inconsistent implicit multiplication

[julinas]

This is on the released version... 2 * 3 3 3 is interpreted as syntax error, but 2 * 3 3 is parsed as 2 * 3 * 3. 2 + 2 3 also gives a syntax error instead of 2 + 2 * 3 as expected. Maybe it's no longer a problem in develop?

[FSMaxB]

I didn't even know something like 2 * 3 3 was even possible. But looking at the source code of the parser I found this:

mathjs/lib/expression/parse.js

Lines 880 to 891 in 248d314

	// parse implicit multiplication
	if ((token_type == TOKENTYPE.SYMBOL) ||
	(token == 'in' && (node && node.isConstantNode)) ||
	(token_type == TOKENTYPE.NUMBER && !(node && node.isConstantNode)) ||
	(token == '(' || token == '[')) {
	// symbol: implicit multiplication like '2a', '(2+3)a', 'a b'
	// number: implicit multiplication like '(2+3)2'
	// Note: we don't allow implicit multiplication between numbers,
	// like '2 3'. I'm not sure whether that is a good idea.
	// parenthesis: implicit multiplication like '2(3+4)', '(3+4)(1+2)', '2[1,2,3]'
	node = new OperatorNode('*', 'multiply', [node, parseMultiplyDivide()]);
	}

    // parse implicit multiplication
    if ((token_type == TOKENTYPE.SYMBOL) ||
        (token == 'in' && (node && node.isConstantNode)) ||
        (token_type == TOKENTYPE.NUMBER && !(node && node.isConstantNode)) ||
        (token == '(' || token == '[')) {
      // symbol:      implicit multiplication like '2a', '(2+3)a', 'a b'
      // number:      implicit multiplication like '(2+3)2'
      //              Note: we don't allow implicit multiplication between numbers,
      //              like '2 3'. I'm not sure whether that is a good idea.
      // parenthesis: implicit multiplication like '2(3+4)', '(3+4)(1+2)', '2[1,2,3]'
      node = new OperatorNode('*', 'multiply', [node, parseMultiplyDivide()]);
    }

The comments suggest that implicit multiplication between numbers wasn't intended at all.

In my understanding this means one of two things:

1. It is a bug that 2 * 3 3 works, according to the comments it shouldn't.
2. It is a bug that 2 * 3 3 3 doesn't work and that 2 3 doesn't work.

But I don't quite understand the parser at this point.

@josdejong: What is your take on this.

[josdejong]

Thanks for bringing this up.

The comments suggest that implicit multiplication between numbers wasn't intended at all.

That's correct. Implicit multiplication between two numbers should not be supported, so "2 * 3 3" should throw an error instead of evaluating.

Reason for not allowing implicit multiplication between values was that I thought it could easily lead to misreading expressions, you can easily confuse it with a list of values (how do you think [1 2 3] would evaluate? as [1, 2, 3] or as [1 * 2 * 3]?).

[josdejong]

This inconsistent behavior is fixed now in the develop branch.