r - Subtract values of a single row from all relevant columns in a data frame -

i have following data set:

foo=data.frame(index=rep(1:10,3),                type=rep(c("a","b","c"),each=10),                ping=rnorm(30),                pong=runif(30)) 

i want subtract values of columns ping , pong index==5 , type=="b", whole columns ping , pong. works:

vec=matrix(subset(foo,index==5 & type=="b",select=ping:pong),2,1) foo[,c("ping","pong")]=foo[,c("ping","pong")]-vec 

however, i'm surprised had specify vec column vector, instead row vector. have thought need subtract same row vector (similar subsets of the) rows of foo. can explain why is? also, if same result can obtained simpler or cleaner code, please let me know.

you want this:

myselect <- with(foo, index ==5 & type == "b") mycol <- c('ping','pong')  foo[, mycol] <- foo[, mycol] - as.list(foo[myselect, mycol]) 

vec should list, substraction of lists done element element. want, , you're doing actually:

first of all, don't specify vec matrix. if use matrix() instead of as.matrix() on list, list. , data frame list, matrix() gives list attribute "dim". attribute makes matrix, but:

> str(vec) list of 2  $ : num 0.704  $ : num 0.164  - attr(*, "dim")= int [1:2] 2 1 

what use here, side effect of function matrix(). drops other attributes, removes data.frame information of vec , makes list. if vec still data frame, wouldn't work. can use mathematical operator when both data frames have same size. , not case here.

> vec=subset(foo,index==5 & type=="b",select=ping:pong) > foo[,c("ping","pong")]-vec error in ops.data.frame(foo[, c("ping", "pong")], vec) :    ‘-’ defined equally-sized data frames 

you shouldn't make matrix. if do, r recycle matrix , dataframe column-wise. means substracts first value of vec first of foo$ping, second value of vec second of foo$ping, first value of vec again third value of foo$ping , forth. doesn't matter in direction put matrix, it's same (wrong!) result:

mytest<- matrix(c(-10,10), nrow = 1) mytest2 <- t(mytest) myfoo <- foo[,c('ping','pong')] all.equal(myfoo - mytest, myfoo - mytest2)