add rank and rank correlation methods

[CRP]

I do not know what scope you have in mind for DataFrame methods, but rank and spearman correlation would sure be handy ;)

[wesm]

Do you mean rank in the sense of matrix rank? You make a good point so I just added kendall/spearman correlation to the corr methods, check it out

[CRP]

no, I do not mean matrix rank, but rank such as that provided in matlab by tiedrank. Actually, that function is a bit limited as there are many definitions of rank: http://en.wikipedia.org/wiki/Ranking

Generally, it would be nice to have some basic data transformation features built into pandas, such as rank, percentile rank, groupings (where each observation is assigned to a class, eg deciles) etc. I might try to implement some of these, though I am not sure my coding abilities are up to pandas' standards ;)

Another thing that might be useful is a isnan method that handles different datatypes transparently. Not sure what nan would be for non-float objects

[wesm]

There is already isnull and notnull which is an "isnan" for all data types

[wesm]

Well, your wish may have come true-- I implemented very fast rank functinos on Series and DataFrame (with proper tie breaking etc... a la scipy.stats.rankdata, except about 20x faster :) ). Will be useful to have