CODEWITHSUNDEEP
r
uday
uday

Reputation: 6713

how can I use apply on two variables simultaneously

suppose I have 

# test case
a<-matrix(rep(c(1,2,3,4,5,6),3),ncol=3)
b<-t(matrix(c(1,3,4)))

a

     [,1] [,2] [,3]
[1,]    1    1    1
[2,]    2    2    2
[3,]    3    3    3
[4,]    4    4    4
[5,]    5    5    5
[6,]    6    6    6

b

     [,1] [,2] [,3]
[1,]    1    3    4

Each element of b "corresponds" to a column of a and needed in a function as follows:

I have a function myFun 

myfun<-function(x,n) {
   x[1:n]<-NA
   x
}

and I want to use it to return 

t<-cbind(myfun(a[,1],b[1]), myfun(a[,2],b[2]), myfun(a[,3],b[3]))

t
     [,1] [,2] [,3]
[1,]   NA   NA   NA
[2,]    2   NA   NA
[3,]    3   NA   NA
[4,]    4    4   NA
[5,]    5    5    5
[6,]    6    6    6

How can I achieve the above using apply / lapply / sapply without using a for loop?

For instance, I try apply(a,2,FUN=myfun,b), then the whole b is passed to myfun instead of b[1] or b[2] etc.

How can I rewrite apply(a,2,FUN=myfun,b) such that myfun will only pick up that element of b corresponding to the respective column of a?

Upvotes: 1

Views: 176

Answers (1)

James
James

Reputation: 66834

You can just use vector subsetting for this task:

(NAcells <- unlist(sapply(seq_along(b), function(i) seq(b[i])+(i-1)*nrow(a))))
[1]  1  7  8  9 13 14 15 16

a[NAcells] <- NA
a
     [,1] [,2] [,3]
[1,]   NA   NA   NA
[2,]    2   NA   NA
[3,]    3   NA   NA
[4,]    4    4   NA
[5,]    5    5    5
[6,]    6    6    6

Upvotes: 1

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