CODEWITHSUNDEEP
pythonarraysnumpy
Leigh
Leigh

Reputation: 49

Adding values to a new array

I have an existing python array instantiated with zeros. How do I iterate through and change the values?

I can't iterate through and change elements of a Python array?

num_list = [1,2,3,3,4,5,]
mu = np.mean(num_list)
sigma = np.std(num_list)
std_array = np.zeros(len(num_list))

for i in std_array:
        temp_num = ((i-mu)/sigma)
        std_array[i]=temp_num

This the error: only integers, slices (:), ellipsis (...), numpy.newaxis (None) and integer or boolean arrays are valid indices

Upvotes: 0

Views: 72

Answers (2)

jyalim
jyalim

Reputation: 3429

In your code you are iterating over the elements of the numpy.array std_array, but then using these elements as indices to dereference std_array. An easy solution would be the following.

num_arr = np.array(num_list)
for i,element in enumerate(num_arr):
    temp_num = (element-mu)/sigma
    std_array[i]=temp_num

where I am assuming you wanted to use the value of the num_list in the first line of the loop when computing temp_num. Notice that I created a new numpy.array called num_arr though. This is because rather than looping, we can use alternative solution that takes advantage of broadcasting:

std_array = (num_arr-mu)/sigma

This is equivalent to the loop, but faster to execute and simpler.

Upvotes: 4

Chris
Chris

Reputation: 29742

You i is an element from std_array, which is float. Numpy is therefore complaining that you are trying slicing with float where:

only integers, slices (:), ellipsis (...), numpy.newaxis (None) and integer or boolean arrays are valid indices

If you don't have to use for, then numpy can broadcast the calculations for you:

(std_array - mu)/sigma
# array([-2.32379001, -2.32379001, -2.32379001, -2.32379001, -2.32379001,
   -2.32379001])

Upvotes: 0

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