如何在数组数组中获取特定值

Question

I have an array (outside array) that contains three arrays (inside arrays), each of which have three elements.

array = [[a, b, c], [d, e, f], [g, h, i]]

I want to select the specific inside array using an index of the outside array and then select the value within the selected inside array based off its index. Here is what I tried:

array.each_index{|i| puts "letter: #{array[i[3]]} " } 

I was hoping that would give me the following output

letter: c letter: f letter: i

but instead, I get

letter: [[a, b, c], [d, e, f], [g, h, i]]

I also tried

array.each_index{|i| puts "letter: #{array[i][3]} " } 

but I get the same result. Please any suggestions are very appreciated. I need a simple explanation.

Answer 1

each_index is an Enumerable which goes through all indices and performs an action on each one. When it's done it will return your original collection as it's not its job to change it. If you want to output stuff on the screen via puts / print then each_index is fine.

If you want to create a new collection as a result of going through all the elements of an original collection, you should use map .

eg

array = [['a', 'b', 'c'], ['d', 'e', 'f'], ['g', 'h', 'i']]
new_array = array.map {|el| el[2]}
=> ["c", "f", "i"]

array.map iterates through array's elements so in every step |el| is an element, not an index, as in: ['a', 'b', 'c'] in the first iteration, ['d', 'e', 'f'] in the second one and so on...

Just pointing this out since I don't know what's the goal of what you're trying to do.

Answer 2

do it like this:

array.each_index{|i| puts "letter: #{array[i][2]} " } 

Since you want letter at index 2, not 3.

Also array should be defined like this:

array = [['a', 'b', 'c'], ['d', 'e', 'f'], ['g', 'h', 'i']]

Answer 3

You could use map like so:

a = [['a', 'b', 'c'], ['d', 'e', 'f'], ['g', 'h', 'i']]
a.map(&:last)
# => ["c", "f", "i"]

Or if you really want the puts and not the collected values:

a.each {|v| puts "letter: #{v.last}"}

You could also use Ruby's Matrix class if there's more Matrix-y stuff you want to do:

require 'matrix'
m = Matrix[['a', 'b', 'c'], ['d', 'e', 'f'], ['g', 'h', 'i']]
last_column = m.column_vectors.last
# => Vector["c", "f", "i"]

You can now use any of the Vector methods on last_column , including to_a , which will put you back in familiar territory with the last column's values.

Answer 4

Arrays in ruby are indexed from 0, not from 1. So:

array.each_index{|i| puts "letter: #{array[i][2]} " }

should give you what you want.

Answer 5

You could try the below also:

p RUBY_VERSION

arr = [[1,2,3],[4,5,6],[11,12,13]]

arr.each{|x| p x; x.each_index{|i| p "Digit :: #{x[i]}" if i == 2} }

output:

"2.0.0"
[1, 2, 3]
"Digit :: 3"
[4, 5, 6]
"Digit :: 6"
[11, 12, 13]
"Digit :: 13