when rownames do not match all columnnames in a matrix, find the missing variable and add it as a new column of zeros in r

# Create a sample matrix
mat <- matrix(1:9, nrow = 3)
rownames(mat) <- c("row1", "row2", "row3")
colnames(mat) <- c("A", "B", "C")

# Define the missing variable
missing_var <- setdiff(rownames(mat), colnames(mat))

# Add the missing variable as a new column of zeros
mat <- cbind(mat, matrix(0, nrow = nrow(mat), ncol = length(missing_var), dimnames = list(NULL, missing_var)))
mat
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