[R] R Newbie Question

[Nordlund, Dan (DSHS/RDA)]

> -----Original Message-----
> From: r-help-bounces at r-project.org 
> [mailto:r-help-bounces at r-project.org] On Behalf Of Johannes Bauer
> Sent: Monday, October 20, 2008 10:42 AM
> To: r-help at r-project.org
> Subject: [R] R Newbie Question
> 
> Hello list,
> 
> I just started R today and tried something quite simple. I wanted to
> create a colored plot and eventually after hours of fiddling 
> around got
> it working. However, my solution seems very suboptimal and I'd really
> appreciate your hints on how to improve. I believe that R 
> already offers
> many functions I coded (e.g. distance between two vectors, vector
> length, vector normalization and so on). I generally didn't 
> even figure
> out how to create a simple vector 

See ?c (i.e. type "?c" without quotes at R prompt)

star1center = c(-0.5, 0)

or how to extract a row 
> from a matrix
> so the result is a vector (to get scalars, I use the "sum" functions,
> which is an incredibly ugly workaround).

In your code a[] is not a matrix, it is a data frame.  However, you should be able to index it just like you would a matrix.  For your code,

b[x, y] = gravitation(a[x,1], a[y,1])

Without the need for the sum() function.

> 
> To sum the problem up: one has a binary star system with a large star
> (e.g. red giant) and a small star (e.g. white dwarf). Gravitation
> between them is directly proportional to the mass and indirectly
> proportional to the square of the distance. If correctly plotted, one
> should be able to see the inner lagrange point L1 which is the point
> where the gravitational potentials of the stars "cancel out", e.g. an
> object would not be attracted to any star. Well, enough background
> information, here's my rookie code - please feel free to comment on
> anything :-)
> 
> Kind regards,
> Johannes
> 
> 
> 
<<<snip>>>

I have edited your code to simplify some of your functions and use vectorized computations.  I'm sure others could provide even more optimized code.  I will leave it to others to respond concerning already written functions for norm, len, etc.  You could try using the various search functions with key terms (like distance) to see what may be available.

star1center = c(-0.5, 0)
star1mass = 30

star2center = c(0.5, 0)
star2mass = 1

sqr = function(x) {
	return(x * x)
}

distance = function(a, b) {
	return(sum((a-b)^2)^0.5)
}

len = function(x) {
	return(sum(x^2)^0.5)
}

norm = function(x) {
	return(x / len(x))
}

gravitation = function(invecx, invecy) {
	invec = c(invecx, invecy)
	vec1 = star1mass * norm(star1center - invec) / sqr(distance(invec,star1center))
	vec2 = star2mass * norm(star2center - invec) / sqr(distance(invec,star2center))
	return(len(vec1 + vec2))
}

vmin = -1
vmax = 1
step = 0.1
vals = ((vmax - vmin) / step) + 1

xvals = seq(vmin, vmax, step)
yvals = seq(vmin, vmax, step)

a = expand.grid(seq(vmin, vmax, step), seq(vmin, vmax, step))
b = matrix(seq(1, vals*vals), vals)

for (x in 1:vals) {
	for (y in 1:vals) {
		b[x, y] = gravitation(a[x,1], a[y,1])
	}
}
filled.contour(xvals, yvals, z = b, color = heat.colors, ylim = c(-1,
1), xlim = c(-1, 1), zlim = c(0, 100), nlevels = 100)

Hope this is helpful,

Dan  

Daniel J. Nordlund
Washington State Department of Social and Health Services
Planning, Performance, and Accountability
Research and Data Analysis Division
Olympia, WA  98504-5204