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Re: ReplaceAll behavour

• To: mathgroup at smc.vnet.net
• Subject: [mg82161] Re: ReplaceAll behavour
• From: Bill Rowe <readnewsciv at sbcglobal.net>
• Date: Sat, 13 Oct 2007 04:00:09 -0400 (EDT)

On 10/12/07 at 2:59 AM, maderri2 at gmail.com (magma) wrote:

>I do not understand what Mathematica 6 is doing here:

>a=5
>a/.a->3

>gives 3

>But...

>a+2/. a->3

>gives 7

>Why is that?

What is happening in each case is evaluation occurs before replacement.

=46or a/.a-> evaluation causes this to become

5/.5->3 then the replacement occurs to give the result 3.

=46or a+2/.a->3 evaluation gives

5+2/.5->3

Arithmetic operators such as + or - have higher precedence than
patter replacement. So, this becomes
7/.5->3. And finally since there is no 5 to replace, the result
is 7.

You can easily verify this is the way things work by looking at
the output from

Trace[a/.a->3] and
Trace[a+2/.a->3]
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