# matmult: 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 of the same length, it will return the inner product (as a matrix).

### Usage

 `1` ```x %*% y ```

### Arguments

 `x, y` numeric or complex matrices or vectors.

### 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.

### Note

Since R 3.2.0, promotion of a vector to a 1-row or 1-column matrix happens in even more cases, when one of the two choices allows `x` and `y` to get conformable dimensions.

### References

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

For matrix crossproducts, `crossprod()` and `tcrossprod()` are typically preferable. `matrix`, `Arithmetic`, `diag`.
 ```1 2 3 4 5 6 7 8 9``` ```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 ```