array.map的不直观行为

Question

So this behaviour seems to be logical

> array = (1..4).to_a
=> [1, 2, 3, 4]
> array.map {|a| [array.delete(array.first), array.delete(array.last)]}
=> [[1, 4], [2, 3]]
> array
=> []

But then when we increase the range it doesn't work:

> array = (1..6).to_a
=> [1, 2, 3, 4, 5, 6]
> array.map {|a| [array.delete(array.first), array.delete(array.last)]}
=> [[1, 6], [2, 5]]
> array
=> [3, 4]

And it does the same weird behaviour with odd numbers:

> array = (1..9).to_a
 => [1, 2, 3, 4, 5, 6, 7, 8, 9]
> array.map {|a| [array.delete(array.first), array.delete(array.last)]}
 => [[1, 9], [2, 8], [3, 7]]
> array
 => [4, 5, 6] 

This was not what we expected, why does it behave this way?

Answer 1

This is a guess since I am not sure if this is the exact case but I can't put this as a comment.

So lets go step by step:

you defined an array:

> array = (1..6).to_a
 => [1, 2, 3, 4, 5, 6]

Now defined an iterator:

array.map {|a| [array.delete(array.first), array.delete(array.last)]}

so for first iteration it will take array[0] as the value for for variable a and do the operation and hence deleting first and last element of the array.

After first iteration array becomes [2,3,4,5] .

For second iteration it will take array[1] as the value for for variable a and do the same as in first iteration and delete first and last element. Now array is [3,4] .

Now for 3rd iteration, well it would not have tried for 3rd iteration if your original array was only size of 2 ie like [2,3] but since you modified your original array during iteration so it will attempt 3rd iteration since original array had length 6 .

For 3rd iteration it needs array's third element ie array[2] but your current array does not have that index so it returns the remaining array without any other operation.

It is really confusing to modify array while being iterated.