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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```

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