[R] matrix indexing and update

[Dimitris Rizopoulos]

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
> 
> 
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-- 
Dimitris Rizopoulos
Assistant Professor
Department of Biostatistics
Erasmus Medical Center

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