Matrix Multiplication

Description

Multiplies two matrices, if they are conformable. If one argument is a vector, it will be promoted to either a row or column matrix to make the two arguments conformable. If both are vectors it will return the inner product (as a matrix).

Usage

x %*% y

Arguments

Details

When a vector is promoted to a matrix, its names are not promoted to row or column names, unlike as.matrix.

This operator is S4 generic but not S3 generic. S4 methods need to be written for a function of two arguments named x and y.

Value

A double or complex matrix product. Use drop to remove dimensions which have only one level.

References

Becker, R. A., Chambers, J. M. and Wilks, A. R. (1988) The New S Language. Wadsworth & Brooks/Cole.

See Also

matrix, Arithmetic, diag.

Examples

x <- 1:4
(z <- x %*% x)    # scalar ("inner") product (1 x 1 matrix)
drop(z)             # as scalar

y <- diag(x)
z <- matrix(1:12, ncol = 3, nrow = 4)
y %*% z
y %*% x
x %*% z