I want to define an array based on condition from two other arrays in python

Question

Say I have 2 arrays

X = np.array([1.,2.,3.,4.,5.,])
Y = np.array([6.,7.,8.,9.,10.,])

and I want to define an array that takes a value of say 1 wherever X < 3 or Y = 9 and takes a value of 0 everywhere else. I used

Z=[1 if i < 3 or j==9 else 0 for i in X and j in Y]
print(Z)

I expect an array that looks like

[1,1,0,1,0]

but I got this error:

ValueError: The truth value of an array with more than one element is 
ambiguous. Use a.any() or a.all()

Jans · Accepted Answer · 2018-11-06 02:18:23Z

2

Try this:

np.where((X < 3) | (Y == 9) , 1, 0)

answered Nov 6, 2018 at 2:18

Jans

11.3k4 gold badges39 silver badges45 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

Oh maaaan, i did where but i just missed the parenthesis of each condition for the or statement, gave me an error, so i use logical_or!!!

Yeah, the need of parenthesis is a result of operator precedence.

blhsing · Accepted Answer · 2018-11-06 02:16:30Z

1

You can use zip to iterate over X and Y in pairs:

Z=[1 if i < 3 or j==9 else 0 for i, j in zip(X, Y)]

Z becomes:

[1, 1, 0, 1, 0]

answered Nov 6, 2018 at 2:16

blhsing

109k9 gold badges88 silver badges132 bronze badges

U13-Forward · Accepted Answer · 2018-11-06 02:18:41Z

1

Use np.logical_or, much more numpy-ish, much faster:

print(np.logical_or(X<3,Y==9).astype(int).tolist())

Output:

[1, 1, 0, 1, 0]

I recommend this because it's fast.

answered Nov 6, 2018 at 2:18

U13-Forward

71.8k15 gold badges100 silver badges125 bronze badges