How does this simple Max(set) implementation work?

[Steve Ravet]

I'm going through the examples at learntla.com and one asks to write an operator that finds the maximum value in a set. I did it using the SetReduce operator from somewhere else in the tutorial, but the given answer is simpler:

Max(S) == CHOOSE x \in S : \A y \in S : y <= x

I can't quite "read" this. Since CHOOSE picks any value from the set where the condition is true, it seems like this would return a value from S that is larger than at least one other value, not larger than all other values. What makes it keep going to find the largest value?

[Hillel Wayne]

Hi Steve,

\A is "for all", so it will choose a value from S that is ≥ all values. If you use \E instead, then it will pick a value that is ≥ at least one value. Note that this includes the value itself, so \E y \in S: y <= x is trivially true for all values in the set.

H

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