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JavaScript

Posted by David Padilla
David Padilla
5,255 Points

Why isn't the (higherVal - lowerVal + 1) in the equation not following the order of operations method PEMDAS?

In PEMDAS, "add" goes before "subtract" when determining the order of operations in a math equation. Why isn't that the case here?

For example, if you run this alert(Math.floor(Math.random() * (higherVal - lowerVal + 1)) + lowerVal), JavaScript will subtract higherVal and lowerVal before adding + 1, why?

3 Answers

andren
andren
28,558 Points

Addition does not come before subtraction in PEMDAS.

The PEMDAS order is the following:

  1. Parentheses
  2. Exponents
  3. Multiplication and Division (left-to-right)
  4. Addition and Subtraction (left-to-right)

Addition and Subtraction are grouped together because they are considered to have the same priority, and as such when they are mixed together you are supposed to solve whatever appears first from left-to-right. So 5 - 3 + 1 becomes 5 - 3 then 2 + 1. The same is true for Multiplication and Division. 10 / 2 * 6 becomes 10 / 2 then 5 * 6. That's why inputting 10 / 2 * 6 into a proper calculator results in 30 rather than 0.83.

David Padilla
David Padilla
5,255 Points

Thank you Andren! That makes complete sense! :-)

Alexander Besse
seal-mask
.a{fill-rule:evenodd;}techdegree seal-36
Alexander Besse
Full Stack JavaScript Techdegree Graduate 35,115 Points
Alexander Besse
.a{fill-rule:evenodd;}techdegree seal-36
Alexander Besse
Full Stack JavaScript Techdegree Graduate 35,115 Points

I'm not sure what's going on,

alert(Math.floor(Math.random() * (higherVal - (lowerVal + 1))) + lowerVal)

Try this. I wrapped the lowerVal + 1 in parentheses so that it will recognize to do that first.

Dave StSomeWhere
Dave StSomeWhere
19,870 Points
Dave StSomeWhere
Dave StSomeWhere
19,870 Points

If you check the documentation in MDN Docs you see that both addition and subtraction are precedence level 13 and its associativity is left to right.