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# Re: Altering terms of an Expression
*To*: mathgroup@smc.vnet.net
*Subject*: [mg11866] Re: Altering terms of an Expression
*From*: "C. Woll" <carlw@u.washington.edu>
*Date*: Fri, 3 Apr 1998 03:45:26 -0500
*Organization*: University of Washington
*References*: <6fsjtm$gpn@smc.vnet.net>
Hi,
How about the following.
1) If your terms are being added together, why don't you just do
Do[ answer=Expand[gm[[k,l]]*f[k]*f[l]];,
{k,1,n},{l,k+1,n}]
To answer you specific question about Length, you could try creating
your own function
SRlength[a_Plus]:=Length[a]
SRlength[_]:=1
2,3) I think the following code does what you ask. Define a function g
which takes two arguments, the index and the string, with the following
properties
g /: g[n_,"newstring"] g[n_,a_] := g[n,a] g /: g[n_,a_] g[n_,b_] := g[n,
a<>b]
You could also use format to have the output appear the way you wanted,
that is
Format[g[n_,a_]] := a[n]
Then, define a fill variable
fill = Product[g[n,"newstring"],{n,4}]
where you can replace 4 with the maximum allowed index.
Now,
g[3,"astring"] g[1,"bstring"] g[1,"cstring"] fill
produces
bstringcstring[1] newstring[2] astring[3] newstring[4]
Internally, the output is represented as
g[1,"bstringcstring"] g[2,"newstring"] g[3,"astring"] g[4,"newstring"]
Is this what you were looking for?
Carl Woll
Dept. of Physics
U. of Washington
On 1 Apr 1998, S. Rempe wrote:
> Hi,
> There are 3 things I want to do with an expression.
> The expression may have 1 or more terms added together, each
> term is composed of a product of elements.
>
>
> 1). I want to multiply each term in the expression by a product of
> functions. For example, the expression labelled by integers k,l
> might look like
>
> gm[[k,l]]= a*b*c
>
> I want it to become
>
> gm[[k,l]]= a*b*c*f[k]*f[l]
>
>
> I have written a Do loop that doesn't work because I wish that
> Length would output the number of terms added in the expression
> (1 term in above expression), but Length gives 3 for the expression
> above.
>
> Do[{
> answer=gm[[k,l]][[i]]*f[k]*f[l];
> },{k,1,n},{l,k+1,n},{i,1,Length[gm[[k,l]]]} ];
>
>
> This works fine when the expression contains 2 or more terms.
> How do I make it work (automatically) even in the case of 1
> term in the expression?
>
> 2). I want to order each term in the expression by the index.
> For example, say the expression is in alphabetical order,
>
> astring[3]*bstring[1]
>
> I want it ordered numerically
>
> bstring[1]*astring[3]
>
>
> 3). Then in each term of the expression, I want to combine
> the strings that refer to the same index and multiply in
> a new string if the index is missing.
>
> For example, the expression in numerical order from the
> last step might be,
>
> bstring[1]*cstring[1]*astring[3]
>
> I want it to become
>
> bstringcstring[1]*newstring[2]*astring[3]
>
>
> To summarize:
>
> In Step 1 Length[expression] gives the number of terms
> summed together in expression except when there is only 1 term.
> I need to know how to get around that.
>
> The result of Step 3, merging strings with the same index, is the
> other thing I want. Numerical ordering of the indices is not
> required in the result, I just thought it might make Step 3
> easier to accomplish.
>
> Thanks.
>
> Susan Rempe
> rempe@u.washington.edu
>
>
>
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