make copy expression easier

Question

When one wants to copy expression, (s)he needs to do something like

ex:=a b;
ex1:=@(ex).
substitute(_,$a->c$);

but it's not direct, is there a more direct way to do this? For example,

ex:=a b;
substitute(@(ex),$a->c$);

Answer 1

I would only like to add that a possible variant is

substitute($@[ex]$, $a -> c$)

Using square brackets also creates a copy and does not affect the original expression.

Comments

Thanks, this is more natural!

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In fact, ( ) also works.

Answer 2

Cadabra's philosophy has always been that algorithms act on expressions in-place, for reasons that have been discussed elsewhere. That's not everyone's preferred mode of operation, but changing it now would not be a good idea.

The notation you propose is an option to get closer to 'act on a copy. I would personally probably prefer something slightly more explicit, e.g.

substitute( copy(ex), $a->c$ );

Answer 3

Hi Eureka and Kasper.

I just wanted to point out that your suggested way to substitute on a copy has the disadvantage that it is not possible to retreive the result, buecause it lacks a name!

I'd suggest the following modification to Kasper's solution:

ex:=a b;
ex1 = substitute(ex.copy(), $a->c$);

In whose case the result of the substitution can be recalled by ex1;.

Best wishes, Dox.

Comments

Yes, you of course need to assign to a new expression if you want to continue with it. Furthermore, we indeed already have ex.copy() which is preferable over my copy(ex). Thanks.

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And to be precise: this already works now, without any changes, whereas the @ notation would require internal changes.

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Thanks. This is what I want. It's direct enough.