Reputation: 4577
I am trying to understand what happens in the following python code:
import numpy as np
numberList1 = [1,2,3]
numberList2 = [[4,5,6],[7,8,9]]
result = np.dot(numberList2, numberList1)
# Converting iterator to set
resultSet = set(result)
print(resultSet)
Output:
{32, 50}
I can see that it is multiplying each element in numberList1
by the element in the same position in each array within numberList2
- so {1*4 + 2*5 + 3*6 = 32},{1*7+2*8+3*9 = 50}
.
But, if I change the arrays to:
numberList1 = [1,1,1]
numberList2 = [[2,2,2],[3,3,3]]
Then the output I see is
{9, 6}
Which is the wrong way around...
and, if I change it to:
numberList1 = [1,1,1]
numberList2 = [[2,2,2],[2,2,2]]
Then the output I see is just
{6}
From the documentation:
If a is an N-D array and b is a 1-D array, it is a sum product over the last axis of a and b.
I am not enough of a mathematician to understand quite what this is telling me; or why the order of the outputs swaps around sometimes.
Upvotes: 3
Views: 923
Reputation: 46921
a set
is an unordered data type - and it will remove your duplicates. np.dot
does not return an iterator (as mentioned in your code) but an np.ndarray
which will be in the order you expect:
import numpy as np
numberList1 = [1, 2, 3]
numberList2 = [[4, 5, 6], [7, 8, 9]]
result = np.dot(numberList2, numberList1)
# [32 50]
# <class 'numpy.ndarray'>
# numberList1 = [1, 1, 1]
# numberList2 = [[2, 2, 2], [3, 3, 3]]
# -> [6 9]
Upvotes: 1