From bug-octave-request at bevo dot che dot wisc dot edu Thu Dec 11 23:09:47 2003
Subject: more indexing weirdness
From: "John W. Eaton" <jwe at bevo dot che dot wisc dot edu>
To: Paul Kienzle <pkienzle at users dot sourceforge dot net>
Cc: bug-octave at bevo dot che dot wisc dot edu
Date: Thu, 11 Dec 2003 23:09:38 -0600

On 11-Dec-2003, Paul Kienzle <pkienzle at users dot sourceforge dot net> wrote:

| octave-2.1.52, Debian linux.
| 
| Shouldn't the following add 5?
| 
| 	octave:24> x=eye(2); x([1,1,1,1,1])+=1
| 	x =
| 	  2  0
| 	  0  1
| 
| 	octave:25> x=[1:4]; x([1,1,1,1,1])+=1
| 	x =  2  2  3  4

No, because this expression is equivalent to 

  x([1,1,1,1,1]) = x([1,1,1,1,1]) + 1

for x = [1:4], the RHS evaluates to

  [ 2  2  2  2  2 ]

It is equivalent to writing

  [x(1), x(1), x(1), x(1), x(1)] + 1

If you do something like

  x([1,1,1,1,1]) =  [ 2  2  2  2  2 ]

then you copy the RHS elements to the corresponding element of X.  It
is equivalent to writing

  x(1) = 2
  x(1) = 2
  x(1) = 2
  x(1) = 2
  x(1) = 2

I know there are other possible interpretations of what this syntax
could mean, but I think this is what Matlab does.  (I know, it does
not have +=, but if we agree that x OP= y is equivalent to x = x OP
y, then I think this is how things must work.)

jwe



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