NamNamNam NamNamNam - 1 year ago 74
Ruby Question

Ruby didn't loop all elements?

I am trying to remove all

s from an array:

a = [1,2,3,4,5,7,7,7,7,7,7,7,7,7,9,10,11,12,13]

I did:

a.each_with_index do |item, index|
if item == 7
a # => [1, 2, 3, 4, 5, 7, 7, 7, 7, 9, 10, 11, 12, 13]

How did this happen?

Answer Source

The fact that only about half (5/9) of your items have disappeared is a dead giveaway that the problem is deleting while iterating over the collection.

The iteration will be processing indexes 1, 2, 3, 4 and so on. If you delete index 2 while processing it, that will shift all later indexes down by one.

So, when you then move on to index 3 in the next iteration, you will skip the original index 3 because that will have been shifted down to index 2.

In other words, let's start with a simpler example with two consecutive items to remove:

index | 0 | 1 | 2 | 3 |
value | 1 | 7 | 7 | 9 |

You check the first index and the value is 1 so you do nothing. You then check the second index and the value is 7 so you delete it, giving:

index | 0 | 1 | 2 |
value | 1 | 7 | 9 |

You then check the third index and the value is 9 so you do nothing. You've also reached the end so it stops.

So you can see there that you actually skipped the second item you wanted to delete, because you shifted things around while iterating. This isn't a Ruby-specific problem, a great many languages have this same issue.

In general, each complete pair of adjacent items will only have the first of the pair deleted while an item on its own (not followed by another of the same value) will be deleted normally. That's why only 5/9 of your 7s are deleted, one for each of the four pairs and the final standalone one.

The correct way (in Ruby) to delete all items of a single given value is to use the array delete method:


You can also use conditional delete for more complex conditions such as deleting everything greater than 7:

a.delete_if {|val| val > 7}
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