Matrix-exponentiation operator: Difference between revisions
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→Infix operator: Improved syntax.
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===Infix operator===
The task wants the implementation to be "as an operator". Given that R lets us define new infix operators, it seems fitting to show how to do this. Ideally, for a matrix a and int n, we'd want to be able to use a^n. R actually has this already, but it's not what the task wants:
<lang rsplus>a <- matrix(c(1, 2, 3, 4), 2, 2)
a^1
a^2</lang>
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[2,] 4 16</pre>
As we can see, it instead returns the given matrix with its elements raised to the nth power. Overwriting the ^ operator would be dangerous and rude. However, R's base library suggests an alternative. %*% is already defined as matrix multiplication, so why not use %^% for exponentiation?
<lang rsplus>`%^%` <- function(mat, n)
{
is.wholenumber <- function(x, tol = .Machine$double.eps^0.5)
if(is.matrix(mat) && is.numeric(n) && is.wholenumber(n))
{
if(n==0)
else if(n == 1)
else if(n > 1)
else stop("Invalid n.")
}
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}
#For output:
a %^% 0
a %^% 1
a %^% 2
a %*% a %*% a#Base R's equivalent of a %^% 3
a %^% 3
nonSquareMatrix <- matrix(c(1, 2, 3, 4, 5, 6), nrow = 2, ncol = 3)
nonSquareMatrix %^% 1
nonSquareMatrix %^% 2#R's %*% will throw the error for us</lang>
{{out}}
<pre>> a %^% 0
[,1] [,2]
[1,] 1 0
[2,] 0 1
> a %^% 1
[,1] [,2]
[1,] 1 3
[2,] 2 4
> a %^% 2
[,1] [,2]
[1,] 7 15
[2,] 10 22
> a %*% a %*% a#Base R's equivalent of a %^% 3
[,1] [,2]
[1,] 37 81
[2,] 54 118
> a %^% 3
[,1] [,2]
[1,] 37 81
[2,] 54 118
> nonSquareMatrix <- matrix(c(1, 2, 3, 4, 5, 6), nrow = 2, ncol = 3)
> nonSquareMatrix %^% 1
[,1] [,2] [,3]
[1,] 1 3 5
[2,] 2 4 6
> nonSquareMatrix %^% 2#R's %*% will throw the error for us
Error in mat %*% (mat %^% (n - 1)) : non-conformable arguments</pre>
Our code is far from efficient and could do with more error-checking, but it demonstrates the principle. If we wanted to do this properly, we'd use a library - ideally one that calls C code. Following the previous submission's example, we can just do this:
<lang rsplus>library(Biodem)
`%^%` <- function(mat, n) Biodem::mtx.exp(mat, n)</lang>
And it will work just the same, except for being much faster and throwing an error on nonSquareMatrix %^% 1.
=={{header|Racket}}==
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