Greater of two numpy arrays, element-wise

Question

I have two numpy arrays and I'm trying to find the greater of them (element wise, i.e. all elements should be greater)

import numpy as np

a = np.array([4,5,6])
b = np.array([7,8,9])

if b > a:
    print 'True'

But I'm not getting the desired output and getting an error

possible duplicate of [Comparing two numpy arrays for equality, element-wise](http://stackoverflow.com/questions/10580676/comparing-two-numpy-arrays-for-equality-element-wise) — smci, Jan 31 '15 at 22:40

score 2 · Answer 1 · answered Jan 31 '15 at 21:22

2

Use np.all()

In [1]: import numpy as np

In [2]: a = np.array([4,5,6])

In [3]: b = np.array([7,8,9])

In [4]: np.all(b > a)
Out[4]: True

answered Jan 31 '15 at 21:22

Akavall

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Jeff · Accepted Answer · 2015-01-31T22:35:28.833

if all(b>a):
   print 'True'

For multi-dimensional arrays, use:

if np.all(b>a):
   print 'True'

However all() is faster for single dimension arrays and may be useful if your arrays are very large:

>>> timeit('a = np.array([4,5,6]); b = np.array([7,8,9]); all(a>b)',number=100000,setup='import numpy as np')
0.34104180335998535
>>> timeit('a = np.array([4,5,6]); b = np.array([7,8,9]); np.all(a>b)',number=100000,setup='import numpy as np')
0.9201719760894775

Alex Riley · Answer 3 · 2015-02-01T11:03:12.903

b > a produces an array containing True/False values.

However, Python can't determine whether NumPy arrays with more than one element should be True or False. How should an array such as array([True, False, True]) be evaluated? A ValueError is raised because of the potential ambiguity.

Instead, you need to check whether all of the values in b > a are True. Use NumPy's all() to do this:

if (b > a).all():
    print 'True'

Greater of two numpy arrays, element-wise

3 Answers3