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So I have a 2D list and want to sort it using a second file of keys. Does anyone know how I would go about doing that?

Heres an example input:

And here is an example input file:

first_nm,last_nm,gender,cwid,cred_hrs,qual_pts,gpa
John,Roe,M,44444444,40,150,3.75
Jane,Roe,F,66666666,100,260,2.6
John,Doe,M,22222222,50,140,2.8
Jane,Doe,F,88888888,80,280,3.5
Penny,Lowe,F,55555555,40,140,3.5
Lenny,Lowe,M,11111111,100,280,2.8
Denny,Lowe,M,99999999,80,260,3.25
Benny,Lowe,M,77777777,120,90,0.75
Jenny,Lowe,F,33333333,50,90,1.8
Zoe,Coe,F,0,50,130,2.6

Here are the keys to sort it(there could be more or less, depending on how you want to sort it)

gender,ascend,string
gpa,descend,float
last_nm,ascend,string

And here would be the output for that input and keys:

first_nm,last_nm,gender,cwid,cred_hrs,qual_pts,gpa
Jane,Doe,F,88888888,80,280,3.5
Penny,Lowe,F,55555555,40,140,3.5
Zoe,Coe,F,00000000,50,130,2.6
Jane,Roe,F,66666666,100,260,2.6
Jenny,Lowe,F,33333333,50,90,1.8
John,Roe,M,44444444,40,150,3.75
Denny,Lowe,M,99999999,80,260,3.25
John,Doe,M,22222222,50,140,2.8
Lenny,Lowe,M,11111111,100,280,2.8
Benny,Lowe,M,77777777,120,90,0.75

I was thinking of just using the built in sort() but was not sure if I would be able to use it if I am sorting 3 different times. I think I would have to sort backwards? (last_nm, then gpa, then gender)

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2 Answers 2

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You can return a tuple from your key function to create complex sorts. And as a quick trick, multiply numeric values by -1 for a reverse sort. Your example would look something like this:

lists.sort(key = lambda x: (x[2], x[6] * -1, x[1]))
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@ncerice: lambda x: (x[2], x[6] * -1, x[1]) is a function that gets passed a row of your 2D list and returns a tuple consisting of element 2 of the row, element 6 multiplied by -1, and element 1. sort() uses this tuple as the key to sort on, rather than the row's value. The key for each row is only calculated once, even though the keys need to be examined multiple times to perform the sort, so this key transformation operation is quite efficient.
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The list sort() method takes a boolean parameter reverse, but it applies to the whole key; you can't say that you want some parts of the key to use ascending sort and others to use descending. Sadly, there isn't a simple way to extend g.d.d.c's trick of multiplying by -1 to non-numeric data.

So if you need to handle arbitrary combinations of ascending and descending then yes, you will have to sort multiple times, working backwards over your list of keys, like you mention in your question. The built-in Python sorting algorithm, timsort, is a stable sort, which means each time you sort your 2D list with a different key the previous sort results won't get scrambled.

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