Re: Function Doesn't Evaluate Local Variables?

• To: mathgroup at smc.vnet.net
• Subject: [mg124676] Re: Function Doesn't Evaluate Local Variables?
• From: Brentt <brenttnewman at gmail.com>
• Date: Tue, 31 Jan 2012 05:35:28 -0500 (EST)
• Delivered-to: l-mathgroup@mail-archive0.wolfram.com
• References: <CAFJC7Xr9ACFiAvBapMcyxMzXgpwRymVc_XMTKkXFeu=MbQi3rw@mail.gmail.com>

```Sorry this
ReplacePart[#1, n -> p #1[[n]] + q #1[[m]]&

should be this

ReplacePart[#1, n -> p #1[[n]] + q #1[[m]]]&

On Mon, Jan 30, 2012 at 10:37 AM, Brentt <brenttnewman at gmail.com> wrote:

> Hello, this result is puzzling me.
> I'm trying to make a function that outputs a pure function.
>
> The inputs are two expressions of the form
>
> input1: p r[n] + q r[m]
> input2: r[n]
>
> And the output should be a pure function of the form:
> ReplacePart[#1, n -> p #1[[n]] + q #1[[m]]&
>
>
> This was my attempt:
>
> row[n_] := Hold[Part[Slot[1], n]];
> rpl[old_, new_] := Block[{index, newrow},
>    index = new[[1]];
>    newrow = old /. r -> row;
>    ReplacePart[#1, index -> newrow] &
>    ];
>
> But when using rpl,  for some reason doesn't evaluate index and newexp,
> and just gives me this
>
> In[1]:= rpl[2 r[1] + 3 r[2] , r[1]]
> Out[1]= ReplacePart[#1, index -> newrow] &
>
> Why did it not give the appropriate expressions for "new row" and "index"
> but instead just output the names of the variables?
>
> In case you are curious, I'm trying to write a program that takes a list
> of elementary row opeartion, specified by a list of the form
> operationSequence ={
> 1 r[1] + 2 r[2] -> r[1],
> r[1] <-> r[2],
> 2 r[1] -> r[1]
> }
> and transforms each row operation in the list into a function, and uses
> compose list to output a nicely formatted sequence of row operations to
> Latex. Any pointers for a more elegant way to do this would be most
> welcome.
>
>
>

```

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