[R] using sapply to apply function to some columns of a dataframe

[mwestphal at worldbank.org]

Hello:

I would like to sum every x columns of a dataframe for each row.  For instance,
if x is 10, then for dataframe df, this function will sum the first ten elements
together and then the next ten:

sapply(list(colnames(df)[1:10], colnames(df)[11:20]),function(x)apply( df[,x],
1, sum))

If the number of columns is quite large (1000's), then manually entering the
list above is not practical.  Any suggestions?

I would also like to do a variant of the above, where I sum every nth element.


Thanks,

Michael

------------------------------------------------------------------------------
Michael I. Westphal, PhD
Africa Region Water Resources (AFTWR)
South Asia Region Sustainable Development (SASSD)
World Development Report 2010: "Development in a Changing Climate"
www.worldbank.org/wdr2010
Room J6-007 (mail stop: J6-603)
Tel: 202.473.1217
The World Bank
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