Clash Royale CLAN TAG#URR8PPP

Identifying if only the diagonal is non-zero

Let's say I have a matrix with all 0's with the exception of the diagonal.

m <- matrix(ncol=3,nrow=3)
m[,1] <- c(1,0,0)
m[,2] <- c(0,1,0)
m[,3] <- c(0,0,1)


What logical check could I use to test if the upper and lower triangle of the matrix are zero?

3 Answers
3

all(m[lower.tri(m)] == 0, m[upper.tri(m)] == 0)


all(m[!diag(nrow(m))] == 0)

Here's a slightly shorter alternative to @GregaKešpret's excellent solution:

m <- diag(11:13)
m

all(m[!diag(nrow(m))] == 0) # TRUE

m[1,2] = 0.01
all(m[!diag(nrow(m))] == 0) # FALSE


The diag function has two major modes: Give it a vector as in diag(11:13) and it puts it on the diagonal. Or give it a number as in diag(3) and it creates a identity matrix of that size. Then I extract all the non-diagonal elements and compare them to 0.

diag

diag(11:13)

diag(3)

Simply check for:

sum(m) == sum(diag(m))


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