Extracting and inserting a "column" from/into an array

Ruby
1

Hi,

For image processing, I have to do the following operations :

1/ I have an array and I want to extract every 4 element into another
array :

r=red, g=green, b=blue, a=alpha

img_data = [ “r1”, “g1”, “b1”, “a1”, "r2, “g2”, “b2”, “a2” ]

=> [ “a1”, “a2” ] # the array of every four elements i.e. alpha data

It’s like extracting the fourth column of a matrix of n rows and 4
columns.

2/ Conversely, I have an array and I need to add elements at every 3
elements

img_data = [ “r1”, “g1”, “b1”, "r2, “g2”, “b2” ]
alpha_data = [ “a1”, “a2” ] # the elements to add

=> [ “r1”, “g1”, “b1”, “a1”, "r2, “g2”, “b2”, “a2” ]

It’s like inserting a column to a matrix of n rows and 3 columns.

I’m wondering if there is an easy or seamless way to do these
operations.
Thanks in advance for any tips.

Cheers.

Chauk-Mean.

2

Array is an Enumerable so when you require ‘enumerator’ you get a bunch
of cool ways of slicing it up.

1/ I have an array and I want to extract every 4 element into another
array :

r=red, g=green, b=blue, a=alpha

img_data = [ “r1”, “g1”, “b1”, “a1”, "r2, “g2”, “b2”, “a2” ]

=> [ “a1”, “a2” ] # the array of every four elements i.e. alpha data

result = img_data.enum_slice(4).map {|arr| arr[3]}

=> [ “r1”, “g1”, “b1”, “a1”, "r2, “g2”, “b2”, “a2” ]
result = img_data.enum_slice(3).zip(alpha_data).flatten

m.

3

Hi Matt,

result = img_data.enum_slice(4).map {|arr| arr[3]}

result = img_data.enum_slice(3).zip(alpha_data).flatten

This is exactly what I was looking for. I just replace enum_slice with
each_slice as I’m using ruby 1.9.
Thanks a lot.

Cheers.

Chauk-Mean.

4

On Jun 16, 9:34 am, Chauk-Mean P. [email protected] wrote:

=> [ “a1”, “a2” ] # the array of every four elements i.e. alpha data
=> [ “r1”, “g1”, “b1”, “a1”, "r2, “g2”, “b2”, “a2” ]

It’s like inserting a column to a matrix of n rows and 3 columns.

I’m wondering if there is an easy or seamless way to do these
operations.

Possibly not as efficient, but if you were representing the data as a
‘2D’ array (array of arrays) alternatively you could .transpose the
array, insert a row, and .transpose it back.