Inefficient summing

beginner zyzhu2000 at gmail.com
Wed Oct 8 22:23:08 CEST 2008


Hi All,

I have a list of records like below:

rec=[{"F1":1, "F2":2}, {"F1":3, "F2":4} ]

Now I want to write code to find out the ratio of the sums of the two
fields.

One thing I can do is:

sum(r["F1"] for r in rec)/sum(r["F2"] for r in rec)

But this is slow because I have to iterate through the list twice.
Also, in the case where rec is an iterator, it does not work.

I can also do this:

sum1, sum2= reduce(lambda x, y: (x[0]+y[0], x[1]+y[1]), ((r["F1"],
r["F2"]) for r in rec))
sum1/sum2

This loops through the list only once, and is probably more efficient,
but it is less readable.

I can of course use an old-fashioned loop. This is more readable, but
also more verbose.

What is the best way, I wonder?


-a new python programmer



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