pas acquis la certitude absolue.

  • Albert Einstein
3

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

4

On 9/13/06, Jason N. [email protected] wrote:

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)})

I am afraid this does not what you want, select returns i’s not the
value of
the block
you had some great ideas I have stolen below though :slight_smile:
I’ll try to fix your interpretation, ah I see you think he just wants
the
indicies, so your solution is (almost) great, you can use [*0…a.size]
but
you might have a problem with f(nil) so use
[0…a.size-1]
OP also stated he wanted g(b[something]) not g(something) it is a riddle
an alternative might be
b.values_at([0…a.size-1].select{|i|f a[i]}.map{|i|
g(b.values_at([(i-n)…(i+n)].reject
etc. etc.

Ruby seems a little verbose on this one.

-j

I still think OP thaught g to be something abstract so I would write

def ranges a, b, n, &block

The following is what you were asking for :wink: 

b.values_at(
*(0..a.length-1).select{|i|  block.call(a[i]) }.map{|i|

[*(i-n)…(i+n)]
}.
flatten.uniq.reject{|i|i<0}).compact

end

thanks to your ideas letting me see my stupid mistakes…

Robert

Peter B. wrote:

Can the block inside Enumerable#collect know it’s position inside the
recipient(s).
Europe

–
Deux choses sont infinies : l’univers et la bêtise humaine ; en ce qui
concerne l’univers, je n’en ai pas acquis la certitude absolue.

  • Albert Einstein
5

Peter B. 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| ii}
b = Array.new(12) {|i| i3}
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] ))
}

6

Peter B. [email protected] writes:

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?

Kind of. But first I’m going to re-arrange your email to answer more
didactically. :slight_smile:

Can the block inside Enumerable#collect know it’s position inside the
Enumerable?

Enumerable::Enumerator – require ‘enumerator’ – can help you out with
this.
It lets you create an Enumerable object which can call any ‘each’-like
method
of any other object. You can in turn call the ‘collect’ method of the
Enumerator object. So:

array.to_enum(:each_with_index).collect { |value, index| ... }

(Using :each_with_index is a common enough case that Enumerator also
adds
#enum_with_index to Enumerable.)

Can one collect two parallel arrays?

In several ways… The most basic is the Enumerable#zip method:

(0..3).zip(('a'..'d')) # => [[0, "a"], [1, "b"], [2, "c"], [3, "d"]]

It returns a new array mapping each element of the receiving Enumerable
to the
corresponding elements of the Enumerables passed as arguments.

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.

Enumerable#inject is the most idiomatic way to perform most Enumerable
iteration not directly supported by other methods. If I understand you
properly, in this case you probably want something like:

a.enum_with_index.inject([]) do |result, (v, n)|
  result << (f(v) ? g(b[n-3..n+3]) : false)
end

Hope this helps!

-Marshall

7

[email protected] 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 :slight_smile:

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

8

Hi –

On Thu, 14 Sep 2006, Peter B. 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?

To answer the first question: no. You are free to join Citizens for
map_with_index, of which I am the founder and president :slight_smile:

David

9

On Fri, Sep 15, 2006 at 06:15:59PM +0900, Marshall T. Vandegrift wrote:

[email protected] 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 :slight_smile:

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!