Delete array element if index position is greater than a specific value

Question

I am trying to delete elements from an array if its index is greater than a certain value. I am looking to do something like this:

a = ["a", "b", "c"]
b = a.delete_if {|x| x.index > 1 }

I took a look at drop, delete_if, etc. I tried completing this using each_with_index like this:

new_arr = []
a.each_with_index do |obj, index|
    if index > 1
        obj.delete
    end
    new_arry << obj
end

How can I delete an array element if it's array position is greater than a certain value?

Cary Swoveland · Accepted Answer · 2017-06-22 14:16:04Z

7

Here are some other ways to return a sans elements at indices >= index, which is probably better expressed as "returning the first index elements". All below return ["a", "b"]).

a = ["a", "b", "c", "d", "e"]
index = 2

Non-destructive (i.e., a is not altered)

a[0,index]
index.times.map { |i| a[i] }

Destructive (a is modified or "mutated")

a.object_id #=> 70109376954280 
a = a[0,index]
a.object_id #=> 70109377839640

a.object_id #=> 70109377699700 
a.replace(a.first(index))
a.object_id #=> 70109377699700

edited Jun 22, 2017 at 14:16

answered Feb 24, 2016 at 19:29

Cary Swoveland

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1 Comment

Jbur43 Over a year ago

exactly what I needed. Thank you for the detailed answer.

squiguy · Accepted Answer · 2016-02-24 20:38:51Z

4

You can use slice! and give it a range. It is a destructive method as indicated by the !, so it will mutate your array.

a = [1, 2, 3, 4]
a.slice!(2..-1)

a = [1, 2]

edited Feb 24, 2016 at 20:38

answered Feb 24, 2016 at 19:13

squiguy

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Comments

sawa · Accepted Answer · 2016-02-24 19:00:13Z

3

Array#first gives you the first n elements.

b = a.first(1)
# => ["a"]

If you want to do it in a destructive way, then this will do:

a.pop(a.length - 1)
a # => ["a"]

answered Feb 24, 2016 at 19:00

sawa

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2 Comments

Jbur43 Over a year ago

Lets say I had an array that was 20 elements long and wanted to delete index positions 9 and up? That's more specific to what I am trying to do

Cary Swoveland Over a year ago

I'm sure @sawa meant n where he wrote 1. (s: n needs backticks.)

Stefan · Accepted Answer · 2016-02-24 21:40:15Z

2

You can append with_index:

a = ["a", "b", "c"]
a.delete_if.with_index { |x, i| i > 1 }
a #=> ["a", "b"]

Another example:

a = ("a".."z").to_a
a.delete_if.with_index { |x, i| i.odd? }
#=> ["a", "c", "e", "g", "i", "k", "m", "o", "q", "s", "u", "w", "y"]

answered Feb 24, 2016 at 21:40

Stefan

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Comments

MattwalOlaide · Accepted Answer · 2016-02-25 15:45:44Z

0

Going by your question, "How can I delete an array element if it's array position is greater than a certain value?".

I assume what you want is that the final array you have should contain only elements before the specified index.

You can just do this:

your_array.select { |element| your_array.index(element) < max_index }

E.g

figures = [1,2,3,4,5,6]
figures.select{ |fig| figures.index(fig) < 3 }
# => [1, 2, 3]

answered Feb 25, 2016 at 15:45

MattwalOlaide

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