[R] Flip Matrix form file?

[Henrik Bengtsson]

Hi.

On Wed, Jul 23, 2008 at 9:23 AM, Chris82 <rubenbauar at gmx.de> wrote:
>
> Hello,
>
> I have a problem to flip a 200x200 matrix, which is imported by a .asc file.
> I want to flip the matrix like in a created example below:
>
>> b <- matrix(1:9,3,3,byrow=T)
>> b
>
>     [,1] [,2] [,3]
> [1,]    1    2    3
> [2,]    4    5    6
> [3,]    7    8    9
>
>> b1 <- apply(t(b),1,rev)
>
>> b1
>     [,1] [,2] [,3]
> [1,]    7    8    9
> [2,]    4    5    6
> [3,]    1    2    3
>

Your flip is nothing but reordering the rows, so your apply(...) is a
bit overkill.  This can be done as:

X <- matrix(1:9, nrow=3, ncol=3, byrow=TRUE);
print(X);
Y <- X[nrow(X):1,];
print(Y);

This does also work for data frames:

Z <- as.data.frame(X);
Z <- Z[nrow(Z):1,];
print(Y);

> but my R Script doesn't work so. I don't know where the problem is.
> What's the different between a created matrix and a imported matrix?
>
> Here is my r Script:
>
> a <- read.table("c:/Test/Test.asc", header=FALSE, sep=" ")
> dim(a)
> b <- matrix(a, nrow=200, byrow=TRUE)
> d <- apply(t(b),1,rev)
> write.table(d, file = "c:/Test/output.asc", sep = " ", row.names = FALSE,
> col.names = FALSE)

Your mistake is most likely that you use byrow=TRUE, which results in
placing the *first column* of data.frame read by read.table() into the
*first row* of the matrix, and so on.  Have you looked what 'a' and
'b' looks like?  I guess that when you look at 'b' it is not what you
expect.

Your probably better off by just reordering the rows as suggested above.

Cheers

Henrik

>
> My output ist like this
>
> input:
>
> 1 2 3
> 4 5 6
> 7 8 9
>
> output:
>
> 3 2 1
> 6 5 4
> 9 8 7
>
> but it should be like this, as in the example:
>
> 7 8 9
> 4 5 6
> 1 2 3
>
> thanks.
> --
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