hockmode - 6 months ago 35

Ruby Question

i'm trying to figure out why this swap does not work properly. I added

`p`

`def partition (array, from, to)`

#declared pivot to last index of array

pivot = array[to]

pIndex = from

for i in from..to-1

if array[i] <= pivot

array[i], array[pIndex] = array[pIndex], array[i]

pIndex += 1

end

end

p pivot

p array[to]

### why doesn't this work properly? pivot is same as array[to]

### array[pIndex], pivot = pivot, array[pIndex]

### the swap below works

array[pIndex], array[to] = array[to], array[pIndex]

p array

return pIndex

end

I have a

`pivot = array[to]`

`array[pIndex], pivot = pivot, array[pIndex]`

`array[pIndex]`

`pivot`

`pivot`

`array[pIndex]`

`array[pIndex], array[to] = array[to], array[pIndex]`

Example with an array:

`arr = [7, 2, 1, 6, 8, 5, 3, 4]`

`partition(arr, 0,7)`

Before the last swap happens the array is

`[2, 1, 3, 6, 8, 5, 7, 4]`

`pivot`

`array[pIndex]`

`[2, 1, 3, 4, 8, 5, 7, 6]`

Answer

Let's break down what parallel assignment is doing here.

Say we have an array:

```
arr = [1, 2]
arr[0], arr[1] = arr[1], arr[0]
# arr => [2, 1]
```

This is the expected behavior - we are simultaneously doing the following two operations:

`arr[0] = arr[1]`

and `arr[1] = arr[0]`

.

Now suppose we do

```
arr = [1, 2]
first = arr[0]
first, arr[1] = arr[1], first
# arr => [1, 1]
# first => 2
```

This is because now we are doing `first = arr[1]`

and `arr[1] = first`

.

`first`

is a variable set to the value found at `arr[0]`

, and changing this variable does NOT mutate the array.