Sundar Dorai-Raj <[EMAIL PROTECTED]> writes:
> Sam R. Smith wrote:
> > if solve(a,b) means to calculate an inverse matrix of
> > a with b, and i wonder why solve(a)%%b will get
> > different result?
> >
>
> It does? Or perhaps your "%%" is not just a typo. It should be "%*%".
>
> > a <- matrix(rnorm(16), 4, 4)
> > b <- matrix(rnorm(4), 4, 1)
> > solve(a, b)
> [,1]
> [1,] -0.8005768
> [2,] 0.5913755
> [3,] -1.8256012
> [4,] 0.8973716
> > solve(a) %*% b
> [,1]
> [1,] -0.8005768
> [2,] 0.5913755
> [3,] -1.8256012
> [4,] 0.8973716
I think the issue is this:
> solve(a, b)
[1] -0.7251033 -0.3903765 0.3212044 -1.2969697
> solve(a)%*% b
[,1]
[1,] -0.7251033
[2,] -0.3903765
[3,] 0.3212044
[4,] -1.2969697
So b gets promoted to a column matrix in one case but not the other.
This is slightly odd, but it's been that way "forever" and in S(-PLUS)
too, so I think it is unchangeable (it's the sort of thing that
there's a 99% chance that some people have actually been relying on).
If you want a vector result from a matrix multiply, there's always
> drop(solve(a)%*% b)
[1] -0.7251033 -0.3903765 0.3212044 -1.2969697
--
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c/ /'_ --- Dept. of Biostatistics PO Box 2099, 1014 Cph. K
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