# crossprod

##### Matrix Crossproduct

Given matrices `x`

and `y`

as arguments, return a matrix
cross-product. This is formally equivalent to (but usually slightly
faster than) the call `t(x) %*% y`

(`crossprod`

) or
`x %*% t(y)`

(`tcrossprod`

).

##### Usage

`crossprod(x, y = NULL)`tcrossprod(x, y = NULL)

##### Arguments

- x, y
numeric or complex matrices (or vectors):

`y = NULL`

is taken to be the same matrix as`x`

. Vectors are promoted to single-column or single-row matrices, depending on the context.

##### Value

A double or complex matrix, with appropriate `dimnames`

taken
from `x`

and `y`

.

##### Note

When `x`

or `y`

are not matrices, they are treated as column or
row matrices, but their `names`

are usually **not**
promoted to `dimnames`

. Hence, currently, the last
example has empty dimnames.

In the same situation, these matrix products (also `%*%`

)
are more flexible in promotion of vectors to row or column matrices, such
that more cases are allowed, since R 3.2.0.

The propagation of NaN/Inf values, precision, and performance of matrix
products can be controlled by `options("matprod")`

.

##### References

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

##### See Also

##### Examples

`library(base)`

```
# NOT RUN {
(z <- crossprod(1:4)) # = sum(1 + 2^2 + 3^2 + 4^2)
drop(z) # scalar
x <- 1:4; names(x) <- letters[1:4]; x
tcrossprod(as.matrix(x)) # is
identical(tcrossprod(as.matrix(x)),
crossprod(t(x)))
tcrossprod(x) # no dimnames
m <- matrix(1:6, 2,3) ; v <- 1:3; v2 <- 2:1
stopifnot(identical(tcrossprod(v, m), v %*% t(m)),
identical(tcrossprod(v, m), crossprod(v, t(m))),
identical(crossprod(m, v2), t(m) %*% v2))
# }
```

*Documentation reproduced from package base, version 3.5.1, License: Part of R 3.5.1*

### Community examples

**afonso.lenzi@gmail.com**at Nov 2, 2018 base v3.5.1

#example to use crossprod to calculate de variance of this famous dataset #http://genomicsclass.github.io/book/pages/matrix_algebra_examples.html library(UsingR) y <- father.son$sheight N <- length(y) Y<- matrix(y,N,1) A <- matrix(1,N,1) barY=crossprod(A,Y) / N print(barY) r <- y - barY crossprod(r)/N