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Python

Posted by Timothy Tseng
Timothy Tseng
3,292 Points

Can some explain how this expression is evaluated in python?

Can someone explain what's going on "behind the scenes" in the below expression? 

arr[i], mx = mx, max(arr[i], mx)

Take for example :

arr[i] = 5
mx = -1

So in the "first" part of the expression arr[i] gets set to -1, but what I'm having trouble grasping is why arr[i] remains 5 when evaluating the second part of the expression.

1 Answer

Chris Freeman
MOD
Chris Freeman
Treehouse Moderator 68,423 Points
Chris Freeman
Chris Freeman
Treehouse Moderator 68,423 Points 
# Given
arr[i] = 5
mx = -1
arr[i], mx = mx, max(arr[i], mx)
# becomes
arr[i], mx = -1, max(5, -1)
# becomes
arr[i], mx = -1, 5
# tuple on right is assigned to tuple on left
# arr[i] becomes -1
# mx becomes 5

>>> arr = [0, 5, 10, 15]
>>> i = 1
>>> arr[i]
5
>>> mx = -1
>>> arr[i], mx = mx, max(arr[i], mx)
>>> arr
[0, -1, 10, 15]
>>> mx
5

Post back if you need more help. Good luck!!

Timothy Tseng
Timothy Tseng
3,292 Points

That explains everything. Thank you so much!!