From help-octave-request at bevo dot che dot wisc dot edu Fri Jan 25 03:57:13 2002
Subject: Recursion / global variables trouble
From: "John W. Eaton" <jwe at bevo dot che dot wisc dot edu>
To: Pieter Thysebaert <pieter dot thysebaert at intec dot rug dot ac dot be>
Cc: help-octave at bevo dot che dot wisc dot edu
Date: Fri, 25 Jan 2002 03:56:32 -0600

On 25-Jan-2002, Pieter Thysebaert <pieter dot thysebaert at intec dot rug dot ac dot be> wrote:

| Hello,
| 
| I'm new to this mailing list, but I've already got some questions ;-)
| 
| The version of octave I'm using is
| GNU Octave, version 2.0.16 (i386-pc-linux-gnu)
| on Debian GNU/Linux 2.2
| 
| I needed to plot a function which is defined as N-1 nested sums (over the 
| variables x1,...,x(N-1) say).
| These values do not range over some interval independently of one another, 
| but their sum must equal a certain fixed number, say Q (so there are N 
| variables, but the last one can be calculated from the first N-1 vars)
| 
| So x1 ranges from 0 to Q, for every x1 value, x2 ranges from 0 to Q-x1, ....
| 
| However, N is a parameter and I would like to be able to plot this function 
| for different values of N.
| 
| The approach I came up with is to use a global vector of elements, to be used 
| to store the actual values of the iteration variables in some iteration.
| 
| Specifically, what I wrote looks like:
| 
| function res=calc_term_in_function_of_iteration_vars(z)
|    global iterationvars;
|    % calculate useful things and store into res 
| endfunction
| 
| function r=forloop(z,depth)
|     global iterationvars; %vector that holds the variable values
|     global N;
|     global Q;
| 
|     if (depth==N)
|         r = calc_term_in_function_of_iteration_vars(z);
|     else
|         for iterationvars(depth)=0:Q-sum(iterationvars(1:depth-1)) 
|            %for x1, where depth = 1,  sum(iterationvars(1:0)) will return 0=OK
|            %DEBUG printf
|            printf("Value of x%d is %d\n\r",depth,iterationvars(depth));
|            r = r+forloop(z,depth+1); 
|         endfor                
|     endif  
| endfunction
| 
| To evaluate the function in a point z, I call forloop(z,1) (after having 
| initialized the needed global variables as global)
| 
| As you can guess from where I inserted the printf, things don't work like I 
| expected them to do: 
| I have tried it with N=2 and Q=2, and then for each iteration at depth 1 
| (there are three terms : x1 = 0,1,2=Q) I get
| Value of x1 = 0
| 
| So somehow  iterationvars(depth) is not adjusted properly (in the for...line) 
| Can anybody help me with this parameter passing /global variable issue ?

I'm not sure I follow what you're trying to do, but it seems to me
that there is a bug in Octave 2.0.x that prevents code like

  for x(i) = 1:n

from working correctly, though it seems to work in the current
development version (2.1.35):

  2.0.16:

    octave:4> x = ones (1,3); i = 3; n = 5; for x(i) = 1:5 x(i) end
    ans = 1
    ans = 1
    ans = 1
    ans = 1
    ans = 1
    octave:5> x
    x =

      1  1  1


  2.1.35 (this is the behavior I would expect):

    octave2.1:1> x = ones (1,3); i = 3; n = 5; for x(i) = 1:5 x(i) end
    ans = 1
    ans = 2
    ans = 3
    ans = 4
    ans = 5
    octave2.1:2> x
    x =

      1  1  5


I suppose a workaround would be to write

  for tmp = 1:n
    x(i) = tmp;
    ...


If this isn't the problem, then can you please post a complete working
example that demonstrates the problem?

Thanks,

jwe



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