Variable Type conversions and ATAN function

["Jason P. Meyers" <jpm7934(at)cis.rit.edu>]

Hello,

	I recently discovered an interesting curiosity while trying to debug
one of my homework problems.  It turns out the problem was partly my
fault (that's now fixed) and partly IDL's as best I can figure out. 
Below, you will find part of a journal file I was working with which
demonstrates the curiosity I discovered.  The curiosity is that
sometimes ATAN(0.0,0.0) = 0.0 and other times ATAN(0.0,0.0) = 3.14159. 
I can't figure out what causes the ATAN to return 0.0 vs 3.14159.  In
the following example, the third element of FK1 vs FK2 is a good
example.  I have tried this with different fn vectors and get similar
results.  Any and all insight would be greatly appreciated.

Thanks,
Jason Meyers
Ph.D. Student, Center for Imaging Science
Rochester Institute of Technology
jpm7934@rit.edu


Program listing:

; IDL Version 5.3.1 (Win32 x86)
; Journal File for jason@BLACKBIRD
; Working directory: E:\Jason\Classes\Dip\idlhw\hw6
; Date: Fri Nov 03 12:33:29 2000

; Create a simple vector
fn=float([1,1,1,1,1,0,0,0,0,0])
; Take its FFT
fk=fft(fn)

; Here is where the fun begins.  I am attempting to set a zero threshold
value
; for the complex vector fk.
;
; NOTE:  The following line of code should produce the same output as
the
;        for loop.  However, it does not.  The difference is not
detectable
;        until the FK array is passed to the ATAN function later on.
fk1=fk*complex((abs(fk) gt 1e-4),0.0)
; make a copy
fk2=fk
for ii = 0,N_elements(fn)-1 do if abs(fk2[ii]) lt 1.0e-4 then
fk2[ii]=complex(0,0)

; Calculate the phase of each element in both vectors.
fkph1=atan(imaginary(fk1),float(fk1))
fkph2=atan(imaginary(fk2),float(fk2))
; Print the various arrays
print,'FK:'
print,fk
print,'FK1:'
print,fk1
print,'FK2:'
print,fk2
print,'Phase of FK1:'
print,fkph1
print,'Phase of FK2:'
print,fkph2
print,'Difference:  FK1 - FK2:'
print,fk1 - fk2


IDL Output:

IDL> @test
FK:
(     0.500000,     0.000000)(     0.100000,   
-0.307768)(-3.72529e-009,     0.000000)(     0.100000,   -0.0726543)(
7.45058e-009,     0.000000)
(     0.100000,     0.000000)( 7.45058e-009,     0.000000)(    
0.100000,    0.0726543)(-3.72529e-009,     0.000000)(     0.100000,    
0.307768)
FK1:
(     0.500000,     0.000000)(     0.100000,    -0.307768)(    
0.000000,     0.000000)(     0.100000,   -0.0726543)(     0.000000,    
0.000000)
(     0.100000,     0.000000)(     0.000000,     0.000000)(    
0.100000,    0.0726543)(     0.000000,     0.000000)(     0.100000,    
0.307768)
FK2:
(     0.500000,     0.000000)(     0.100000,    -0.307768)(    
0.000000,     0.000000)(     0.100000,   -0.0726543)(     0.000000,    
0.000000)
(     0.100000,     0.000000)(     0.000000,     0.000000)(    
0.100000,    0.0726543)(     0.000000,     0.000000)(     0.100000,    
0.307768)
Phase of FK1:
     0.000000     -1.25664      3.14159    -0.628318     0.000000    
0.000000     0.000000     0.628318     0.000000      1.25664
Phase of FK2:
     0.000000     -1.25664     0.000000    -0.628318     0.000000    
0.000000     0.000000     0.628318     0.000000      1.25664
Difference:  FK1 - FK2:
(     0.000000,     0.000000)(     0.000000,     0.000000)(    
0.000000,     0.000000)(     0.000000,     0.000000)(     0.000000,    
0.000000)
(     0.000000,     0.000000)(     0.000000,     0.000000)(    
0.000000,     0.000000)(     0.000000,     0.000000)(     0.000000,    
0.000000)