[R] need technique for speeding up R dataframe individual element insertion (no deletion though)

[Bill.Venables at csiro.au]

Why do you need an explicit loop at all?

(Also, your loop goes over i in 1:length(cam$end_date) but your code refers to cam$end_date[i+1] -->||<--!!)


Here is a suggestion.  You want to identify places where the date increases but the volume does not change.  OK, where?

ind <- with(cam, {
           dx <- as.numeric(diff(strptime(end_date, "%d/%m/%Y")))
           dt <- diff(vol)
           which(dx > 0 & dt == 0)
})

Now adjust the new data frame

cap <- within(cam, {
             levels[ind] <- 1
             levels[ind+1] <- 1
})

Of course this is untested code, so caveat emptor!

Bill Venables.

________________________________________
From: r-help-bounces at r-project.org [r-help-bounces at r-project.org] On Behalf Of Ishwor [ishwor.gurung at gmail.com]
Sent: 13 August 2009 22:07
To: r-help at r-project.org
Subject: [R] need technique for speeding up R dataframe individual element      insertion (no deletion though)

Hi fellas,

I am working on a dataframe cam and it involves comparison within the
2 columns - t1 and t2 on about 20K rows and 14 columns.

###
cap = cam; # this doesn't take long. ~1 secs.


for( i in 1:length(cam$end_date))
  {
    x1=strptime(cam$end_date[i], "%d/%m/%Y");
    x2=strptime(cam$end_date[i+1], "%d/%m/%Y");

    t1= cam$vol[i];
    t2= cam$vol[i+1];

    if(!is.na(x2) && !is.na(x1) && !is.na(t1) && !is.na(t2))
    {
      if( (x2>=x1) && (t1==t2) ) # date and vol
      {
        cap$levels[i]=1; #make change to specific dataframe cell
        cap$levels[i+1]=1;
      }
    }
  }
###

Having coded that, i ran a timing profile on this section and each
1000'th row comparison is taking ~1.1 minutes on a 2.8Ghz dual-core
box (which is a test box we use).
This obviously computes to ~21 minutes for 20k which is definitely not
where we want it headed. I believe, optimisation(or even different way
to address indexing inside dataframe) can be had inside the innermost
`if' and specifically in `cap$levels[i]=1;' but I am a bit at a loss
having scoured the documentation failing to find anything of value.
So, my question remains are there any general/specific changes I can
do to speed up the code execution dramatically?

Thanks folks.

--
Regards,
Ishwor Gurung

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