Message-ID: <1346196068.95295.YahooMailNeo@web142604.mail.bf1.yahoo.com>
Date: 2012-08-28T23:21:08Z
From: arun
Subject: return first index for each unique value in a vector
In-Reply-To: <CADTt-8vVmyfXDYGXeeuSf=0f6qx438tJjJwywp78m-kwgth=dw@mail.gmail.com>
HI,
I was thinking about duplicated().? But, Bert already posted the solution.? The solution below is not very efficient.
A<-c(9,2,9,5)
unik<-as.numeric(names(table(A)))
match(unik,A)
#[1] 2 4 1
#Bert's solution wins here.
system.time({
set.seed(1)
A<-sample(1:5,1e6,replace=TRUE)
unik <- !duplicated(A)? ## logical vector of unique values
seq_along(A)[unik]? ## indices
A[unik]})
?user? system elapsed
? 0.040?? 0.016?? 0.056
#My solution
system.time({
set.seed(1)
A<-sample(1:5,1e6,replace=TRUE)
#unik<-as.numeric(names(table(A)))
match(as.numeric(names(table(A))),A)})
?user? system elapsed
?0.344?? 0.036?? 0.383
#Robert's solution
?system.time({
set.seed(1)
A<-sample(1:5,1e6,replace=TRUE)
as.numeric(rownames(unique(data.frame(A)[1])))})
?user? system elapsed
? 0.056?? 0.012?? 0.069
A.K.
----- Original Message -----
From: Bronwyn Rayfield <bronwynrayfield at gmail.com>
To: r-help at r-project.org
Cc:
Sent: Tuesday, August 28, 2012 3:58 PM
Subject: [R] return first index for each unique value in a vector
I would like to efficiently find the first index of each unique value in a
very large vector.
For example, if I have a vector
A<-c(9,2,9,5)
I would like to return not only the unique values (2,5,9) but also their
first indices (2,4,1).
I tried using a for loop with which(A==unique(A)[i])[1] to find the first
index of each unique value but it is very slow.
What I am trying to do is easily and quickly done with the "unique"
function in MATLAB (see
http://www.mathworks.com/help/techdoc/ref/unique.html).
Thank you for your help,
Bronwyn
??? [[alternative HTML version deleted]]
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