Re: [tlaplus] How to set a default for CHOOSE?

Thanks a lot.

On Wednesday, August 8, 2018 at 10:54:49 PM UTC+8, Stephan Merz wrote:
Hello,

your approach is correct. Instead of checking emptiness via cardinality, I’d rather use a direct set comparison. Also, you can avoid some repetition using a LET definition, like so:

get_element(key) ==
LET matches == { x \in some_set : x.key = key }
IN IF matches = {} THEN [ key |-> 0 ]
ELSE CHOOSE m \in matches : TRUE

Regards,
Stephan

> On 8 Aug 2018, at 12:42, wang...@xxxxxxxxx wrote:
>
> I'm using CHOOSE, but it will catch error when no element is satisfied by p.
>
> In my situation, I want to choose one element from a set or get a default element.
>
> Like this:
>
> 
> some_set == {[key |-> 1, x |-> 1], [key |-> 2, x |-> 3], ...}
>
> get_element(key) ==
>  IF Cardinality({x \in some_set : x.key = key}) /= 0
>  THEN CHOOSE x \in some_set : x.key = key
>  ELSE [x |-> 0]
>
> val == get_element(9).x
> 
>
> Is there a shorter way to represent get_element?
>
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