On Dec 16, 2008, at 7:19 AM, Antje wrote:
Hi David,
thanks a lot for your proposal. I got a lot of useful hints from all
of you :-)
David Winsemius schrieb:
It's not entirely clear what you are asking for, since
which(within.interval(a, -0.5, 0.5)) is actually longer than
which(a > -0.5 & a < 0.5).
Right but in case 'a' is something with a long name and '0.5' is a
variable you might end up with something like this (for the data
frame example):
DF[which( DF$myReallyLongColumnName > -myReallyLongThreshold & DF
$myReallyLongColumnName < -myReallyLongThreshold ), ]
I see your point, but I must point out that no cases would ever
satisfy that construction.
instead of:
DF[which( within.interval(DF$myReallyLongColumnName,
myReallyLongThreshold), ]
That would be a different within.interval function than I suggested,
but you could certainly create one which accepted a vector.
within.interval <- function(x, y) { min(y) < x & x < max(y) }
----------
> within.interval2 <- function(x,y) { min(y) < x & x < max(y)}
> y <- c(-.1, -.2, .1,.2)
> which(within.interval2(DF$a,y))
[1] 7 13 14 17
You mention that you want a solution that applies to
dataframes. Using indexing you can get entire rows of dataframes
that satisfy multiple conditions on one of its columns:
>> DF <- data.frame(a = rnorm(20), b= LETTERS[1:20], c =
letters[20:1], stringsAsFactors=FALSE)
> DF[which( DF$a > -0.5 & DF$a < 0.5 ), ]
# note that one needs to avoid DF[which(a > -0.5 & a<0.5) , ]
# the "a" vector is not the same as the "a" column vector within DF
a b c
3 -0.47310672 C r
6 -0.49784460 F o
9 0.02571058 I l
10 0.16893759 J k
11 -0.11963322 K j
12 0.39378887 L i
16 0.03712263 P e
Could get the indices that satisfy more than one condition:
> which(DF$a > 0.5 & DF$b < "K")
[1] 1 2 6 10
Or you can get rows of DF that satisfy conditions on multiple
columns with the subset function:
> subset(DF, a > 0.5 & b < "K")
a b c
1 2.2500997 A t
2 0.7251357 B s
6 0.7845355 F o
10 1.0685649 J k
Or if you wanted a within.interval function
> within.interval <- function(x,a,b) { x > a & x < b}
> which(within.interval(DF$a, -0.5, 0.5))
[1] 3 4 7 8 9 13 14 17 20
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