Sum() in python

Question

I have been trying to be comfortable with the sum() in python, I did understand the basic function of sum but as a mathematica backgroud,I was just inquistive to know can we use sum in python in the same way like we do in mathematica for example consider this mathematica module:

Sq[a_, b_] := Module[{m, n}, m = Max[a, b]; n = Min[a, b];Sum[(m - r + 1) (n - r + 1), {r, 1, n}]]

Now,could it be possible to write the sum part like that? I mean:

Sum[(m - r + 1) (n - r + 1), {r, 1, n}]

Trying to covert this in python,I think of something like this:

sum((m - r + 1) (n - r + 1) in xrange(1,n+1))

but doesn't seems to be working! so my question how to get it work?

Sebastian Paaske Tørholm · Accepted Answer · 2011-05-15 10:26:51Z

13

sum((m - r + 1) * (n - r + 1) for r in xrange(1,n+1))

There's no implicit multiplication between integers, so you need the *.
f(x) for x in xes is the general format of a list comprehension, where you want x to iterate through every element of xes, and give back the value f(x).

answered May 15, 2011 at 10:26

Sebastian Paaske Tørholm

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2 Comments

Quixotic Over a year ago

I just happen to figure it out that sum((m - r + 1) * (n - r + 1) for r in range(1,n+1)) too works.

ncoghlan Over a year ago

The difference between the two is that in Python 2.x, range returns an actual list, which may be quite wasteful of memory if the list is large. xrange is an iterable which generates the numbers in order rather than actually returning a real list. (In Python 3, xrange goes away and range becomes a memory efficient iterable)

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