Luc Devroye from McGill University's School of Computer Science has provided pdf copies of his book, "Non-Uniform Random Variate Generation," on-line.

http://cg.scs.carleton.ca/~luc/rnbookindex.html

Most common distributions are already built into R, but sometimes you need to build your own -- Luc provides the algorithms to do it.

## Thursday, May 17, 2007

### Repeat elements of a vector

Two methods. The first produces a vector, the second a one column matrix.

x=1:10

# Method 1

rep(x,each=3)

# Method 2

matrix(t(matrix(x,length(x),3)))

x=1:10

# Method 1

rep(x,each=3)

# Method 2

matrix(t(matrix(x,length(x),3)))

### Convert image to matrix in R

This is how to use the Pixmap library to read in an image as a matrix.

To get info on your new object:

Although included in the previous output, the size of the image can be extracted by:

Then to extract the red channel from the image for the first ten rows:

Or to extract the entire red channel to an actual matrix:

> library(pixmap)

# the next command may only work on Linux

> system("convert foo.tiff foo.ppm")

> img <- read.pnm("foo.ppm")

To get info on your new object:

> str(img)

Although included in the previous output, the size of the image can be extracted by:

>img@size

Then to extract the red channel from the image for the first ten rows:

> myextract <- img@red[1:10,]

Or to extract the entire red channel to an actual matrix:

> red.mat<-matrix(NA,img@size[1],img@size[2])

> red.mat<-img@red

## Wednesday, May 16, 2007

### Calculate turning angles and step lengths from location data

anglefun <- function(xx,yy,bearing=TRUE,as.deg=FALSE){## calculates the compass bearing of the line between two points

## xx and yy are the differences in x and y coordinates between two points

## Options:

## bearing = FALSE returns +/- pi instead of 0:2*pi

## as.deg = TRUE returns degrees instead of radians

c = 1

if (as.deg){

c = 180/pi

}

b<-sign(xx)

b[b==0]<-1 #corrects for the fact that sign(0) == 0

tempangle = b*(yy<0)*pi+atan(xx/yy)

if(bearing){

#return a compass bearing 0 to 2pi

#if bearing==FALSE then a heading (+/- pi) is returned

tempangle[tempangle<0]<-tempangle[tempangle<0]+2*pi

}

return(tempangle*c)

}

bearing.ta <- function(loc1,loc2,loc3,as.deg=FALSE){

## calculates the bearing and length of the two lines

## formed by three points

## the turning angle from the first bearing to the

## second bearing is also calculated

## locations are assumed to be in (X,Y) format.

## Options:

## as.deg = TRUE returns degrees instead of radians

if (length(loc1) != 2 | length(loc2) != 2 | length(loc3) !=2){

print("Locations must consist of either three vectors, length == 2,

or three two-column dataframes")

return(NaN)

}

c = 1

if (as.deg){

c = 180/pi

}

locdiff1<-loc2-loc1

locdiff2<-loc3-loc2

bearing1<-anglefun(locdiff1[1],locdiff1[2],bearing=F)

bearing2<-anglefun(locdiff2[1],locdiff2[2],bearing=F)

if(is.data.frame(locdiff1)){

dist1<-sqrt(rowSums(locdiff1^2))

dist2<-sqrt(rowSums(locdiff2^2))

}else{

dist1<-sqrt(sum(locdiff1^2))

dist2<-sqrt(sum(locdiff2^2))

}

ta=(bearing2-bearing1)

ta[ta < -pi] = ta[ta < -pi] + 2*pi

ta[ta > pi] = ta[ta > pi] - 2*pi

return(list(bearing1=unlist(bearing1*c),bearing2=unlist(bearing2*c),

ta=unlist(ta*c),dist1=unlist(dist1),dist2=unlist(dist2)))

}

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