[R] matrix indexing and update

[murali.menon at fortisinvestments.com]

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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