[R] counting row repetitions without loop
Waterman, DG (David)
david.waterman at diamond.ac.uk
Thu Feb 7 16:18:12 CET 2008
Hi Harold,
Thanks for your suggestion, which worked very well. I had to modify the
pattern as I'm really dealing with larger grids than 10x10 so I needed a
way to distinguish between for example position 1,11 and 11,1. The new
function based on your suggestion runs in ~40% of the time compared with
the original for loop construction, so it's an important saving! For the
record, the new function is essentially as follows:
pixellate <- function(input, xPxNum = 128, yPxNum = 128, gain = 1){
temp <- matrix(data = 0, ncol = yPxNum, nrow = xPxNum)
input <- subset(input, x <= xPxNum & x > 0 & y <= yPxNum & y > 0)
#strip illegal pixel values
input$patt <- input$x + (input$y - 1) * xPxNum #make unique identifier
for each pixel
counts <- as.data.frame(with(input, table(patt)))
input <- merge(input, counts, by='patt')
index <- matrix(c(input$x, input$y), ncol=2) #make an index matrix
temp[index] <- input$Freq * gain
temp
}
where input is a data frame consisting of x and y coordinates of events
on a grid, 1 row per event, e.g.
>input
x y
1 86 63
2 75 16
3 122 34
4 8 17
5 1 7
6 115 81
7 103 125
8 76 54
9 114 117
10 96 18
...and so on for many rows.
Cheers
David.
-----Original Message-----
From: Doran, Harold [mailto:HDoran at air.org]
Sent: 06 February 2008 15:03
To: Doran, Harold; Waterman, DG (David); r-help at r-project.org
Subject: RE: [R] counting row repetitions without loop
Sorry, word wrap made that incomprehensible, I think
x y
4 5
6 7
6 6
7 5
5 7
6 7
4 5
6 7
7 6
5 6
dat <- read.table('clipboard', header=TRUE)
dat$patt <- paste(dat$x,dat$y, sep='')
mm <- as.data.frame(with(dat, table(patt)))
dat <- merge(dat, mm, by='patt')
mat <- matrix(0, ncol=10, nrow=10)
gg <- matrix(c(dat$x, dat$y), ncol=2)
mat[gg] <- dat$Freq
> -----Original Message-----
> From: r-help-bounces at r-project.org
> [mailto:r-help-bounces at r-project.org] On Behalf Of Doran, Harold
> Sent: Wednesday, February 06, 2008 9:56 AM
> To: Waterman, DG (David); r-help at r-project.org
> Subject: Re: [R] counting row repetitions without loop
>
> I think this does what you want, but there may be a more efficient way
>
> x y
> 4 5
> 6 7
> 6 6
> 7 5
> 5 7
> 6 7
> 4 5
> 6 7
> 7 6
> 5 6
> dat <- read.table('clipboard', header=TRUE) # copy sample data above
> dat$patt <- paste(dat$x,dat$y, sep='') mm <- as.data.frame(with(dat,
> table(patt))) dat <- merge(dat, mm,
> by='patt') mat <- matrix(0, ncol=10, nrow=10) gg <- matrix(c(dat$x,
> dat$y), ncol=2) mat[gg] <- dat$Freq
>
> > -----Original Message-----
> > From: r-help-bounces at r-project.org
> > [mailto:r-help-bounces at r-project.org] On Behalf Of Waterman, DG
> > (David)
> > Sent: Wednesday, February 06, 2008 9:08 AM
> > To: r-help at r-project.org
> > Subject: [R] counting row repetitions without loop
> >
> > Hi,
> >
> > I have a data frame consisting of coordinates on a 10*10 grid, i.e.
> >
> > > example
> > x y
> > 1 4 5
> > 2 6 7
> > 3 6 6
> > 4 7 5
> > 5 5 7
> > 6 6 7
> > 7 4 5
> > 8 6 7
> > 9 7 6
> > 10 5 6
> >
> > What I would like to do is return an 10*10 matrix
> consisting of counts
> > at each position, so in the above example I would have a
> matrix where,
> > for example, cell [4,5] contains 2 and [6,7] contains 3. At
> the moment
> > I have implemented this using a for loop over the rows of the data
> > frame, however the data frames I want to process are very
> long so the
> > loop takes many minutes to complete. Can I do this in a
> more efficient
> > way?
> >
> > Cheers,
> > David
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Any opinions expressed within this e-mail are those of the individual and not necessarily of Diamond Light Source Ltd.
Diamond Light Source Ltd. cannot guarantee that this e-mail or any attachments are free from viruses and we cannot accept liability for any damage which you may sustain as a result of software viruses which may be transmitted in or with the message.
Diamond Light Source Limited (company no. 4375679). Registered in England and Wales with its registered office at Diamond House, Harwell Science and Innovation Campus, Didcot, Oxfordshire, OX11 0DE, United Kingdom
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