按行名和列名访问矩阵，如果不可用，则返回零

Question

I have a matrix with rownames and colnames as:

a = matrix(1:4,2,2)
dimnames(a) = list(c("x","y"),c("x","y"))

I can have access to matrix elements by rownames and colnames, for example,

a["x","y"]

When I type a["x","z"], it gives me an error "Error in a["x", "z"] : subscript out of bounds", which should be.

My question is how can I get zero instead of that error. More precisely, when I type wrong rownames or colnames that are not in rownames(a) or colnames(a), it returns a fixed value such as zero. For example, zero for a["x","z"], a["z","t"], ... .

Answer 1

Wrap it in tryCatch . No packages are used:

tryCatch(a["x", "y"], error = function(e) 0)
## [1] 3

tryCatch(a["x", "w"], error = function(e) 0)
## [1] 0

Answer 2

We can wrap with a tryCatch or possibly from purrr to make this happen

library(purrr)
f1 <- possibly(function(mat, indx1, indx2) mat[indx1, indx2], otherwise = 0)
f1(a, 'x', 'z')
#[1] 0

f1(a, 'x', 'y')
#[1] 3

Answer 3

You could use match to make sure an NA is returned instead:

a[match("x",rownames(a)), match("y",colnames(a))]
#[1] 3

a[match("x",rownames(a)), match("z",colnames(a))]
#[1] NA