From bug-octave-request at che dot utexas dot edu  Wed Oct 13 15:23:39 1993
Subject: exist bug?
From: donoho at gandalf dot Berkeley dot EDU (David Donoho)
To: bug-octave at che dot utexas dot edu
Date: Wed, 13 Oct 93 13:22:20 PDT


For a library of signal processing stuff in matlab
I developed tools which use the idiom

if exist('n')==0,
    n = input('n?');
end

to set variables which are so far unset. These don't seem
to work in OCTAVE. Example:
%======= TrialSignals.m =========
% TrialSignals: generate various signals for
%    time-frequency analysis ...
%
if(exist('n') == 0),
     n = input('signal length ?')
end
t = (1:n) ./n;
% Sinusoids
HiSine = sin( pi * (n * .6902) .* t);
LoSine = sin( pi * (n * .3333) .* t);
% Chirps
LinChirp = sin(250 .* t .^2);
TwoChirp = sin(250 .* t .^2) + sin(100 .* t .^2);
QuadChirp = sin( 100 .* (2 .* t) .^3);
%   Singularities
Leopold = (t == floor(.37 * n)/n);  % Kronecker
Ramp = t - (t >= .37);
Crease = exp(-4*abs(t - .37));
Cusp = sqrt(abs(t - .37));
k = floor(n * .37);
Sing = 1 ./abs(t - (k+.5)/n);



===========================================
this m-file generates the following script

gandalf.donoho% octave
Octave, version 0.76.  Copyright (C) 1992, 1993, John W. Eaton.
This is free software with ABSOLUTELY NO WARRANTY.
For details, type `warranty'.

octave:1> startoct
LOADPATH = ~/octave:.:/lion/g/app/octave/scripts
octave:2> TrialSignals
error: `n' undefined near line 4 column 8
error: invalid null value in colon expression near line 4 column 7
error: evaluating assignment expression near line 4, column 3
octave:3> who
octave:4> exist('n')
ans = 1
octave:5> for i=1:10, exist('n'),end
ans = 1
ans = 1
ans = 1
ans = 1
ans = 1
ans = 1
ans = 1
ans = 1
ans = 1
ans = 1
octave:6> who

*** local symbols:

i

octave:7> n
error: `n' undefined near line 7 column 1
octave:8> for i=1:10,exist('Beufort'), end
ans = 0
ans = 0
ans = 0
ans = 0
ans = 0
ans = 0
ans = 0
ans = 0
ans = 0
ans = 0
octave:9> for i=1:10, exist('blotz'), end
ans = 0
ans = 0
ans = 0
ans = 0
ans = 0
ans = 0
ans = 0
ans = 0
ans = 0
ans = 0
octave:10> exist('n')
ans = 0
octave:11> TrialSignals
error: `n' undefined near line 4 column 8
error: invalid null value in colon expression near line 4 column 7
error: evaluating assignment expression near line 4, column 3
octave:12> exist('n')
ans = 1
octave:13> n = 2048
n = 2048
octave:14> TrialSignals
octave:15> who

*** local symbols:

Crease     HiSine     LinChirp   QuadChirp  Sing       i          n
Cusp       Leopold    LoSine     Ramp       TwoChirp   k          t

octave:16> quit
gandalf.donoho% 

========================================================

In short, even when Octave returns an empty who, exist('n') returns
1.    But sometime later, it starts to return 0.

Could it be that variables which are referred to in an m-file
are thought by Octave to ``exist'' from the moment the m-file is entered,
even if no code referring to them has yet been processed?

Could it be that if m-file execution is terminated due to
error, that symbols have ghost lives that make them continue
to ``exist''?

Dave