R Matrices
A matrix holds values of one type in rows and columns. It is a grid where every row has the same number of cells.
Fill down the columns
The values one to six fill a grid with two rows. They run down the first column before they move to the next one.
m <- matrix(1:6, nrow = 2)
print(m)Fill across the rows
byrow set to TRUE changes the fill direction. The values now run across the first row before they drop to the second one.
m <- matrix(1:6, nrow = 2, byrow = TRUE)
print(m)Read one cell
The brackets hold a row number and a column number. They pick the cell in row two and column one.
m <- matrix(1:6, nrow = 2)
print(m[2, 1])Sums and size
colSums adds each column and rowSums adds each row. nrow and ncol then report two rows and three columns.
m <- matrix(1:6, nrow = 2)
print(colSums(m))
print(rowSums(m))
print(nrow(m))
print(ncol(m))Example
grid <- matrix(c(4, 8, 2, 6), nrow = 2, byrow = TRUE)
print(grid)
print(rowSums(grid))Key points
- matrix() builds a matrix from a vector of values.
- Values fill down each column first by default.
- byrow = TRUE fills values across each row instead.
- One cell is read with its row and column inside brackets.
- colSums() adds each column and rowSums() adds each row.
- nrow() counts the rows and ncol() counts the columns.
In this example, byrow fills the grid across the rows. rowSums then reports 12 for the first row and 8 for the second row.
Try it Yourself
Edit the code, then press Run.
Running is simulated · rdrr.io R Snippets
Exercise
How does matrix(1:6, nrow = 2) place the value 2?