I have 2d array
arr = [[1, 2, 1, 1],
[1, 1, 1, 1],
[1, 4, 4, 4],
[1, 4, 8, 4],
[1, 4, 4, 4]]
What is the best way to extract that array with lots of 4s when I know only the top left corner indexes?
arr[1][2]
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2What exactly are you trying to achieve?Uri Agassi– Uri Agassi2014-12-01 09:52:56 +00:00Commented Dec 1, 2014 at 9:52
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Make a new smaller 3x3 array.Kano– Kano2014-12-01 09:53:56 +00:00Commented Dec 1, 2014 at 9:53
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Do you, perchance, look to find a submatrix from a 2D array?Amadan– Amadan2014-12-01 09:54:00 +00:00Commented Dec 1, 2014 at 9:54
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1Why the rush to select an answer? For one, it's a discourtesy to others who are still preparing answers.Cary Swoveland– Cary Swoveland2014-12-01 10:20:02 +00:00Commented Dec 1, 2014 at 10:20
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3 Answers
arr[2, 3].map { |row| row[1, 3] }
# => [[4, 4, 4], [4, 8, 4], [4, 4, 4]]
Get three rows starting from the second, for each row get three elements starting from the first (0-based).
Comments
The method Matrix#minor is taylor-made for this.
Code
require 'matrix'
def pull_subarray(arr, row_range, col_range)
Matrix[*arr].minor(row_range, col_range).to_a
end
Examples
pull_subarray(arr, 1..-1, 2..-1)
#=> [[1, 1],
# [4, 4],
# [8, 4],
# [4, 4]]
pull_subarray(arr, 1..2, 2..3)
#=> [[1, 1],
# [4, 4]]
Comments
Another option:
arr[2..-1].map { |a| a[1..-1] }
# => [[4, 4, 4], [4, 8, 4], [4, 4, 4]]
