| Re: where help [message #7999 is a reply to message #7998] |
Mon, 27 January 1997 00:00  |
davidf
Messages: 2866
Registered: September 1996
|
Senior Member |
|
|
Phil Williams <williams@irc.chmcc.org> writes:
> How do I get the x,y coords from the result of where?
>
> Here's what I did:
> IDL> t = where(slice eq max(slice))
I'm really glad you asked this question! I am sure there is
some nifty algorithm that everyone learns in CS101. But
since I taught myself how to program, I don't know it.
And I've always wanted to figure it out. So here goes...
Here is what I know (for a 2D array!). Arrays are stored in
row order, so the column index varies the fastest. Suppose
I create a 3 column by 4 row array and get its size,
like this:
data = FINDGEN(12)
data = REFORM(data, 3, 4)
PRINT, data
0.00000 1.00000 2.00000
3.00000 4.00000 5.00000
6.00000 7.00000 8.00000
9.00000 10.0000 11.0000
s = SIZE(data)
OK, suppose I now do this:
index = WHERE(data EQ 6)
The WHERE function returns the 1D index into the 2D array.
PRINT, index
6
So, the number 6 is located in index 6 (the 7th number in
the array with zero-based subscripting). What is its
2D subscript? Well, if I divide index by how many columns
there are in the array, and then take the whole part of that
number, I will know its row number. In IDL terms:
row = FIX(FLOAT(index)/s(1))
PRINT, row
2
To find the column number, I multiply the row number times
the number of columns in the array, and subtract that value
from the index. Again, in IDL terms:
col = index - (row * s(1))
PRINT, col
0
That seem's right. Let's test it in the real world.
seed = -1L
data = RANDOMU(seed, 50, 100)
s = SIZE(data)
index = 124
PRINT, data(index)
0.973124
Now for the test:
row = FIX(FLOAT(index)/s(1))
col = index - (row * s(1))
PRINT, col, row
24 2
PRINT, data(col, row)
0.973124
Hooray!
I don't know if its the algorithm they teach in CS101,
but it works for me. :-)
Cheers!
David
-----------------------------------------------------------
David Fanning, Ph.D.
Fanning Software Consulting
2642 Bradbury Court, Fort Collins, CO 80521
Phone: 970-221-0438 Fax: 970-221-4762
E-Mail: davidf@dfanning.com
Coyote's Guide to IDL Programming: http://www.dfanning.com
-----------------------------------------------------------
|
|
|
|
comp.lang.idl-pvwave archive
Members
Search
Help
Login
Home
