How to change the position of an array element?

Question

I have a question on how I can change the index of a array element, so that it doesn\'t come at the 7. position but at position 2 instead...

Is there a function to h

Answer 1

The answers here don't cover both possible scenarios. While the question dealt with an origin index higher than the destination, if the reverse is true then the solution below won't work:

array.insert 7, array.delete_at(2)

This is because deleting the value at 2 shifts everything (above 2) down the array by 1. Now our destination index of 7 is pointing at what used to be at index 8.

To fix this, we need to check if the origin is less than the destination and if so, deduct 1 from the destination.

origin = 2
destination = 7

destination -= 1 if origin < destination
array.insert destination, array.delete_at(origin)

Answer 2

Isn't better to use:

irb> a = [2,5,4,6]
#=> [2, 5, 4, 6]
irb> a.insert(1, a.pop)
#=> [2, 6, 5, 4]

?

Answer 3

If you don't care about the position of the other elements in the array you can use the .rotate! (note that the ! at the end of this method changes the actual array) method.

arr = [1, 2, 3, 4, 5, 6, 7, 8]
arr.rotate! -3
arr = [6, 7, 8, 1, 2, 3, 4, 5]

This takes the element 8 which is at index 7, and rotates it to an index of 2.

Answer 4

Explanation for beginners

These answers are great. I was looking for a little more explanation on how these answers work. Here's what's happening in the answers above, how to switch elements by value, and links to documentation.

# sample array
arr = ["a", "b", "c", "d", "e", "f", "g", "h"]

# suppose we want to move "h" element in position 7 to position 2 (trekd's answer)
arr = arr.insert(2, arr.delete_at(7))
=> ["a", "b", "h", "c", "d", "e", "f", "g"]

This works because arr.delete_at(index) deletes the elements at the specified index ('7' in our example above), and returns the value that was in that index. So, running arr.delete_at(7) would produce:

# returns the deleted element
arr.delete_at(7) 
=> "h"           

# array without "h"
arr
=> ["a", "b", "c", "d", "e", "f", "g"]

Putting it together, the insert method will now place this "h" element at position 2. Breaking this into two steps for clarity:

# delete the element in position 7
element = arr.delete_at(7)  # "h"
arr.insert(2, element)
=> ["a", "b", "h", "c", "d", "e", "f", "g"]

Switching elements by value

Suppose you wanted to move the element in the array whose value is "h", regardless of its position, to position 2. This can easily be accomplished with the index method:

arr = arr.insert(2, arr.delete_at( arr.index("h") ))

Note: The above assumes that there's only one value of "h" in the array.

Documentation

• Array#delete_at
• Array#insert
• Array#index

Answer 5

Best way..

array = [4, 5, 6, 7]

array[0], array[3] = array[3], array[0]

array # => [7, 5, 6, 4]

Answer 6

irb> a = [2,5,4,6]
=> [2, 5, 4, 6]
irb> a.insert(1,a.delete_at(3))
=> [2, 6, 5, 4]