Re: Help-octave Digest, Vol 111, Issue 27

[Dave Cottingham]

---------- Forwarded message ----------
From: Vic Norton <address@hidden>
To: help-octave Octave <address@hidden>
Cc: 
Date: Wed, 10 Jun 2015 07:03:03 -0400
Subject: Re: a try-if construction

On Jun 9, 2015, at 5:32 PM, Vic Norton <address@hidden> wrote:

> The perl code
>
>   my @K= qw( 1   30   48 );
>   my $n = 109;
>   my ($k0, $k1);
>   my $i = 0;
>   for (my $i = 0; $i < @K; $i++) {
>     $k0 = $K[$i];
>     unless ($k1 = $K[$i + 1]) { $k1 = $n }
>     printf("i = %d, k0 = %d, k1 = %d\n", $i + 1, $k0, $k1);
>   }
>
> and the octave code
>
>   K = [ 1   30   48 ];
>   n = 109;
>   for i = 1 : length(K)
>     k0 = K(i);
>     try k1 = K(i + 1); end
>     if k1 == k0; k1 = n; endif
>     printf("i = %d, k0 = %d, k1 = %d\n", i, k0, k1);
>   endfor
>
> produce exactly the same results:
>
>   i = 1, k0 = 1, k1 = 30
>   i = 2, k0 = 30, k1 = 48
>   i = 3, k0 = 48, k1 = 109
>
> I am very comfortable with the "unless" line in perl. I am much less comfortable with the "try-if" lines in octave. Is this "try-if construction the best way to do what I want to do?

On Jun 10, 2015, at 3:53 AM, JokerOne <address@hidden> wrote:

> Hi Vic,
>
> to be honest, I do not fully understand what you actually want to do?
>
> For the output you got, maybe something like that is more straightforward?
>
>   K = [ 1   30   48 ];
>   n = 109;
>   K_helper = [K,n]; % "attach" n to K
>
>   for i = 1 : (length(K_helper)-1)
>
>       k0 = K_helper(i);
>       k1 = K_helper(i+1);
>       printf("i = %d, k0 = %d, k1 = %d\n", i, k0, k1);
>
>   endfor

I guess I wasn't very clear. Suppose you have a list of "actions" you want to take and a default action in case one or more of the actions is impossible. In Perl you simply run through the list, taking the default action UNLESS you CAN take the listed action.

How do I do the same thing in Octave? I want to TRY each action and, if I CAN'T take it, take the default. The Perl mechanism is extremely simple and logical. Is there a simple and logical Octave mechanism to implement this process?

Now I think I've answered my question. The following TRY-IF Octave construction seems to duplicate the Perl mechanism.

   K = [ 1   30   48 ];
   n = 109;
   for i = 1 : length(K)
     k0 = K(i);
     k1 = n;                     % take the default action
     try k1 = K(i + 1); end      % unless the action works
     printf("i = %d, k0 = %d, k1 = %d\n", i, k0, k1);
   endfor

with the output

   i = 1, k0 = 1, k1 = 30
   i = 2, k0 = 30, k1 = 48
   i = 3, k0 = 48, k1 = 109

I think what you have in mind could be done by including the "catch" clause of the try-catch statement.

try

  ... some statements here, if they hit an error then ...

catch

  ... these statements get executed ...

end_try_catch

Not sure what the intent of that Perl construction is, but octave's try-catch is aimed at exception handling. Still, it does what you want (if I understand what you want).

 - Dave Cottingham