Create a program that sums the diagonal of a matrix

Question

I'm trying to create a program that sums the diagonal of a matrix that I've created. I used the code below but I don't understand whats wrong with the code I made.

a <- c(1, 2, 3) 
b <- c(3, 5, 6)

x <- matrix(a, b, nrow=3, ncol=3)
x

for (i in 1:nrow(x)) {
  for (j in 1:ncol(x)) {
    if (i=j) {
      diags[i, j] <- sum(x[i, j])
    }
  }
}

You're using `=` (value assignment) instead of `==` (comparison), so `i` becomes `j` and can't be found, but even so `diags` would be the next undefined thing to throw an error, and after that `sum(diags[i, j])` would return each individual diagonal's element's sum instead of the sum along the diagonal. You can obtain the desired result with `sum(diag(x))`. — , Apr 20 '22 at 04:34

score 0 · Accepted Answer · answered Apr 20 '22 at 07:07

It's already great code! However, as was noted in comments, you were trying to sum a scalar when you probably thought you were summing a vector. You need to add the result of x[i, j] to a starting value which is of course zero. So first create diags with value zero and add the result to it in each iteration of the loop.

Notes: for comparisons we use `==`, for assignment in function calls =, and for assignment in the script `<-`. Better use the seq functions such as seq_len(nrow(x)) instead of 1:nrow(x); yields at first glance the same, but has advantages e.g. if nrow(x) is zero.

diags <- 0

for (i in seq_len(nrow(x))) {
  for (j in seq_len(ncol(x))) {
    if (i == j) {
      diags <- diags + x[i, j]
    }
  }
}
diags
# [1] 9

Checking with the diag function.

sum(diag(x))
# [1] 9

Data:

I used a slightly different matrix, that isn't so symmetric and has more distinguishable values.

x <- matrix(1:9 , nrow=4, ncol=3)

Create a program that sums the diagonal of a matrix

1 Answers1