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Please help me avoid loops and conditionals [message #36332]

Tue, 09 September 2003 09:44

pford
Messages: 33
Registered: September 1996

Member

Greetings,

I use IDL just frequently enough to know that my ingrained programming
style is sub optimal for IDL but not frequently enough to clearly see
how to improve it. What I want to do is have one object inside the
other, in this case two concentric ellipses, and fill them with
different values. This function will be invoked thousands of times if
not more, so any small improvement here will show significant..

In the example below I see 3 items that sould slow this down, the
array declaration, the loops and the conditional statements. (Note: I
have not tried running this yet since I don't currently have access to
the machine with IDL, so there are likely typo bugs, etc.)

function elp2, a, b, box_dim, vval, e_a,e_b, I_ratio
x_box = box_dim/2
box = intarr(box_dim,box_dim)
o_val = fix(vval / I_ratio)
v = fix(vval)
for i = 0, box_dim-1 do begin
for j = 0, box_dim-1 do begin
x = float(i - x_box)
y = float(j - x_box)
if( ((x/(a+e_a))^2 + (y/(b+e_b))^2) LE 1.0) then $
if( ((x/a)^2 + (y/b)^2) LE 1.0) then box(i,j) = o_val $
else box(i,j) = v
endfor; j = 0, box_dim-1 do
endfor; i = 0, box_dim-1 do
return, box
end

So how do I go about converting this into a Boolean matrix operation
that avoids all of this? Would it be faster to create a mask array
such as:

x = float((indgen(box_dim) ï¿½ box_dim/2) # replicate(1, box_dim))
y = (transpose(x) / b)^2
x = (x / a)^2
mask = where((x + y) LE 1.0)

?

Thanks

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[Message index]

		Please help me avoid loops and conditionals By: pford on Tue, 09 September 2003 09:44
		Re: Please help me avoid loops and conditionals By: pford on Sat, 13 September 2003 13:58
		Re: Please help me avoid loops and conditionals By: Chris Lee on Fri, 19 September 2003 02:40
		Re: Please help me avoid loops and conditionals By: David Fanning on Wed, 10 September 2003 10:06
		Re: Please help me avoid loops and conditionals By: Chris Lee on Wed, 10 September 2003 07:15

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