Check that a vector is contained in a matrix in R

Question

I can't believe this is taking me this long to figure out, and I still can't figure it out.

I need to keep a collection of vectors, and later check that a certain vector is in that collection. I tried lists combined with %in% but that doesn't appear to work properly.

My next idea was to create a matrix and rbind vectors to it, but now I don't know how to check if a vector is contained in a matrix. %in appears to compare sets and not exact rows. Same appears to apply to intersect.

Help much appreciated!

Answer 1

Do you mean like this:

wantVec <- c(3,1,2)
myList <- list(A = c(1:3), B = c(3,1,2), C = c(2,3,1))
sapply(myList, function(x, want) isTRUE(all.equal(x, want)), wantVec)
## or, is the vector in the set?
any(sapply(myList, function(x, want) isTRUE(all.equal(x, want)), wantVec))

We can do a similar thing with a matrix:

myMat <- matrix(unlist(myList), ncol = 3, byrow = TRUE)
## As the vectors are now in the rows, we use apply over the rows
apply(myMat, 1, function(x, want) isTRUE(all.equal(x, want)), wantVec)
## or
any(apply(myMat, 1, function(x, want) isTRUE(all.equal(x, want)), wantVec))

Or by columns:

myMat2 <- matrix(unlist(myList), ncol = 3)
## As the vectors are now in the cols, we use apply over the cols
apply(myMat, 2, function(x, want) isTRUE(all.equal(x, want)), wantVec)
## or
any(apply(myMat, 2, function(x, want) isTRUE(all.equal(x, want)), wantVec))

If you need to do this a lot, write your own function

vecMatch <- function(x, want) {
    isTRUE(all.equal(x, want))
}

And then use it, e.g. on the list myList:

> sapply(myList, vecMatch, wantVec)
    A     B     C 
FALSE  TRUE FALSE 
> any(sapply(myList, vecMatch, wantVec))
[1] TRUE

Or even wrap the whole thing:

vecMatch <- function(x, want) {
    out <- sapply(x, function(x, want) isTRUE(all.equal(x, want)), want)
    any(out)
}

> vecMatch(myList, wantVec)
[1] TRUE
> vecMatch(myList, 5:3)
[1] FALSE

---

EDIT: Quick comment on why I used isTRUE() wrapped around the all.equal() calls. This is due to the fact that where the two arguments are not equal, all.equal() doesn't return a logical value (FALSE):

> all.equal(1:3, c(3,2,1))
[1] "Mean relative difference: 1"

isTRUE() is useful here because it returns TRUE iff it's argument is TRUE, whilst it returns FALSE if it is anything else.