[R] matrix indexing and update
murali.menon at fortisinvestments.com
murali.menon at fortisinvestments.com
Wed Nov 5 17:30:04 CET 2008
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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