| a BUG or not a BUG in IDL ? [message #6644] |
Thu, 18 July 1996 00:00  |
Luis E. Liziola
Messages: 2
Registered: May 1996
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Junior Member |
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I have a "warning in loops" for you guys...
Check this "stupid" routine:
;---------------------------------------
print,'Integers'
J = 0
for I = 1,10 do begin
print,I
I = I + J
endfor
print,'Float'
J = 0.0
for I = 1,10 do begin
print,I
I = I + J ; Here I am changing the Type of "I"
endfor
print,'Long'
J = 0L
for I = 1,10 do begin
print,I
I = I + J ; Here I am also changing the type of "I"
;if i gt 20 then stop
endfor
end
;----------------------------------
The first loop if OK, the second loop stops after the first
iteration, but the last loop, just keeps going forever...
Any comments ?
At least should be a "WARNING" in the manual...
Luis
University of Colorado
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| Re: a BUG or not a BUG in IDL ? [message #6696 is a reply to message #6644] |
Fri, 26 July 1996 00:00  |
steinhh
Messages: 260
Registered: June 1994
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Senior Member |
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In article <4so62p$42j@reznor.larc.nasa.gov>, g.d.nowicki@larc.nasa.gov (Gregory D. Nowicki) writes:
|>
|> HOLD IT!!!
|>
|> You all seem to be missing the point. There is no language I know of that
|> recommends you modify a control variable within a FOR/DO loop. Results are
|> unpredictable. If fact, there are a fair number of compilers out there
|> that will produce an error if you mess with the control variable, not just a
|> warning.
|>
|> As with any language, there are ways to shoot yourself in the foot. IDL
|> just allows you to do it differently.
|>
As for comments to the original article, I definitely agree with the above.
If you want to mess with a FOR loop variable, make the whole thing into
a WHILE loop (or use GOTOs, I don't care as long as I don't have to maintain
the software). They might have mentioned it in the manual, but who would
read it before finding the problem first?
Stein Vidar Haugan
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