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Pipe Signs and Dollar Signs (Expected Changes for 4.97)

4 months ago
21 July 2023
4 replies
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Userlevel 5

gabiservidone
AIMMSian
82 replies

Beginning on 4.97 AIMMS version, warning and possibly errors will reinforce the differences between the pipe sign (|) and the dollar sign ($).

A new article discusses deeply the historical reasons and the differences, but you can also have a peak of the differences below.

Dollar Sign

The $ is used to impose a condition on a part of an expression as in:

A(i) := B(i) $ C(i);

This is semantically the same as:

A(i) := if C(i) then B(i) else 0 endif;

Pipe Sign

The operator | is used to restrict an index domain. An index domain can be specified at several places in the model:

At the left-hand-side of a multi-dimensional assignment:
A((i,j) | restriction(i,j)) := rhs(i,j)
In an iterative operator:
sum((i,j) | restriction(i,j), term(i,j) )
In the index domain attribute of an identifier:
Index domain: (i,j) | restriction(i,j)
In a for statement:
for (i,j) | restriction(i,j) do ! some code endfor;
In a constructed set expression:
MySet := { i | restriction(i) } ! or MyRelation := { (i,j) | restriction(i,j) }

4 replies

What are the performance differences between the alternatives? Clearly, the operator “|” is better than a “for” loop, but is there a performance benefit of using the operator “$” instead of an “if” clause? I suspect that “$” and “if” are simply a matter of taste and, if so, I find using an if clause is much more transparent and leads to fewer misinterpretations. Thus, “$” is best avoided. Is this a correct view?

Userlevel 2

There is no performance difference between writing either:

x := a $ b;
x := if b then a endif;
x := a onlyif b;

The resulting compiled expression is the same for all three.

So it is indeed a matter of taste which one you prefer. Personally, I think the ‘if’ variant is much too verbose so I have preference for using the $.

However there is a difference between using the ‘if’ expression and the ‘if’ statement. So writing

if a then x:=b; else x:=0; endif;

Is definitely performing worse than writing it as a single assignment like above.

Userlevel 5

Good to know the performance difference between an if-then-else expression and statement in mind. Thanks for pointing out.

I agree that choosing between the three different forms of expression is a matter of taste, but my preference is different. I prefer the if-then variant as it makes it easier to explain a model to stakeholders, as stakeholders are often not familiar with AIMMS, but they are familiar with some programming language.

Hi guys,

Thanks for the discussion. I would add another option which we have found to be significant when handling large matrices. This involves a separate assignment statement and an override statement, both of which enable the AIMMS sparcity logic:

x := b;

x | not a := 0;

(though this assumes “not a” already is defined or can be calculated efficiently). Yet another is

empty x;

x | a := b;

Both of these approaches can be much more efficient when managing very-large multi-dimensional arrays.

Dollar Sign

Pipe Sign

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