Mapping the index of values in an array that match a specific value?

Question

Disclaimer, I'm a beginner.

I have an array that is 16 digits, limited to 0's and 1's. I'm trying to create a new array that contains only the index values for the 1's in the original array.

I currently have:

one_pos = []
    image_flat.each do |x| 
        if x == 1 
            p = image_flat.index(x)
            one_pos << p
            image_flat.at(p).replace(0)
        end
    end

The image_flat array is [0, 0, 0, 1, 0, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0]

With the code above, one_pos returns [3, 3] rather than the [3, 5] that I'd expect.

Where am I going wrong?

Maxim Pontyushenko · Accepted Answer · 2015-04-14 18:24:09Z

6

Where am I going wrong?

When you call

image_flat.index(x)

It only returns first entry of x in image_flat array.

I guess there are some better solutions like this one:

image_flat.each_with_index do |v, i|
  one_pos << i if v == 1
end

edited Apr 14, 2015 at 18:24

answered Apr 14, 2015 at 16:40

Maxim Pontyushenko

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1 Comment

amongmany Over a year ago

Thanks for the explanation Maxim, this really helps me understand why .index wasn't working for me!

Aaron Washburn · Accepted Answer · 2015-04-14 16:42:15Z

2

Try using each_with_index (http://apidock.com/ruby/Enumerable/each_with_index) on your array.

image_flat = [0, 0, 0, 1, 0, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0]

one_pos = []
image_flat.each_with_index do |value, index| 
  if value == 1 
    one_pos << index
  end
end

answered Apr 14, 2015 at 16:42

Aaron Washburn

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Comments

Undo · Accepted Answer · 2015-04-14 17:18:58Z

2

I think this is the most elegant solution here:

image_flat.each_index.select{|i| image_flat[i] == 1}

edited Apr 14, 2015 at 17:18

Undo♦

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answered Apr 14, 2015 at 16:57

Gena Shumilkin

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earth2jason · Accepted Answer · 2019-01-30 00:29:04Z

0

Here is a solution if you are looking for a solution that doesn't reach out of the enumerable block although it does require a chained solution.

image_flat.each_with_index.select { |im,i| im==1 }.map { |arr| arr[1] }

Its chained and will require an additional lookup so Gena Shumilkin's answer will probably be more optimal for larger arrays.

This was what I originally thought Gena Shumilkin was trying to reach until I realized that solution used each_index instead of each_with_index.

answered Jan 30, 2019 at 0:29

earth2jason

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