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MathGroup Archive 2007

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Re: ReplaceList and //.

  • To: mathgroup at smc.vnet.net
  • Subject: [mg73600] Re: ReplaceList and //.
  • From: dh <dh at metrohm.ch>
  • Date: Fri, 23 Feb 2007 04:24:05 -0500 (EST)
  • References: <erjo1u$nn5$1@smc.vnet.net>


Hi Ajit,

ReplaceList applies the first rule in all possible ways, then the 

second,.. and returns a list of all the results where a replacement has 

been done.

//. Applies the first rule, then to the result the second rule,.. After 

this it starts over with the first rule until nothing changes anymore. 

It then returns the last expression.



Mr Ajit Sen wrote:

> Dear Mathgroup,

> 

>  Could someone please enlighten me as to when I should

> use  ReplaceList and when to use //.  in pattern

> matching.

> 

>   Thus, if  Y = a x^2/b^3,  then

> 

>    ReplaceList[Y, a^m_. x^n_. b^p_. :> z 

>     b^(p + 1) a^(m - 1) x^(n - 1)][[1]]

> 

>  as well as 

>    

>   Y //. a^m_. x^n_. b^p_. :> z b^(p + 1) a^(m - 1)

> x^(n - 1)

>   

> give the same result.

> 

>  However,

> 

>      ReplaceList[a*b*c, {(x_)*(y_) -> x^y, (x_)*(y_)

> -> x + y}]

> 

>  and 

>       a*b*c //. {(x_)*(y_) :> x^y, (x_y)*_ :> x + y}

> 

>   yield different outputs.

> 

>  What am I missing here?

> 

>   Thanks in advance.

> 

> 

>   Ajit Sen.

> 

> 

> 

> 

> 

> 		

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