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MathGroup Archive 1996

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Re: Bug??

  • To: mathgroup at smc.vnet.net
  • Subject: [mg3845] Re: Bug??
  • From: danl (Daniel Lichtblau)
  • Date: Mon, 29 Apr 1996 00:32:51 -0400
  • Organization: Wolfram Research, Inc.
  • Sender: owner-wri-mathgroup at wolfram.com

In article <4l794q$1ov at ralph.vnet.net> pillay12 at shrike.und.ac.za (Theo  
Pillay) writes:
> Hi all,
> 
> I came across the following strange (and highly frustrating) behaviour  
while
> using MMA to do some simple substitutions. The relevant parts of the  
text
> file I was using are reproduced below.
> 
> f[x,y] = a[x] y + b[x];
> 
> F1 = f[x,y];
> 
> F2 = y2 (D[f[x,y], y]  - D[r[x], x]) + D[f[x,y],x];
> 
> F3 = r[x];
> 
> eight = -y2 A[x,y] D[F2,y] - 2 y2 B[x,y,y2] D[F3,y] +
>         2 y2 D[F2,y,x] + y2^2 D[F2,{y,2}] - A[x,y] D[F2,x] +
>         B[x,y,y2] D[F2,y2] -
>         2 B[x,y,y2] D[F3,x] - D[B[x,y,y2], y] F1 - D[B[x,y,y2], y2] F2
>         - D[B[x,y,y2],x] F3 + D[F2, {x,2}];  (* 1 *)
> 
> I get different answers for eight depending on the position of line (1).  
As
> shown I get the wrong answer, but it works if I move (1) up to the  
previous
> line (i.e next to F2). This is worse than irritating - anybody know  
what's
> happening here?
> 
> I'm using Ver 2.0 for DOS.
> 
> Thanks,
> Theo
> 
> 

  It appears you are using a text (rather than notebook) interface. The  
problem is that the newline after F2 terminates a command, and so the next  
line of input,
- D[B[x,y,y2],x] F3 + D[F2, {x,2}];   (* 1 *)
is treated as a separate command. By moving the minus sign up a line you  
can avoid this. Another tactic is to begin with a left paren and end with  
a right paren. The parser will of course recognize when they are not yet  
balanced, and hence not terminate the command at an earlier point (see p.  
61 of the ref manual).

Daniel Lichtblau
Wolfram Research, Inc.
danl at wolfram.com


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