CODEWITHSUNDEEP
pythonarraysnumpysorting
Rational-IM
Rational-IM

Reputation: 2682

Why is the change of values in one array is affecting the value on another array (with exact same values) that isn't involved in the operation?

When I run the command "negative_only[negative_only>0]=0" (which should make positive values = 0 on the array "negative_only") the values on a similar array ("positive_only") are also changed. Why is this happening? I'm using Python 3.7 (Windows 10 / Spyder IDE).

The code where the two arrays are being manipulated is below. The "long_dollars_ch" is an array of ~2700 x 60 with some positive values, some negative values and a lot of zeros. This code is part of a loop that is cycling through each row of the array "long_dollars_ch".

# calculations to isolate top contributors to NAV change for audits
top_check = 3 # number of top values changes to track

# calculate dollar change (for longs), and create array with most positive/negative values
long_dollars_ch[c_day,:] = long_shares[c_day,:]*hist_prices_zeros[c_day,:]-long_shares[c_day,:]*hist_prices_zeros[c_day-1,:]
positive_only = long_dollars_ch[c_day,:]
positive_only[positive_only<0]=0 #makes non-positive values zero
idx = np.argsort(positive_only) #creat index representing sorted values for only_positive for c_day
non_top_vals = idx[:-top_check]
negative_only = long_dollars_ch[c_day,:]
negative_only[negative_only>0]=0 #makes non-negative values zero
idx = np.argsort(negative_only) #creat index representing sorted values for only_negative for c_day
non_bottom_vals = idx[:-top_check]

# create array that shows the most positive/negative dollar change for "top-check" securities
long_dollars_ch_pos[c_day,:] = positive_only
long_dollars_ch_pos[c_day,:][non_top_vals] *= 0
long_dollars_ch_neg[c_day,:] = negative_only
long_dollars_ch_neg[c_day,:][non_bottom_vals] *= 0

The objective of this code is to create two arrays. One that has only the top "top_check" positive values (if any) and another that has the bottom "top_check" negative values (if any) for each row of the original array "long_dollars_ch". However, it appears that Python is considering the "positive_only" and "negative_only" the same "variable". And therefore, the operation with one of them affects the values inside the other (that was not part of the operation).

Upvotes: 0

Views: 90

Answers (1)

Mateusz Boruch
Mateusz Boruch

Reputation: 318

its quite simple.

In numpy np.array x = np.array y you do not copy array :)

You make a reference to array x.

In other words, you do not have two array after using "=". You still have one array x, and a reference to that array (y is that reference).

positive_only = long_dollars_ch[c_day,:]
.
.
,
negative_only = long_dollars_ch[c_day,:]

do not make a copy of long_dollars_ch, but only makes references to it. you need to use copy method, or other method (numpy provides few of them) to make it work. Here is a documentation

EDIT: I posted wrong link, now it is ok.

Upvotes: 1

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