Get indices of rows whose elements equal x at all columns indicated by a vector, in R

Question

Let us have a matrix M, e.g.

> M
     [,1] [,2] [,3] [,4]
[1,]   15    0    0    9
[2,]    0    1    8   24
[3,]    4    0    0    0
[4,]    3    2    0    0
[5,]    0    0   56    0

a vector of its column indices ind, e.g.

> ind=c(2,4)
> ind
[1] 2 4

and a value x, e.g. x=0.

How to get the indices of rows of matrix M, whose elements at all columns indicated by ind are equal to x?

The following code returns proper row indices:

> which(M[,2]==0 & M[,4]==0)
[1] 3 5

but I need a solution that will use a vector ind, possibly very long. I tried:

> which(M[,ind]==0)
[1]  1  3  5  8  9 10

but instead I got the entries which have zeros in either of the columns indicated by ind, not in all of them at the same time.

Answer 1

How about

rowSums(M[, ind] == 0) == length(ind)
# [1] FALSE FALSE  TRUE FALSE  TRUE

Let's break the code down step-by-step:

• M[, ind] == 0 - get a logical matrix showing where M[, ind] is zero
• rowSums(.) - determine how many TRUE values are in each row
• . == length(ind) - compare it to the number of columns used

And if you need the numeric indices, wrap it in which().

which(rowSums(M[, ind] == 0) == length(ind))
# [1] 3 5

Data:

M <- structure(c(15L, 0L, 4L, 3L, 0L, 0L, 1L, 0L, 2L, 0L, 0L, 8L, 
0L, 0L, 56L, 9L, 24L, 0L, 0L, 0L), .Dim = c(5L, 4L), .Dimnames = list(
    NULL, c("V1", "V2", "V3", "V4")))
ind <- c(2, 4)