Python, getting max value of an array with a simple loop

Question

I am writing a simple loop to get the max value inside an array.

I know there is the function arr.max() but I'm just curious why the following code does not work.

import numpy as np
arr1 = np.arange(0,9).reshape(3,3)

max_value = arr1[0]
for i in arr1:
    if max_value > i:
        max_value = i
print(max_value)

which gives me:

---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last) <ipython-input-80-9fb1fcfa0948> in <module>
       1 max_value = arr1[0]
       2 for i in arr1:
 ----> 3     if i > max_value:
       4         max_value = i

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

I think you also have a typo in your question. "max_value > i" should be "i > max_value" on line 6. — Oskar Skog
– Oskar Skog, Commented Mar 5, 2020 at 10:44

Stephen Kitt · Accepted Answer · 2020-03-05 10:41:41Z

4

.reshape(3,3) reshapes your array, so it ends up being a two-dimensional 3×3 array:

>>> print(arr1)
[[0 1 2]
 [3 4 5]
 [6 7 8]]

As a result, arr1[0] isn’t a scalar, it’s itself a three-element array. You’d have to iterate over that again to find the maximum value.

answered Mar 5, 2020 at 10:41

Stephen Kitt

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