Option to always use bignumber?

[mockdeep]

We came across the docs for bignumber and were excited to see the bignumber config. Once we played around with it for a while, we were surprised to find it was still using just normal numbers when evaluating formulas. It would be great if there was an option to always cast input values to bignumbers so that we don't have to remember to do it everywhere we calculate a formula.

[josdejong]

The number option currently only applies to parsing expressions with math.eval. I suppose you mean converting to bignumber when using functions like math.add(a,b)? That could indeed be convenient. Not sure if it would be a good idea, let's give it some more thought.

[mockdeep]

I see. In our case we were a bit surprised that math.eval can still return a plain js number sometimes, but a bignumber other times:

math.eval('2 + 2')
>> Object { constructor: Decimal(), s: 1, e: 0, d: Array[1] }
math.eval('F1 / F2', { F1: 5.0, F2: 10.0 })
>> 0.5

One problem we want to solve is weird js math issues:

math.eval('F1 * F2', { F1: 0.575, F2: 124.5 })
>> 71.58749999999999
math.eval('F1 * F2', { F1: math.bignumber(0.575), F2: math.bignumber(124.5) }).toNumber()
71.5875

We'd also like to take advantage of the niceties of bignumber after the fact, like convenient rounding and decimal places. Instead of having to wrap all of our numbers on the way in, it would be great if there were a setting to just always wrap inputs in bignumber. Odds are, if we're using math.eval anywhere in our application we want to make sure we're using bignumber every time.

[josdejong]

The number configuration only applies to numbers parsed by the expression parser. The expression parser does not touch values provided via scope, it just leaves them as is. I understand that this can be confusing.

What you could do is create a a util function to turn all input into BigNumbers:

math.confg({number: 'BigNumber'});

function myEval (expr, scope) {
  // convert numbers in the scope into BigNumbers
  var _scope = {};
  for (var prop in scope) {
    var value = scope[prop];
    _scope[prop] = (typeof value === 'number') ? math.bignumber(value) : value;
  }

  // evaluate
  return math.eval(expr, _scope);
}

[mockdeep]

Yeah, we were thinking of doing something like that. Just seemed like it might be nice to have a castnumber configuration or something on mathjs that does the same thing. Btw, could probably simplify the cast line to:

_scope[prop] = math.bignumber(scope[prop])

Since bignumber handles other types as well.