> ...if I had the closing prices of the
> S&P from 01/01/1990-12/31/1990, how could I get the average price of
> the S&P from 02/01/1990-03/15/1990? Or the average price of the S&P on
> Mondays (assuming a dummy var is created for 1 = Monday, 0 = else).
tapply has already been referred to. You m
I think Simon has provided a good answer to the actual question but as a
refugee from SAS I'd suggest having a look at
www.et.bs.ehu.es/~etptupaf/pub/R/RforSAS&SPSSusers.pdf or getting the book
Muenchen, R. A. (2008). R for SAS and SPSS Users (1st ed.). Springer.
R ans SAS approach things very
You can use the tapply function to do this. You can't type a line into
the mean statement. (See ?mean for what you can type in there). The
general approach is to have a vector of data (stock prices) and a
categorical variable (day of week). Then break up the data vector
according to the levels
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