Question: How to get a list undefined variables?

[tmtron]

e.g. the user may enter a formula like 2 * scale * pi * + offset

• pi is known by MathJs
• but the other 2 variables are unknown and must be provided by the user when we want to evaluate the expression

How can we easily get a list of all unknown variables?

i.e. we can evaluate the expression with an empty scope and it will throw - but this will only return the first undefined symbol

[josdejong]

I think this is a duplicate of #1414. It will take a bit of effort.

[tmtron]

@josdejong thanks for the link that makes sense. Maybe the library should contain a convenience function, since this could be useful in many projects.

[tmtron]

After some testing, I don't see how #1414 can solve this.

e.g. math.parse('var*pi'); returns this tree:

{
    "mathjs": "OperatorNode",
    "op": "*",
    "fn": "multiply",
    "args": [
        {
            "mathjs": "SymbolNode",
            "name": "var"
        },
        {
            "mathjs": "SymbolNode",
            "name": "pi"
        }
    ],
}

Both nodes var and pi are SymbolNodes with paths arg[0]/arg[1] and they have the same parent-node (when we traverse the tree).
So how could we know, that pi is a known symbol and var is an undefined variable, that the user must provide?

[josdejong]

So how could we know, that pi is a known symbol and var is an undefined variable, that the user must provide?

You can check whether the symbol is defined or not, for example:

function isDefined(symbol) {
  return math.expression.mathWithTransform[symbol] !== undefined || math.Unit.isValuelessUnit(symbol)
}

isDefined('pi') // true
isDefined('var') // false
isDefined('foo') // false
isDefined('millimeter') // true

[tmtron]

@josdejong that would also report e.g. math.AccessorNode as defined, which is not true

[josdejong]

Ah, good point. I've updated my example to use math.expression.mathWithTransform[symbol] instead of math[symbol], since the former is the actual object exposed inside the expression parser.