R help - Jun 2009 - Indexing matrix

Hi

imagine the following situation:

a <- runif(100)
plot(a[a>0.5])

plots only the elements in the vector which are larger then 0.5

Now imagine a matrix:

b <- matrix(runif(20000), ncol=40, nrow=50)
image(b[b>0.5])
Error in image.default(b[b > 0.5]) : argument must be matrix-like

Is there a similar way then above to plot an image from the matrix with only
the elements larger then 0.5, and the remaining elements equals NA?

I know I could do something like

image(b, zlim=c(0.5, 1))
but this includes 0.5 in the image,

bb <- b
bb[bb<=0.5] <- NA
image(bb)

but this includes an additional assignment.

This is not of practical use at the moment - the question came up when
preparing a lecture.

Rainer


-- 
Rainer M. Krug, Centre of Excellence for Invasion Biology, Stellenbosch
University, South Africa

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On Tue, Jun 30, 2009 at 9:19 AM, Rainer M Krug<r.m.krug at gmail.com>
wrote:
> Hi
>
> imagine the following situation:
>
> a <- runif(100)
> plot(a[a>0.5])
>
> plots only the elements in the vector which are larger then 0.5
>
> Now imagine a matrix:
>
> b <- matrix(runif(20000), ncol=40, nrow=50)
> image(b[b>0.5])
> Error in image.default(b[b > 0.5]) : argument must be matrix-like
>
> Is there a similar way then above to plot an image from the matrix with
only
> the elements larger then 0.5, and the remaining elements equals NA?
>
> I know I could do something like
>
> image(b, zlim=c(0.5, 1))
> but this includes 0.5 in the image,
>
> bb <- b
> bb[bb<=0.5] <- NA
> image(bb)
>
> but this includes an additional assignment.
Although this does involve an assignment underneath
as well perhaps its sufficient for your purposes:

image(replace(b, b <= 0.5, NA))