When converting a nested array to a matrix in Ruby, the matrix ends up with an extra [] around the values, compared to simply creating a matrix from scratch.
> require 'matrix'
> matrix1 = Matrix[[1,2,3],[4,5,6],[7,8,9]]
> p matrix1
=> Matrix[[1, 2, 3], [4, 5, 6], [7, 8, 9]]
> nested_array = [[1,2,3],[4,5,6],[7,8,9]]
> matrix2 = Matrix[nested_array]
> p matrix2
=> Matrix[[[1, 2, 3], [4, 5, 6], [7, 8, 9]]]
Is there a way to avoid the extra square brackets when building from an array?
matrix2 = Matrix[*nested_array]
p matrix2
=> Matrix[[1, 2, 3], [4, 5, 6], [7, 8, 9]]
The asterisk (*) there is called the "splat operator," and it essentially can be used to treat an array (nested_array in this case) as if it weren't an array, but rather as if its elements were individual elements/arguments.
Related
Is there any short way to access an element of a nested array, passing the array with coordinates? I mean something like:
matrix = [[1,2,3,4],[5,6,7,8]]
array = [1,1]
matrix [array]
# => 6
I just wonder if there is a shorter version than:
matrix [array[0]][array[1]]
I believe you want to use the Matrix class:
require 'matrix'
arr = [[1,2,3,4],[5,6,7,8]]
matrix = Matrix[*arr] #=> Matrix[[1, 2, 3, 4], [5, 6, 7, 8]]
matrix[1,1] #=> 6
matrix.row(1) #=> Vector[5, 6, 7, 8]
c = matrix.column(1) #=> Vector[2, 6]
c.to_a #=> [2, 6]
m = matrix.transpose #=> Matrix[[1, 5], [2, 6], [3, 7], [4, 8]]
m.to_a #=> [[1, 5], [2, 6], [3, 7], [4, 8]]
array.inject(matrix, :fetch)
# => 6
matrix[1][1]
should equal 6. matrix[1] is the 2nd array, matrix[1][1] is the second element in that array.
I have several equally sized arrays containing numbers (matrix), and I want to sum them all by their index (matrix columns).
For example, if I have:
data = [[1, 2, 3, 4], [5, 6, 7, 8]]
I want to get the result:
column_totals = [6, 8, 10, 12]
I understand how to do this imperatively, but how would I do this using functional programming? (Preferably, using built in Enumerable methods.) I wasn't very happy with any of the functional solutions I came up with.
I ended up using the Matrix class:
require 'matrix'
data = [[1, 2, 3, 4], [5, 6, 7, 8]]
matrix = Matrix[*data]
# Added sum method to Vector class.
matrix.column_vectors.map { |column| column.sum }
I'm happy enough with that solution, but am frustrated that I couldn't wrap my mind around a good functional solution without relying on the Matrix class.
Specifically, I was tripped up on the step to transform this:
data = [[1, 2, 3, 4], [5, 6, 7, 8]]
into this:
columns = [[1, 5], [2, 6], [3, 7], [4, 8]]
Any reason to not use Array#transpose?
data.transpose
# => [[1, 5], [2, 6], [3, 7], [4, 8]]
Alternatively, if you only want to use Enumerable methods to iterate, you can do
columns = data.inject(Array.new(data.first.length){[]}) { |matrix,row|
row.each_with_index { |e,i| matrix[i] << e }
matrix }
# => [[1, 5], [2, 6], [3, 7], [4, 8]]
or
columns = data.flatten.group_by.with_index { |e,i| i % data[0].size }.values
# => [[1, 5], [2, 6], [3, 7], [4, 8]]
To sum:
columns.map { |row| row.inject :+ }
# => [6, 8, 10, 12]
Thirdly, if you don't need the intermediate columns:
data.inject { |s,r| s.zip(r).map { |p| p.inject :+ } }
# => [6, 8, 10, 12]
You could use Array#transpose, as #Matt hinted, and then sum the arrays inside:
data.transpose.map {|a| a.reduce(:+) }
I have this array
a = [[1,2,3,4,5],[2,3,6],[2,8,9]]
How can I get the intersection of all the arrays in a? It should be the same as a[0]&a[1]&a[2], which is:
[2]
a = [[1 ,2, 3, 4, 5], [2, 3, 6], [2, 8, 9]]
a.inject(:&)
# => [2]
How can I go from this:
for number in [1,2] do
puts 1+number
puts 2+number
puts 3+number
end
which will return 2,3,4 then 3,4,5 -> 2,3,4,3,4,5. This is just an example, and clearly not the real use.
Instead, I would like it to return 2,3 3,4 4,5 -> 2,3,3,4,4,5. I would like each of the puts to be iterated for each of the possible values of number; In this case 1 and 2 are the two possible values of 'number', before moving on to the next puts.
One way to do this is to create two lists, [2,3,4] and [3,4,5] and then use the zip method to combine them like [2,3,4].zip([3,4,5]) -> [2,3,3,4,4,5].
zip is good. You should also look at each_cons:
1.9.2p290 :006 > [2,3,4].each_cons(2).to_a
=> [[2, 3], [3, 4]]
1.9.2p290 :007 > [2,3,4,5,6].each_cons(2).to_a
=> [[2, 3], [3, 4], [4, 5], [5, 6]]
1.9.2p290 :008 > [2,3,4,5,6].each_cons(3).to_a
=> [[2, 3, 4], [3, 4, 5], [4, 5, 6]]
Because each_cons returns an Enumerator, you can use a block with it, as mentioned in the documentation for it, or convert it to an array using to_a like I did above. That returns the array of arrays, which can be flattened to get a single array:
[2,3,4,5].each_cons(2).to_a.flatten
=> [2, 3, 3, 4, 4, 5]
From the ri docs:
Iterates the given block for each array of consecutive elements. If no
block is given, returns an enumerator.
e.g.:
(1..10).each_cons(3) {|a| p a}
# outputs below
[1, 2, 3]
[2, 3, 4]
[3, 4, 5]
[4, 5, 6]
[5, 6, 7]
[6, 7, 8]
[7, 8, 9]
[8, 9, 10]
Maybe not the most readable code but you could use inject on the first range to create an array based on the summed up second range.
(1..3).inject([]){|m,n| (1..2).each{|i| m<<n+i }; m }
=> [2, 3, 3, 4, 4, 5]
This might be a little more readable
res=[]
(1..3).each{|r1| (1..2).each{|r2| res<<r1+r2 } }
[1, 2, 3].each { |i| [1, 2].each { |y| puts i + y } }
I have a sparse array, for example:
rare = [[0,1], [2,3], [4,5], [7,8]]
I want to plot a chart with these data, each pair are point coordinates.
As you can see I don't have points for x=1, x=3 , x=5, x=6
I want to fill the array with the previous values, so for the above example I will get:
filled = [[0,1], [1,1], [2,3], [3,3], [4,5], [5,5], [6,5], [7,8]
As you can see, for calculating the y value, I simply take the last y value I used.
What is the best aproach to accomplish this ?
Range.new(*rare.transpose.first.sort.values_at(0,-1)).inject([]){|a,i|
a<<[i, Hash[rare][i] || a.last.last]
}
Step-by-step explanation:
rare.transpose.first.sort.values_at(0,-1) finds min and max x ([0,7] in your example)
Range.new() makes a range out of it (0..7)
inject iterates through the range and for every x returns pair [x,y], where y is:
y from input array, where defined
y from previously evaluated pair, where not
Note: here are some other ways of finding min and max x:
[:min,:max].map{|m| Hash[rare].keys.send m}
rare.map{|el| el.first}.minmax # Ruby 1.9, by steenslag
rare = [[0,1], [2,3], [4,5], [7,8]]
filled = rare.inject([]) do |filled, point|
extras = if filled.empty?
[]
else
(filled.last[0] + 1 ... point[0]).collect do |x|
[x, filled.last[1]]
end
end
filled + extras + [point]
end
p filled
# => [[0, 1], [1, 1], [2, 3], [3, 3], [4, 5], [5, 5], [6, 5], [7, 8]]
An inject solution:
filled = rare.inject([]) do |filled_acc, (pair_x, pair_y)|
padded_pairs = unless filled_acc.empty?
last_x, last_y = filled_acc.last
(last_x+1...pair_x).map { |x| [x, last_y] }
end || []
filled_acc + padded_pairs + [[pair_x, pair_y]]
end
More about Enumerable#inject and functional programming with Ruby here.
irb(main):001:0> rare = [[0,1], [2,3], [4,5], [7,8]]
=> [[0, 1], [2, 3], [4, 5], [7, 8]]
irb(main):002:0> r=rare.transpose
=> [[0, 2, 4, 7], [1, 3, 5, 8]]
irb(main):003:0> iv = (r[0][0]..r[0][-1]).to_a.select {|w| !r[0].include?(w) }
=> [1, 3, 5, 6]
irb(main):004:0> r[1][-1]=r[1][-2]
=> 5
irb(main):005:0> p (iv.zip(r[1]) + rare).sort
[[0, 1], [1, 1], [2, 3], [3, 3], [4, 5], [5, 5], [6, 5], [7, 8]]
=> [[0, 1], [1, 1], [2, 3], [3, 3], [4, 5], [5, 5], [6, 5], [7, 8]]