r - How to use Apply function in a column dependent case? -

i trying fill new column in df of 700k records , goes slow loop , therefore want use apply function. not familiar , below attempt doesn't work. please help

myfunc <- function(a,b,c,d) {if (a=="xyz" & b==11) {c=d}} dataf[,'target'] <- apply(dataf, 1, function(dataf) myfunc(dataf[,'col1'],dataf[,'col2'],dataf[,'target'],dataf[,'col3'])) 

adding more description -

what have:

a   b   c   d x   2       p x   2       p x   2       p xyz 11      p xyz 11      p xyz 2       p y   2       p y   2       p y   2       p 

what want achieve:

a   b   c   d x   2       p x   2       p x   2       p xyz 11  p   p xyz 11  p   p xyz 2       p y   2       p y   2       p y   2       p 

given op, guessing want this??

library(data.table) setdt(dataf)[a == "xyz" & b == 11, c := d] 

output:

      b d  c 1:   x  2 p na 2:   x  2 p na 3:   x  2 p na 4: xyz 11 p  p 5: xyz 11 p  p 6: xyz  2 p na 7:   y  2 p na 8:   y  2 p na 9:   y  2 p na 

i highly suggest reading tutorial of data.table super-fast , can used lot of different things. on site find more articles. read them all, need of , lot!!