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Javascript Tut - Variable Operator Precedence

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You can have more than two number for your math, of course. You can even mix variables and actual numbers. Change the total line of your code to this:

total = number1 + number2 / 2;

This time, we have a division symbol and a 2 at the end. What do you think will happen when you run this code? You may think that Javascript adds up the two values in our variables first and then divides by 2. But it doesn't.

To clarify what it is you want to do, you can use round brackets. If you want to do the addition first then the division, surround the addition with round brackets. Like this:

total = ( number1 + number2 ) / 2;

Now Javascript will do the sums between the round brackets first. When it's found an answer it will then divide by 2.

If you wanted to divide the second number by 2 then add that to the first number you just rearrange the round brackets:

total = number1 + ( number2 / 2) ;

The reason you need to do this is because of something called Operator Precedence. This just means which mathematical symbols get taken care of first. Take the following code:

var result = 5 + 3 * 8

The multiplication operator ( * ) has a higher priority than the plus symbol. Because the 3 and the 8 are together, Javascript thinks you want to multiply these two numbers first. After it multiplies 8 by 3 to get 24 it will then add the 5 at the start. Parentheses (round brackets) are used as a way to override this operator precedence. So if you want to add 5 plus three first, then multiply by 8, you'd do it like this:

var result = (5 + 3) * 8

The result would now be 64 ( 5 + 3 equals 8, then multiply by 8).

Division and multiplication have equal priority. These two in turn have priority over addition and subtraction which, again, have equal priority. To clear up what this means, take a look at the following:

var result = 32 - 16 / 8

Because division has a higher priority than subtraction the first thing Javascript does is to divide 16 by 8 to get 2. It then subtracts the 2 from 32 to get an answer of 30.

Now put some round brackets in:

var result = (32 - 16) / 8

This time, Javascript will see the parentheses and do this calculation first. 32 minus 16 is 16. This is then divided by 8 to get an answer of 2.
Now take this sum:

var result = 32 - 16 + 8;

Because addition and subtraction have the same priority the sum is calculated from left to right, giving an answer of 24. However, put some round bracket in and you get a different answer:

var result = 32 - (16 + 8);

In the sum above, Javascript will do the sum between the round brackets first, returning an answer of 24 (16 + 8 = 24). This is then subtracted from 32 to give a result of 8.

The moral of all this, then, is be careful of your mathematical operators: they might not give you the answer you were expecting.

The only other operator we haven't discussed is the modulus operator ( %). If you're not sure what this is, it's when you divide one number by another but keep the remainder. For example, in normal division 31 divided by 10 gives an answer of 3.1. In other words, 10 divides into 31 three times with one left over. Modulus keeps the 1 and throws away the rest. Why is this interesting? Because you can tell if a number is odd or even with modulus. All you need to do is divide any number by 2. If there's no remainder then you know the number is even. If there is a remainder then the number is odd. Here's some code to examine:

var result = 31 % 2;

if (result == 1) {
document.write("Odd number");
else {
document.write("Even number");

Although you haven't done IF statements yet, you should be able to figure out what's going on. First we test a number using the modulus operator. We then say IF 31 MOD 2 has a value of 1 then write "Odd number", ELSE write "Even number".


This was not created by me. Credits to the authors!

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