[R] memory in R

[Spencer Graves]

To expand a bit on Uwe's comments:  R is vectorized and loops are 
notoriously inefficient.  Consider the following: 

start.time <- proc.time()
set.seed(1)
X <- array(rnorm(1000), dim=c(100, 10))
X.1 <- apply(X, 2, mean)
print(var(X.1))
(elapsed.time <- proc.time()-start.time)

start.ttime <- proc.time()
set.seed(1)
X <- array(NA, dim=c(100, 10))
for(j in 1:10)for(i in 1:100)X[i,j] <- rnorm(1)
X. <- rep(0, 10)
X.. <- 0
varX <- 0
for(j in 1:10){
    for(i in 1:100)
        (X.[j] <- X.[j]+X[i,j])
    X.[j] <- X.[j]/100   
    X.. <- X..+X.[j]
    varX <- varX+X.[j]^2
}
X.. <- X../10
varX <- (varX-10*X..^2)/9
print(varX)
(elapsed.time2 <- proc.time()-start.time)

elapsed.time
elapsed.time2

# In R 1.8.1: 
 > elapsed.time
[1] 0.00 0.00 0.54   NA   NA
 > elapsed.time2
[1] 0.05 0.00 1.73   NA   NA
 >
# In S-_lus 8.2: 
 > elapsed.time
[1] 0.04 0.00 0.05 0.00 0.00
 > elapsed.time2
[1] 0.37 0.06 0.44 0.00 0.00

      hope this helps.  spencer graves

Uwe Ligges wrote:

> Raphael Schoenle wrote:
>
>> Hi,
>>  
>> How can I assign memory in R? My simulations are very slow.
>
>
> What do you mean with "assign memory in R"?
> You might want to optimize your code...
>
> Uwe Ligges
>
>
>> Thanks!,
>>  
>> -R
>>
>>     [[alternative HTML version deleted]]
>>
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>
>
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