Ruby array.each with non unique integer values

Question

We have array = [4, 2, 9, 11, 2, 16]

Then we have

indexes = []
for i in array do 
    if i > 0 then indexes << array.find_index(i) else next end 
end

When printing out the result it returns [0, 1, 2, 3, 1, 5]

The problem is with the fourth index. It should be 4, but it's 1 and that's because index 1 and 4 of array have the same value (which is 2).

Isn't for loop (or .each) supposed to go through all the elements one by one? Why is this happening? Why is it picking up second index of array twice?

I would strongly suggest using modern syntax. There are very few cases that for..end syntax is preferred. It is identical to array.each though more verbose and some subtle scope differences that may get you in trouble someday. The else next has no effect either. — ForeverZer0
– ForeverZer0, Commented Oct 13, 2018 at 10:37
array = [0, 2, 9, 0, 2, 16]; array.each_index.reject { |i| array[i].zero? } #=> [1, 2, 4, 5]. — Cary Swoveland
– Cary Swoveland, Commented Oct 13, 2018 at 18:57

Chris Heald · Accepted Answer · 2018-10-13 10:34:29Z

2

array.find_index returns the first index of an element in array matching the passed value.

If you want the index of the value you're looking for then you should be iterating with each_with_index:

indexes = []
array.each_with_index do |value, index|
  indexes << index if value > 0
end

Or more compact (with just a single array allocation):

indexes = array.each_with_object([]).with_index {|(v, o), i| o << v if i > 0 }

Or allowing for multiple allocations:

indexes = array.map.with_index {|v, i| v > 0 ? i : nil }.compact

Or:

indexes.map.with_index.select {|v| v.first > 0 }.map(&:last)

answered Oct 13, 2018 at 10:34

Chris Heald

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2 Comments

lacostenycoder Over a year ago

Do you even refactor!? :) Nice one! But last answer is the most slick but you should call it on the array object array.map.with_index.select {|v| v.first > 0 }.map(&:last) or it just returns []

Sagar Pandya Over a year ago

Alternatively for your last example use select { |a, _| a.positive? }.

koffeinfrei · Accepted Answer · 2018-10-13 10:33:46Z

1

Because Array#find_index returns the index of the first element it finds in the array.

Returns the index of the first object in ary such that the object is == to obj.

answered Oct 13, 2018 at 10:33

koffeinfrei

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