Harry Smith - 10 months ago 30

Python Question

When one converts a list to a set in Python using the

`set()`

`In [1]: list = [1,4,1]`

In [2]: list

Out[2]: [1, 4, 1]

In [3]: set = set(list)

In [4]: set

Out[4]: {1, 4}

This is inconvenient when using sets to calculate properties of the list and converting back, as in the following vector subtraction example.

`In [1]: a = [1,1,1]`

In [2]: a

Out[2]: [1, 1, 1]

In [3]: b = [1,2,3]

In [4]: b

Out[4]: [1, 2, 3]

In [5]: list( set(a) - set(b) )

Out[5]: []

Clearly something went wrong: we were expecting the operation

`{1-1, 1-2, 1-3} = {0, -1, -2}`

`In [6]: set(a)`

Out[6]: {1}

In [7]: set(b)

Out[7]: {1, 2, 3}

This means we are calculating

`{1-1, ?-2, ?-3} = ?`

Is there a way to retain the structure and values of a list when converting to a set in order to avoid this problem?

Clearly I am not talking about subtracting lists: I am talking about subtracting sets.

Answer

I don't see the need for `set`

in this case. You can just `zip`

the lists and do a subtraction like this

```
>>> a = [1,1,1]
>>> b = [1,2,3]
>>> [x-y for x,y in zip(a,b)]
[0, -1, -2]
```