r - How to apply a function over two series of sequentially labelled variables without using column numbers? -

i need apply function using 2 sets of sequentially labelled variables , attach new set of variables data frame. need without referring column numbers in code.

more specifically, here simple task trying do:

dat <- data.frame(sec1 = sample(c(0:3),10,replace=t) , sec2 = sample(c(0:4),replace=t) , sec3 = sample(c(0:4),replace=t),pri1 = sample(c(0:3),10,replace=t) , pri2 = sample(c(0:4),replace=t) , pri3 = sample(c(0:4),replace=t) ) dat$rel1 <- ifelse(dat$pri1>0,dat$sec1/dat$pri1,na) dat 

i want repeat "ifelse" function shown above without typing repeatedly each set of variables.

i must say, asked similar questions , received helpful answers (eg1 , eg2) in case responses either used column number in code, or example on single set of sequentially labelled variable. not manage revise suggested code solve particular problem.

any suggestion appreciated.

dat_n <- cbind(dat, mapply(function(x, y) ifelse(y>0,x/y,na) ,dat[grepl("sec",names(dat))], dat[grepl("pri",names(dat))])) > dat_n    sec1 sec2 sec3 pri1 pri2 pri3      rel1      sec1      sec2 sec3 1     2    1    2    3    3    0 0.6666667 0.6666667 0.3333333   na 2     3    3    4    0    2    4        na        na 1.5000000 1.00 3     1    0    3    1    4    4 1.0000000 1.0000000 0.0000000 0.75 4     2    4    1    3    3    2 0.6666667 0.6666667 1.3333333 0.50 5     2    0    2    3    4    1 0.6666667 0.6666667 0.0000000 2.00 6     1    1    2    1    3    0 1.0000000 1.0000000 0.3333333   na 7     1    3    4    0    2    4        na        na 1.5000000 1.00 8     1    0    3    1    4    4 1.0000000 1.0000000 0.0000000 0.75 9     3    4    1    2    3    2 1.5000000 1.5000000 1.3333333 0.50 10    1    0    2    2    4    1 0.5000000 0.5000000 0.0000000 2.00