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Hi,
I noticed the following behavior with regard to missing values:
The formula
false and missingshort circuits and evaluates tofalse. This seems expected since even though the value of the righthand side is unknown, thefalseis enough to determine the overall expression must befalse.However, with
missing and false, theandoperator seems to propagate the missing value, instead of determining that the result must befalse. Is this intended?Is this because missing values are considered falsy, and
andshort circuits and evaluates to the lefthand side if it is falsy?Thanks,
Thomas
Hello Thomas,
What an interesting question! The short answer is that expressions are evaluated as Javascript expressions and this is the behavior in Javascript. Here is a snippet I copied from a node session that confirms this:
> var missing = “”;
undefined
> [missing && false, false && missing, true || missing, missing || true];
[ ”, false, true, true ]This is consistent with your explanation, just putting it another way.
You have probably also discovered that there are other places where the commutative property of ‘and’ and ‘or’ is not observed:
Although this behavior is consistent with expressions in the Javascript language, it is not consistent with boolean logic and likely to be confusing to students. I will file a bug. The delegation of CODAP’s expression language to Javascript is pretty fundamental to CODAP’s formula engine, so it may take a while to explore the implications elsewhere in CODAP.
Jonathan
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