How do I subtract lists from each other?

Question

I have two functions that output 2 lists respectively using what I've coded below. I'm trying to subtract one list from another.

def ok(n):
    results = []
    for n in range (2, n+1):
        s = Sup(n)
        results.append(s)
    return(results)


def uk(m):
    result = []
    for m in range (2, m+1):
        t = Sdown(m)
        result.append(t)
    return(result)
print(ok(7))
print(uk(7))

uk(7) - ok(7)

When I call, ok(7) I get:

[1.0833333333333333, 1.7178571428571427, 2.380728993228994, 3.05849519543652, 3.7438909037057684, 4.433147092589173]

Similarly for uk(7), I get:

[2.083333333333333, 2.7178571428571425, 3.380728993228993, 4.058495195436521, 4.743890903705768, 5.433147092589174]

I've tried then performing the operation: uk(7) - ok(7) but get the following error:

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-23-3aa3eb52f7a8> in <module>
     18 print(uk(7))
     19 
---> 20 uk(7) - ok(7)

TypeError: unsupported operand type(s) for -: 'list' and 'list'

How can I fix this?

It's not something to _fix_. If you want element-wise operation, try: `[l1-l2 for l1,l2 in zip(uk(7), ok(7))]` or use `numpy` — Chris, Apr 08 '19 at 05:05
To help you understand why Python is throwing an error here, notice it's not immediately obvious what the `-` operation should do to two lists. Maybe element-wise subtraction (subtract `a[0]` from `b[0]`, ...). But what if the lists are different lengths? What if one list is empty? You'll need to implement the behavior you're seeking yourself. — jayelm, Apr 08 '19 at 05:09
Possible duplicate of [Python list subtraction operation](https://stackoverflow.com/questions/3428536/python-list-subtraction-operation) — Amit Gupta, Apr 08 '19 at 05:14
Unless OP was intending to emulate set difference by doing `uk(7) - ok(7)` (which is unlikely), the question is not a duplicate to [Python list subtraction operation](https://stackoverflow.com/questions/3428536/python-list-subtraction-operation). — TrebledJ, Apr 08 '19 at 05:34

score 2 · Answer 1 · answered Apr 08 '19 at 05:10

You Cannot subtract list from another list. Try using numpy or Zip

>>> l1 = [1.0833333333333333, 1.7178571428571427, 2.380728993228994, 3.05849519543652, 3.7438909037057684, 4.433147092589173]
>>> l2 = [2.083333333333333, 2.7178571428571425, 3.380728993228993, 4.058495195436521, 4.743890903705768, 5.433147092589174]

>>> import numpy as n
>>> n.array(l2) - n.array(l1)
    array([ 1.,  1.,  1.,  1.,  1.,  1.])

Thierry Lathuille · Answer 2 · 2019-04-08T05:19:35.357

Use zip to pair the elements of your lists, and a list comprehension to generate the output list:

difference = [u - o for u, o in zip(uk(7), ok(7))]

zip yields tuples by combining the elements of your two lists uk(7) and ok(7):

(<first item of uk(7)>, first item of ok(7)>)
(<second item of uk(7)>, second item of ok(7)>)
...

In the for loop, the two values in the tuple are unpacked to u and o, and the difference list is built of the resulting u - o values.

Search about 'list comprehension' if you don't know about it, you'll find plenty of information.

How do I subtract lists from each other?

2 Answers2