William Dunlap pisze:
-----Original Message-----
From: r-help-boun...@r-project.org
[mailto:r-help-boun...@r-project.org] On Behalf Of Jarek Jasiewicz
Sent: Tuesday, September 29, 2009 11:36 AM
To: Erik Iverson
Cc: R-help@r-project.org
Subject: Re: [R] Equivalent for Matematica function Which...
well function arguments are in square brackets. z is result (new
vector). I show Matematica syntax, but cannot explain what I
expect. Sorry
The example is wrong because it can be replaced by R cut
function. The
arguments are: condition,action.... and can be replaced by
ste of ifelse
formulas:
if (x<10) x<-0.7
else if (x<30 && x=>10) x<-x^2/(x-1)
etc...
but that solution is slow for vectors with millions of numbers
ifelse is different than if-then-else. Your if-then-else needs
to be in a loop but ifelse is vectorized. Try something like
z <- ifelse(x<10,
0.7, # result for x's less than 10
ifelse(x<30 & x>=10, # x>=10 is redundant in this
branch
x^2/(x-1), # result for x's >=10 and x<30
1)) # result for x's>=30
That evaluates all the arguments for each value in x, even those
for which the answer will not be used. That wastes some time
and sometimes causes warnings or even errors. In that case you
can use more flexible but less convenient syntax like:
z <- NA * x # initialize z to be like x but filled with NA's
cond <- x<10
z[cond] <- 0.7
cond <- x>=10 & x<30
z[cond] <- x[cond]^2/(x[cond]-1) # or (function(y)y^2/(y-1))(x[cond])
cond <- x>=30
z[cond] <- 1.0
Thanks! it seems very good approach
Jarek
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