Operation on numpy arrays contain rows with different size

Question

I have two lists, looking like this:

a= [[1,2,3,4], [2,3,4,5],[3,4,5,6,7]], b= [[5,6,7,8], [9,1,2,3], [4,5,6,7,8]]

which I want to subtract from each other element by element for an Output like this:

a-b= [[-4,-4,-4,-4],[7,2,2,2],[-1,-1,-1,-1,-1]]

In order to do so I convert each of a and b to arrays and subtract them I use:

np.array(a)-np.array(b)

The Output just gives me the error:

Unsupported Operand type for-: 'list' and 'list'

What am I doing wrong? Shouldn't the np.array command ensure the conversion to the array?

Answer 1

Here is a Numpythonic way:

>>> y = map(len, a)  
>>> a = np.hstack(np.array(a))
>>> b = np.hstack(np.array(b))
>>> np.split(a-b, np.cumsum(y))
[array([-4, -4, -4, -4]), array([-7,  2,  2,  2]), array([-1, -1, -1, -1, -1]), array([], dtype=float64)]
>>> 

Since you cannot subtract the arrays with different shapes, you can flatten your arrays using np.hstack() then subtract your flattened arrays then reshape based on the previous shape.

Answer 2

You can try:

>>> a= [[1,2,3,4], [2,3,4,5],[3,4,5,6,7]]
>>> b= [[5,6,7,8], [9,1,2,3], [4,5,6,7,8]]
>>> 
>>> c =[]
>>> for i in range(len(a)):
    c.append([A - B for A, B in zip(a[i], b[i])])


>>> print c
[[-4, -4, -4, -4], [-7, 2, 2, 2], [-1, -1, -1, -1, -1]]

Or 2nd method is using map:

from operator import sub
a= [[1,2,3,4], [2,3,4,5],[3,4,5,6,7]]
b= [[5,6,7,8], [9,1,2,3], [4,5,6,7,8]]
c =[]
for i in range(len(a)):
    c.append(map(sub, a[i], b[i]))  
print c
[[-4, -4, -4, -4], [-7, 2, 2, 2], [-1, -1, -1, -1, -1]]

Answer 3

The dimensions of your two arrays don't match, i.e. the first two sublists of a have 4 elements, but the third has 5 and ditto with b. If you convert the lists to numpy arrays, numpy silently gives you something like this:

In [346]: aa = np.array(a)
In [347]: bb = np.array(b)
In [348]: aa
Out[348]: array([[1, 2, 3, 4], [2, 3, 4, 5], [3, 4, 5, 6, 7]], dtype=object)
In [349]: bb
Out[349]: array([[5, 6, 7, 8], [9, 1, 2, 3], [4, 5, 6, 7, 8]], dtype=object)

You need to make sure that all your sublists have the same number of elements, then your code will work:

In [350]: a = [[1,2,3,4], [2,3,4,5],[3,4,5,6]]; b = [[5,6,7,8], [9,1,2,3], [4,5,6,7]] # I removed the last element of third sublist in a and b
In [351]: np.array(a) - np.array(b)
Out[351]: 
array([[-4, -4, -4, -4],
       [-7,  2,  2,  2],
       [-1, -1, -1, -1]])