Subtracting 2 lists in Python
PythonListTuplesVectorPython Problem Overview
Right now I have vector3 values represented as lists. is there a way to subtract 2 of these like vector3 values, like
[2,2,2] - [1,1,1] = [1,1,1]
Should I use tuples?
If none of them defines these operands on these types, can I define it instead?
If not, should I create a new vector3 class?
Python Solutions
Solution 1 - Python
If this is something you end up doing frequently, and with different operations, you should probably create a class to handle cases like this, or better use some library like Numpy.
Otherwise, look for list comprehensions used with the zip builtin function:
[a_i - b_i for a_i, b_i in zip(a, b)]
Solution 2 - Python
Here's an alternative to list comprehensions. Map iterates through the list(s) (the latter arguments), doing so simulataneously, and passes their elements as arguments to the function (the first arg). It returns the resulting list.
import operator
map(operator.sub, a, b)
This code because has less syntax (which is more aesthetic for me), and apparently it's 40% faster for lists of length 5 (see bobince's comment). Still, either solution will work.
Solution 3 - Python
If your lists are a and b, you can do:
map(int.__sub__, a, b)
But you probably shouldn't. No one will know what it means.
Solution 4 - Python
I'd have to recommend NumPy as well
Not only is it faster for doing vector math, but it also has a ton of convenience functions.
If you want something even faster for 1d vectors, try vop
It's similar to MatLab, but free and stuff. Here's an example of what you'd do
from numpy import matrix
a = matrix((2,2,2))
b = matrix((1,1,1))
ret = a - b
print ret
>> [[1 1 1]]
Boom.
Solution 5 - Python
import numpy as np
a = [2,2,2]
b = [1,1,1]
np.subtract(a,b)
Solution 6 - Python
If you have two lists called 'a' and 'b', you can do: [m - n for m,n in zip(a,b)]
Solution 7 - Python
A slightly different Vector class.
class Vector( object ):
def __init__(self, *data):
self.data = data
def __repr__(self):
return repr(self.data)
def __add__(self, other):
return tuple( (a+b for a,b in zip(self.data, other.data) ) )
def __sub__(self, other):
return tuple( (a-b for a,b in zip(self.data, other.data) ) )
Vector(1, 2, 3) - Vector(1, 1, 1)
Solution 8 - Python
If you plan on performing more than simple one liners, it would be better to implement your own class and override the appropriate operators as they apply to your case.
Taken from Mathematics in Python:
class Vector:
def __init__(self, data):
self.data = data
def __repr__(self):
return repr(self.data)
def __add__(self, other):
data = []
for j in range(len(self.data)):
data.append(self.data[j] + other.data[j])
return Vector(data)
x = Vector([1, 2, 3])
print x + x
Solution 9 - Python
For the one who used to code on Pycharm, it also revives others as well.
import operator
Arr1=[1,2,3,45]
Arr2=[3,4,56,78]
print(list(map(operator.sub,Arr1,Arr2)))
Solution 10 - Python
The combination of map
and lambda
functions in Python is a good solution for this kind of problem:
a = [2,2,2]
b = [1,1,1]
map(lambda x,y: x-y, a,b)
zip
function is another good choice, as demonstrated by @UncleZeiv
Solution 11 - Python
Many solutions have been suggested.
If speed is of interest, here is a review of the different solutions with respect to speed (from fastest to slowest)
import timeit
import operator
a = [2,2,2]
b = [1,1,1] # we want to obtain c = [2,2,2] - [1,1,1] = [1,1,1
%timeit map(operator.sub, a, b)
176 ns ± 7.18 ns per loop (mean ± std. dev. of 7 runs, 1000000 loops each)
%timeit map(int.__sub__, a, b)
179 ns ± 4.95 ns per loop (mean ± std. dev. of 7 runs, 1000000 loops each)
%timeit map(lambda x,y: x-y, a,b)
189 ns ± 8.1 ns per loop (mean ± std. dev. of 7 runs, 1000000 loops each)
%timeit [a_i - b_i for a_i, b_i in zip(a, b)]
421 ns ± 18.4 ns per loop (mean ± std. dev. of 7 runs, 1000000 loops each)
%timeit [x - b[i] for i, x in enumerate(a)]
452 ns ± 17.2 ns per loop (mean ± std. dev. of 7 runs, 1000000 loops each
%timeit [a[i] - b[i] for i in range(len(a))]
530 ns ± 16.7 ns per loop (mean ± std. dev. of 7 runs, 1000000 loops each)
%timeit list(map(lambda x, y: x - y, a, b))
546 ns ± 16.1 ns per loop (mean ± std. dev. of 7 runs, 1000000 loops each)
%timeit np.subtract(a,b)
2.68 µs ± 80.9 ns per loop (mean ± std. dev. of 7 runs, 100000 loops each)
%timeit list(np.array(a) - np.array(b))
2.82 µs ± 113 ns per loop (mean ± std. dev. of 7 runs, 100000 loops each)
%timeit np.matrix(a) - np.matrix(b)
12.3 µs ± 437 ns per loop (mean ± std. dev. of 7 runs, 100000 loops each)
Using map
is clearly the fastest.
Surprisingly, numpy
is the slowest. It turns out that the cost of first converting the lists a
and b
to a numpy
array is a bottleneck that outweighs any efficiency gains from vectorization.
%timeit a = np.array([2,2,2]); b=np.array([1,1,1])
1.55 µs ± 54.9 ns per loop (mean ± std. dev. of 7 runs, 1000000 loops each)
a = np.array([2,2,2])
b = np.array([1,1,1])
%timeit a - b
417 ns ± 12.8 ns per loop (mean ± std. dev. of 7 runs, 1000000 loops each)
Solution 12 - Python
This answer shows how to write "normal/easily understandable" pythonic code.
I suggest not using zip
as not really everyone knows about it.
The solutions use list comprehensions and common built-in functions.
Alternative 1 (Recommended):
a = [2, 2, 2]
b = [1, 1, 1]
result = [a[i] - b[i] for i in range(len(a))]
Recommended as it only uses the most basic functions in Python
Alternative 2:
a = [2, 2, 2]
b = [1, 1, 1]
result = [x - b[i] for i, x in enumerate(a)]
BioCoder):
Alternative 3 (as mentioned bya = [2, 2, 2]
b = [1, 1, 1]
result = list(map(lambda x, y: x - y, a, b))
Solution 13 - Python
arr1=[1,2,3]
arr2=[2,1,3]
ls=[arr2-arr1 for arr1,arr2 in zip(arr1,arr2)]
print(ls)
>>[1,-1,0]
Solution 14 - Python
Very easy
list1=[1,2,3,4,5]
list2=[1,2]
list3=[]
# print(list1-list2)
for element in list1:
if element not in list2:
list3.append(element)
print(list3)
Solution 15 - Python
use a for loop
a = [3,5,6]
b = [3,7,2]
c = []
for i in range(len(a)):
c.append(a[i] - b[i])
print(c)
output [0, -2, 4]
Solution 16 - Python
Try this:
list(array([1,2,3])-1)