How to return only the "TRUE" values from sapply?-CodePudding

I have a very large list of matrices and need to check if a new matrix is identical to any of the matrices in my existing list. My code looks like this:

sapply(matrix_list, identical, new_matrix)

I'm getting a very long list of False's, which is to be expected, but it's so long that I can't go through to see if there are any matches (TRUE's). Does anyone know how I can run this sapply function and only have it return TRUES, if there are any? Bonus points if you can tell me how I can locate the exact matrix that is associated with the TRUE.

Thank you!

CodePudding user response：

If you want to keep just true values then wrap the whole statement in which, here's a simple example:

which(sapply(letters, function(x) identical(x, "a")))

a 
1

CodePudding user response：

like so:

## list of ten random 2 x 2 matrices:
haystack <- lapply(1:10, \(n) matrix(sample(1:100, 4), 2, 2))
## repeat 8th element of matrix:
haystack <- c(haystack, haystack[8])

## pick 8th element of matrix list as the new matrix
needle <- haystack[[8]]

which(
    sapply(haystack, \(i) identical(needle, i)) 
)