[AMPL 2797] indexing expressions

[pioneer]

Hi,

I have a variable, say a[i,j] where i and j reside in sets A and B.

Suppose now that i create an index parameter which I call "point"
defined as follows

param k_ default 1;
for {i in A}{
for {j in B}{
let point[k_]:={(i,j)};
let k_:=k_+1;
}}
delete k_;

Next, I want to evaulate a[] at point. I tried a[point[k]] , however
this doesnt seem to work as point[k] is of the form (i,j) but
apparently you need to index variables with comma-separated lists. The
following ugly trick worked:

( j is fixed )
sum{(k,l) in point[j]} a[k,l]   .... this evaulated a[] at the correct
point (the sum is over one single element only because j is fixed)

Is there a better way to do this?


Thanks

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[pietro belotti]

This is probably uglier but, in my opinion, more readable: define TWO
indexed parameters, point1 and point2. Then

param k_ default 1;

for {i in A} {
  for {j in B} {
    let point1 [k_] := i;
    let point2 [k_] := j;
    let k_ := k_ + 1;
  }
}

Now "a [point1 [k], point2 [k]]" is what you look for.

Hope this helps.

On Sep 7, 3:13 am, pioneer <spencer.m...@...> wrote:

> Hi,
>
> I have a variable, say a[i,j] where i and j reside in sets A and B.
>
> Suppose now that i create an index parameter which I call "point"
> defined as follows
>
> param k_ default 1;
> for {i in A}{
> for {j in B}{
> let point[k_]:={(i,j)};
> let k_:=k_+1;}}
>
> delete k_;
>
> Next, I want to evaulate a[] at point. I tried a[point[k]] , however
> this doesnt seem to work as point[k] is of the form (i,j) but
> apparently you need to index variables with comma-separated lists. The
> following ugly trick worked:
>
> ( j is fixed )
> sum{(k,l) in point[j]} a[k,l]   .... this evaulated a[] at the correct
> point (the sum is over one single element only because j is fixed)
>
> Is there a better way to do this?
>
> Thanks

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