[R] using sapply to apply function to some columns of a dataframe
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 1818 H St NW, Washington DC 20433, USA __ R-help@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code.
Re: [R] using sapply to apply function to some columns of a dataframe
On 17/02/2009 4:42 PM, mwestp...@worldbank.org wrote: 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. I think the easiest way to do this is to convert the dataframe into an array with 3 indices, and sum over one of them. For example: rows - 20 cols - 120 df - matrix(1:(rows*cols), rows, cols) # in your case, df - as.matrix( df ) arr - array( df, c(rows, 10, cols/10)) sums - apply( arr, c(1,3), sum) Duncan Murdoch __ R-help@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code.
