R help - Nov 2008 - matrix indexing and update

Folks,

I have a matrix:

set.seed(123)
a <- matrix(rnorm(100), 10)

And a vector:

b <- rnorm(10)

Now, I want to switch the signs of those rows of a corresponding to 
indices in b whose values exceed the 75 %-ile of b

which(b > quantile(b)[4])
[1] 2 6 10

so I want, in effect:

a[2, ] <-  -a[2, ]
a[6, ] <- -a[6, ]
a[10, ] <- -a[10, ]

I thought I could do

a[which(b > quantile(b)[4]), ] <- -a

but that's clearly wrong. 

I came up with an sapply():

t(sapply(1 : NROW(a), function(n) ifelse(b > quantile(b)[4], -a[n, ], a[n, 
])))

Ugh.

What's a good way to achieve this? 

Thanks,

Murali


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you almost have it, e.g., check the following:

set.seed(123)
a <- matrix(rnorm(100), 10)
b <- rnorm(10)

ind <- b > quantile(b, 0.75)
a[ind, ] <- -a[ind, ]
a


I hope it helps.

Best,
Dimitris


murali.menon at fortisinvestments.com wrote:
> Folks,
> 
> I have a matrix:
> 
> set.seed(123)
> a <- matrix(rnorm(100), 10)
> 
> And a vector:
> 
> b <- rnorm(10)
> 
> Now, I want to switch the signs of those rows of a corresponding to 
> indices in b whose values exceed the 75 %-ile of b
> 
> which(b > quantile(b)[4])
> [1] 2 6 10
> 
> so I want, in effect:
> 
> a[2, ] <-  -a[2, ]
> a[6, ] <- -a[6, ]
> a[10, ] <- -a[10, ]
> 
> I thought I could do
> 
> a[which(b > quantile(b)[4]), ] <- -a
> 
> but that's clearly wrong. 
> 
> I came up with an sapply():
> 
> t(sapply(1 : NROW(a), function(n) ifelse(b > quantile(b)[4], -a[n, ],
a[n,
> ])))
> 
> Ugh.
> 
> What's a good way to achieve this? 
> 
> Thanks,
> 
> Murali
> 
> 
> ---------------------------------------------------------------------------
> This e-mail is confidential and may be privileged. If you have received it
by mistake, please notify the sender by return e-mail and delete it from your
system. You should not disclose, copy or use it for any purpose. The information
in this e-mail is not contractual. Fortis Investments provides no guarantee as
to the correctness of this information and accepts no responsibility for any
action taken on the basis of it. Fortis Investments is the trade name for all
entities within the Fortis Investment Management group.
> 
> ______________________________________________
> R-help at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide
http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
> 
-- 
Dimitris Rizopoulos
Assistant Professor
Department of Biostatistics
Erasmus Medical Center

Address: PO Box 2040, 3000 CA Rotterdam, the Netherlands
Tel: +31/(0)10/7043478
Fax: +31/(0)10/7043014

Dear Murali,
Here is one way:


set.seed(123)
a <- matrix(abs(rnorm(100)), 10)
b <- rnorm(10)
apply(a,2,function(x) ifelse(b>quantile(b,probs=0.75),-x,x))


HTH,

Jorge





On Wed, Nov 5, 2008 at 11:30 AM, <murali.menon@fortisinvestments.com>
wrote:
> Folks,
>
> I have a matrix:
>
> set.seed(123)
> a <- matrix(rnorm(100), 10)
>
> And a vector:
>
> b <- rnorm(10)
>
> Now, I want to switch the signs of those rows of a corresponding to
> indices in b whose values exceed the 75 %-ile of b
>
> which(b > quantile(b)[4])
> [1] 2 6 10
>
> so I want, in effect:
>
> a[2, ] <-  -a[2, ]
> a[6, ] <- -a[6, ]
> a[10, ] <- -a[10, ]
>
> I thought I could do
>
> a[which(b > quantile(b)[4]), ] <- -a
>
> but that's clearly wrong.
>
> I came up with an sapply():
>
> t(sapply(1 : NROW(a), function(n) ifelse(b > quantile(b)[4], -a[n, ],
a[n,
> ])))
>
> Ugh.
>
> What's a good way to achieve this?
>
> Thanks,
>
> Murali
>
>
> ---------------------------------------------------------------------------
> This e-mail is confidential and may be privileged. If you have received it
> by mistake, please notify the sender by return e-mail and delete it from
> your system. You should not disclose, copy or use it for any purpose. The
> information in this e-mail is not contractual. Fortis Investments provides
> no guarantee as to the correctness of this information and accepts no
> responsibility for any action taken on the basis of it. Fortis Investments
> is the trade name for all entities within the Fortis Investment Management
> group.
>
> ______________________________________________
> R-help@r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide
> http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
>
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