How does this max() expression in Python work?

Question

Here's the code:

a = [1,2,3,4]
b = {}
b[1] = 10
b[2] = 8
b[3] = 7
b[4] = 5
print max(a,key=lambda w: b[w])

This prints out 1.

I don't understand how max(a,key=lambda w: b[w]) is being evaluated here though; I'm guessing for each value i in a, it finds the corresponding value b[i] by

saving the current value of i as w in the lambda function
getting the corresponding value from b[i] and storing it in key.

But then why does it print out 1 instead of 11? Or why doesn't it print out 10, since that's really the maximum number?

Almost afraid to ask, but why did you imagine it might produce 11? 1 + 10? — John Machin
– John Machin, Commented Dec 16, 2009 at 20:45

unutbu · Accepted Answer · 2009-12-16 03:07:03Z

9

max(a,...) is always going to return an element of a. So the result will be either 1,2,3, or 4. For each value w in a, the key value is b[w]. The largest key value is 10, and that corresponds with w equalling 1. So max(a,key=lambda w: b[w]) returns 1.

answered Dec 16, 2009 at 3:07

unutbu

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Comments

enrnpfnfp · Accepted Answer · 2009-12-16 12:49:34Z

-3

Try:

a = [1,2,3,4]
b = {}
b[1] = 10
b[2] = 8
b[3] = 7
b[4] = 5
c = a + b.values()
print max(*c)

answered Dec 16, 2009 at 12:49

enrnpfnfp

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4 Comments

enrnpfnfp Over a year ago

why is bad? you can also do the same without concatenate the values if a and b are huge.

SilentGhost Over a year ago

because it's so far from what OP is trying to do, that it's just frustrating

Vlad the Impala Over a year ago

why the max(*c)? Are you trying to dereference c?

John Machin Over a year ago

@Goose Bumper: if c refers to [1, 42, 666] then max(*c) is equivalent to max(1, 42, 666) (see http://docs.python.org/reference/expressions.html#calls). My downvote is because that is obscure and inefficient as well; max(c) gives the same result (see http://docs.python.org/library/functions.html#max).

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