R help - Nov 2018 - very slow "memoise"

Hi,

  I want to compute a lot of values and I have tried to use
memoise::memoise to speed-up the computation.
  However it is much slower using the memoised version.

I guess I have misunderstood how to use the package memoise or the
purpose of the package.

The code takes more than 2 minutes to finish but if I remove the line:
"nextstep <- memoise(nextstep)" the code runs in less than 1
second. I
was expecting a
total different result.

Here are the code:

library(memoise)

nextstep <- function(num) {
    if (num %% 2 == 0) {
        return(num/2)
    }
    num*3+1
}

nextstep <- memoise(nextstep)

for (idx in 1:1e4) {
    steps <- 0
    current <- idx
    while (current != 1) {
      steps <- steps + 1
      current <- nextstep(current)
    }
    cat(idx,steps,"\n")
}


Regards
Martin

Yes, you have misunderstood what memoise is for.

First, it is for when you call your function with the same inputs frequently as
part of your calling-level algorithm. For your iterative calculation you would
have  a stuck (cycling) process if the same value of current were  to be
revisited... ever.

Second, it is for when your nextstep function is very slow to compute, since the
memoisation process is non-trivial... certainly it takes much more work to keep
the previous results around than your example nextstep algorithm would take to
simply recompute the answer.

On November 15, 2018 4:44:05 AM PST, "Martin M?ller Skarbiniks
Pedersen" <traxplayer at gmail.com> wrote:
>Hi,
>
>  I want to compute a lot of values and I have tried to use
>memoise::memoise to speed-up the computation.
>  However it is much slower using the memoised version.
>
>I guess I have misunderstood how to use the package memoise or the
>purpose of the package.
>
>The code takes more than 2 minutes to finish but if I remove the line:
>"nextstep <- memoise(nextstep)" the code runs in less than 1
second. I
>was expecting a
>total different result.
>
>Here are the code:
>
>library(memoise)
>
>nextstep <- function(num) {
>    if (num %% 2 == 0) {
>        return(num/2)
>    }
>    num*3+1
>}
>
>nextstep <- memoise(nextstep)
>
>for (idx in 1:1e4) {
>    steps <- 0
>    current <- idx
>    while (current != 1) {
>      steps <- steps + 1
>      current <- nextstep(current)
>    }
>    cat(idx,steps,"\n")
>}
>
>
>Regards
>Martin
>
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