Matrices

Matrices

A matrix is a 2D array of elements, all of the same type. Create one with matrix():

m <- matrix(1:6, nrow = 2, ncol = 3)
# Creates:
#      [,1] [,2] [,3]
# [1,]    1    3    5
# [2,]    2    4    6

By default, R fills matrices column by column. Use byrow = TRUE to fill by row:

m <- matrix(1:6, nrow = 2, byrow = TRUE)
# Creates:
#      [,1] [,2] [,3]
# [1,]    1    2    3
# [2,]    4    5    6

Indexing

Use [row, col] to access elements:

m <- matrix(1:9, nrow = 3, byrow = TRUE)
cat(m[1, 2], "\n")  # Row 1, Col 2 -> 2
cat(m[2, ], "\n")   # Entire row 2 -> 4 5 6
cat(m[, 3], "\n")   # Entire col 3 -> 3 6 9

Matrix Operations

m <- matrix(1:4, nrow = 2)
cat(nrow(m), "\n")  # 2
cat(ncol(m), "\n")  # 2
cat(t(m), "\n")     # Transpose (flattened output)

Arithmetic works element-wise, just like vectors:

m <- matrix(1:4, nrow = 2)
cat(m * 2, "\n")  # 2 4 6 8

Your Task

Create a 3x3 matrix filled by row with the values 1 through 9. Print the element at row 2, column 3, then print the sum of all elements.