[R] problems with ifelse??
Berend Hasselman
bhh at xs4all.nl
Mon Jul 11 10:02:01 CEST 2011
David Winsemius wrote:
>
> On Jul 10, 2011, at 9:09 PM, mousy0815 wrote:
>
>> I have the following code to determine the probability of a t-cell
>> encountering an antigen after "m" steps.
>>
>> probability <- function(N, f, m, b, x, t) { #using ifelse instead of
>> if/else
>> #N is the number of lymph nodes
>> #f is the fraction of Dendritic cells (in the correct node) that
>> have the
>> antigen
>> #m is the number of time steps
>> #b is the starting position (somewhere in the node or somewhere in
>> the gap
>> between nodes. It is a number between 1 and (x+t))
>> #x is the number of time steps it takes to traverse the gap
>> #t is the number of time steps it takes to traverse a node.
>> A <- 1/N
>> B <- 1-A
>> C <- 1-f
>> D <- (((m+b-1)%%(x+t))+1)
>>
>> ifelse(b<=t, ########starts inside node
>> ifelse( (m<=(t-b)),
>> return(B + A*(C^m)), # start & end in first node
>> ifelse (D<=t, # we finish in a node
>> return((B + A*(C^(t-b)))*((B + A*(C^t))^(floor(m/(x
>> +t))-1))*(B +
>> A*(C^D))),
>> probability(N, f, (m-1), b, x, t)
>> )
>> ),
>> ifelse( ########starts outside node
>> m<=(x+t-b),
>> return(1), #also end in the gap,
>> ifelse (
>> (D<=t), #end in a node
>> (return(((B + A*(C^t))^(floor((m/(x+t)))))*(B + (A*(C^D))))),
>> probability(N, f, (m-1), b, x, t)#outside node
>> )
>> )
>> )
>> }
>>
>> But I do:
>>> m<- c(1:3)
>>> probability(10, 0.1, m, 3, 4, 5)
>> 0.9900000 0.9810000 0.9729000
>>
>> but if you do each number separately you get
>>> probability(10, 0.1, 1, 3, 4, 5)
>> 0.99
>>> probability(10, 0.1, 2, 3, 4, 5)
>> 0.981
>>> probability(10, 0.1, 3, 3, 4, 5)
>> 0.981
>
> Your function is probably not fully vectorized. Try comparing with
>
> sapply(1:3, function(x) probability( m =x, N=0.1, f=3, b=3, x=4,
> t=5) )
>
Shouldn't that be
sapply(1:3, function(x) probability(m =x, N=10, f=0.1, b=3, x=4, t=5))
@mousy
Your function uses ifelse which is a function that takes a vector as first
argument.
In the inner ifelse's you do a return().
So if any element of the condition vector implies that a return-branch must
be taken, your function
will immediately return a vector which is filled with values which may not
be correct for other entries in the condition vector.
Is m supposed to be a vector? If not then it may be a good idea to test for
that.
If not then why are you using ifelse? if {} else {} would be better.
Berend
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