How do I transpose multiple arrays unless an array element is empty?

Question

I have the following code:

[array1, array2, array3].transpose

I want to do something like this:

[array1, array2, array3].transpose unless array2[value] == ""

I want to transpose each array, but when the value for array2 that is being transposed is blank I want to skip to the next set of values altogether, ensuring that none of the arrays get transposed.

How can I handle that within Ruby 1.9.3?

Suppose you have [["a","b"], ["", "c"], ["d", "e"]]. On tranpose, do you want the result as ["b", "c", "e"] ? — kiddorails
– kiddorails, Commented Oct 22, 2013 at 17:56

kiddorails · Accepted Answer · 2013-10-22 18:04:36Z

3

Consider this :

arr1 = ["a", "b"]
arr2 = ["",  "c"]
arr3 = ["d", "e"]

Now, as per your requirement,; you want those tranposed arrays where arr2 blank value is not present.

Use #reject to do that as:

[arr1, arr2, arr3].transpose.reject{ |x| x[1].empty? }

Here, x[1] corresponds to the second element in each transposed array; and it comes from arr2; so here, we rejected all those instances where "" was present in arr2.

Hope it helps

answered Oct 22, 2013 at 18:04

kiddorails

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1 Comment

Luigi Over a year ago

.reject - That does it. Never cease to be amazed...Thanks for the help.

Cary Swoveland · Accepted Answer · 2013-10-23 18:09:45Z

1

If you had large arrays, and memory were a concern, you could reduce the size of the array before transposing it. That's straightfoward. Here's one way (i being the row containing one or more instances of bad_obj):

def tr(arr, i, bad_obj)
  bad_cols = []
  arr[i].each_with_index {|e,i| bad_cols << i if e==bad_obj}
  arr.map {|row| row.reject.with_index {|e,i| bad_cols.include?(i)}}.transpose
end
arr = [[1,2,3], [4,'',6], [7,8,9]]  
tr(arr, 1, "") # => [[1,4,7], [3,6,9]]

construct an array bad_cols that contains the indices of the columns to be skipped.
remove the bad_cols columns from each row, then apply transpose.

Edit: replaced

bad_cols = arr[i].each_with_index.reduce([]) {|a,e| a << e.last if e.first==bad_obj; a}

with the first two lines of def tr(). Simpler is better.

edited Oct 23, 2013 at 18:09

answered Oct 22, 2013 at 23:02

Cary Swoveland

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1 Comment

Luigi Over a year ago

It's always helpful and eye-opening to see other methods like this. No doubt this will help in the future - Thanks!

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