Array handling - Python

Question

I am a beginner in Python. I try to store the max value of two array in an another array. The length of array is known so I used c=[]*len(a)

 a=[3,4,6,8]
 b=[9,4,5,10]
 c=[]*len(a)
 for i in range(len(a)):
    if (a[i]>b[i]):
      c.append(a[i])
    else:
      c.append(b[i])

I got the following output, which is correct.

 c=[9,4,6,10]

If I have arrays like

 a=[[2,4],[6,8]]
 b=[[1,7],[5,9]]

How should I proceed this to store the max value of each elements in an another array? Thank for your help in advance.

Answer 1

You can use zip() to zip together each list and each sublist to compare them element-wise:

Make an iterator that aggregates elements from each of the iterables.

Returns an iterator of tuples, where the i-th tuple contains the i-th element from each of the argument sequences or iterables. [...].

>>> def max_value(lst1, lst2):
        for subl1, subl2 in zip(lst1, lst2):
            for el1, el2 in zip(subl1, subl2):
                yield max(el1, el2)

            
>>> 
>>> a=[[2,4],[6,8]]
>>> b=[[1,7],[5,9]]
>>> 
>>> list(max_value(a, b))
[2, 7, 6, 9]

If using NumPy, you can use numpy.maximum():

Element-wise maximum of array elements.

Compare two arrays and returns a new array containing the element-wise maxima. [...].

>>> import numpy as np
>>> 
>>> a = np.array([[2,4],[6,8]])
>>> b = np.array([[1,7],[5,9]])
>>> 
>>> np.maximum(a, b)
array([[2, 7],
       [6, 9]])
>>> 

Answer 2

In single line solution you can use map and lambda. For Example in this case the solution can be

a=[[2,4],[6,8]]
b=[[1,7],[5,9]]
map(lambda x,y : map(lambda p,q : max(p,q),x,y),a,b)
[[2, 7], [6, 9]]

Since a and b both are array of arrays the first lambda input are arrays and then next map takes the maximum of the individual element.