Mathjs: Correct support of the sin(PI)

[zagovorichev]

Why no one not support sin(PI) = 0
For example: I have equation y=x*sin(PI), I try to check knowledges of somebody, and propose to introduce the variables which satisfy: Enter x: __ y: __. User enter y=0, x = 100, and result is false.

[zagovorichev]

for now I use if (result >=-1e-10 && result <= 1e-10), but, as for me, it's a dirty hack. Maybe has sense to create static library, with preloaded results?

[josdejong]

Calculations with irrational numbers and trigonometric functions result in round off errors. The relational functions of math.js like math.equal reckon with round-off errors.

See relevant docs:
http://mathjs.org/docs/datatypes/numbers.html#roundoff-errors
http://mathjs.org/docs/datatypes/numbers.html#equality
http://mathjs.org/docs/reference/functions/equal.html

[zagovorichev]

@josdejong Thanks for response, but it's not work for me

math.equal(0,132*math.sin(math.pi)) === false
console.log(math.equal(0,math.eval('11234*sin(pi)')));

[josdejong]

Ah, yes that's true. We can't compare against zero since the function can't know what "scale" to compare the value against. It's explained in the docs:

http://mathjs.org/docs/datatypes/numbers.html#equality

To work around this you could add for example 10 to both values and compare that:

var offset = 10;
math.equal(0 + offset, 132*math.sin(math.pi) + offset); // true

[zagovorichev]

Thank you! Now it's clear

[zagovorichev]

But it's doesn't work with big numbers

var offset = 10;
math.equal(0 + offset, 999999999*math.sin(math.pi) + offset); // false

[josdejong]

You will have to rewrite this check to allow using bignumbers too. Replacing the operators + and * with math.add and math.multiply should do the trick.