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
using sapply to apply function to some columns of a dataframe
2 messages · mwestphal at worldbank.org, Duncan Murdoch
On 17/02/2009 4:42 PM, mwestphal at 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