RE: constant -> constant
- To: mathgroup at smc.vnet.net
- Subject: [mg41963] RE: [mg41945] constant -> constant
- From: "David Park" <djmp at earthlink.net>
- Date: Wed, 11 Jun 2003 13:17:58 -0400 (EDT)
- Sender: owner-wri-mathgroup at wolfram.com
Carlos,
It is not a bug. I think it is correct behavior. Having set b to a value you
can't expect it to behave like a symbol without a value.
a = b = 0;
a /. b -> 1 // Trace
{{a, 0}, {{b, 0}, 0 -> 1, 0 -> 1}, 0 /. 0 -> 1, 1}
a was zero, but then you told Mathematica to changes 0's to 1's. Mathematica
evaluates the lhs of rules before applying them. When you preserve b as a
symbol by using HoldPattern, you do get 0 as a result.
a /. HoldPattern[b] -> 1 // Trace
{{a, 0}, {HoldPattern[b] -> 1, HoldPattern[b] -> 1},
0 /. HoldPattern[b] -> 1, 0}
If you expect to use a symbol for symbolic manipulation don't Set it to a
value. Use rules in the first place for value substitution, or use it as a
parameter in a function definition.
David Park
djmp at earthlink.net
http://home.earthlink.net/~djmp/
From: Carlos Felippa [mailto:carlos at colorado.edu]
To: mathgroup at smc.vnet.net
I traced a bug in a large program to an unexpected
substitution. This gives the idea:
a=b=0; Print [a/.b->1];
1
Is this correct behavior and, is there a way to guard
against this happening?