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From: Leonid Shifrin on 10 Jun 2010 08:07
Hi Stuart,

for completeness, let me mention that my function will fail in case when
some of the <aelement[i]> has some value at the moment when we create our
delayed assignment. Observe:

In[1]:=
Table[aelement[i] = i, {i, 3}]

Out[1]= {1, 2, 3}

In[2]:=
Scan[assignElements[#, "element", 3] &, {"a", "b"}]

In[3]:= ?a

Global`a

a:={1,2,3}

This is probably not what you would like. Here is a somewhat more
sophisticated version that is free from this flaw:

Clear[assignElementsFull];
assignElementsFull[name_String, elemName_String, length_Integer] :=
With[{heldName = ToExpression[name, InputForm, Hold]},
Clear @@ heldName;
With[{heldElementBaseName =
ToExpression[name <> elemName, InputForm, Hold]},
Block @@ Append[Map[List, heldElementBaseName],
Unevaluated[
Evaluate[First(a)heldName] :=
Evaluate[
Table[First[heldElementBaseName][i], {i, length}]]]]]];

For example:

In[6]:=
Clear[a, aelement];
Table[aelement[i] = i, {i, 3}];
assignElementsFull["a", "element", 3]

In[9]:= ?a

Global`a

a:={aelement[1],aelement[2],aelement[3]}

It will work even in this case:

In[10]:=
aelement = 1;

In[11]:= assignElementsFull["a", "element", 3]

In[11]:= ?a

Global`a

a:={aelement[1],aelement[2],aelement[3]}


Regards,
Leonid





On Wed, Jun 9, 2010 at 4:20 AM, Stuart Nettleton <
Stuart.Nettleton(a)uts.edu.au> wrote:

> Hi Leonid,
> What a wonderful little function!
> Just the thing.
> Many thanks indeed,
> Stuart
>
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From: Stuart Nettleton on 10 Jun 2010 08:09
Hi Lenoid,
Your extended function is much appreciated. It will be even more useful
now you have generalised it. I have avoided using Hold until now because
it seemed to be quite an advanced topic. As you will see from my next
question, I am still a bit vague when it comes to this function.
My intended use for your assignElementsFull function is in a modelling
application that I simplify to the following for illustration:
Clear[Prev, eqns, a, b];
Prev[vec_] := Flatten[{First[vec], Most[vec]}];
eqns = {a == Prev[b], b == Table[1, {3}]};
lhs = Map[ToString[#] &, Cases[eqns, Equal[m_, n_] -> m]]
Scan[assignElementsFull[#, "element", 3] &, lhs]
eqns2 = Flatten[Map[Thread[#] &, eqns]]
Solve[eqns2, Flatten[lhs]]

I would like to isolate the left hand side of the equations without
evaluation, and then use these variables to create element vectors. Then I
would like to use the variables in the vector form. My experiments using
Hold to eliminate the errors have not yet been satisfactory. Please excuse
my laborious syntax!
Would you be able to suggest a way forward?
Many thanks,
Stuart

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From: Stuart Nettleton on 10 Jun 2010 08:10
Hi Leonid,
I have found a possible solution:

ClearAll[Prev, eqns, a, b];
Prev[vec_] := Flatten[{First[vec], Most[vec]}];
eqns = Hold[
a == Prev[b],
b == Table[1, {3}]
];
lhs = Map[ToString, Cases[eqns, Equal[m_, n_] -> m]]
Scan[assignElementsFull[#, "element", 3] &, lhs]
eqns2 = Apply[List, Map[ReleaseHold, eqns]]
lhs2 = Cases[eqns2, Equal[m_, n_] -> m]
Solve[eqns2, Flatten[lhs2]]

Stuart

On Thu, 10 Jun 2010 10:28:21 +1000, Stuart Nettleton
<Stuart.Nettleton(a)uts.edu.au> wrote:

> Hi Lenoid,
> Your extended function is much appreciated. It will be even more useful
> now you have generalised it. I have avoided using Hold until now because
> it seemed to be quite an advanced topic. As you will see from my next
> question, I am still a bit vague when it comes to this function.
> My intended use for your assignElementsFull function is in a modelling
> application that I simplify to the following for illustration:
> Clear[Prev, eqns, a, b];
> Prev[vec_] := Flatten[{First[vec], Most[vec]}];
> eqns = {a == Prev[b], b == Table[1, {3}]};
> lhs = Map[ToString[#] &, Cases[eqns, Equal[m_, n_] -> m]]
> Scan[assignElementsFull[#, "element", 3] &, lhs]
> eqns2 = Flatten[Map[Thread[#] &, eqns]]
> Solve[eqns2, Flatten[lhs]]
>
> I would like to isolate the left hand side of the equations without
> evaluation, and then use these variables to create element vectors. Then
> I would like to use the variables in the vector form. My experiments
> using Hold to eliminate the errors have not yet been satisfactory.
> Please excuse my laborious syntax!
> Would you be able to suggest a way forward?
> Many thanks,
> Stuart


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PhD, FCPA, MBA, MEngSci, BEng(Hons), GradDipAICD
Senior Lecturer
School of Management & Systems
Faculty of Engineering & IT
Energy Policy Research Centre

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