From: Tim Hunter <cyclists@...>
Date: 2006-04-02T09:13:44+09:00
Subject: Re: Newbie! Programming Calculations

Indifferent wrote:
> Im new to programming, im learning ruby.
> 
> Here is the guide I am working through:
> http://pine.fm/LearnToProgram/?Chapter=01
> 
> At the bottom of the page there are these sample calculations:
> puts 5 * (12-8) + -15 (is that + minus15 or does the + and the - mean
> something combined? same for the one below with * and -52)
> puts 98 + (59872 / (13*8)) * -52
> 
> What do they do? What are the brackets for?
> I sort of remember doing something simular to this when I was using
> excel in a course i did. Is the number is the brackets calculated first?
> Can someone explain this to me? Or give me a link were I can find more
> information.
> 
> Extreamly confused newbie! Please help!
> 

When you have an expression that uses more than one arithmetic operator, 
such as 6 + 12 / 3, it's not always obvious which operations should be 
performed first. Should we add 6 and 12 first, or divide 12 by 3 and 
then add 6?

Ruby follows a couple of very simple rules in this case.

Rule 1: First do all the multiplications and divisions, then all the 
additions and subtractions.

Rule 2: If there are more than 1 multiplication or division, or more 
than one addition or subtraction, do them from left to right.

So, for 6 + 12 / 3, Ruby divides 12 by 3 first, which gives it 4, and 
then adds 6 and 4, so the result is 10.

Now it's possible that you may want Ruby to actually add 6 and 12 and 
then divide the result by 3, so Ruby gives you a way to tell it to do it 
that way. To do this, enclose 6 + 12 in parentheses: (6 + 12) / 3. Ruby 
will do the addition, getting 18, and then divide 18 / 3, giving 6.

You can nest the parentheses, in which case the most deeply nested 
expressions are evaluated first: ((3+5)/2)+6 means "add 3 and 5, divide 
by 2, and add 6".

For rule 2, given the expression 24 / 2 * 3, since division and 
multiplication are equally "important," Ruby just goes left to right. 
First it divides 24 / 2, giving 12, then multiplies by 3, giving 36. 
Similarly, for 12 + 3 - 5, Ruby adds 12 and 3, giving 15, and then 
subtracts 5, giving 10.

Regarding -15, or -52, consider that most arithmetic operations are 
"binary," that is, the operation produces a result by combining two 
numbers. However, we can say that a number is negative by preceding it 
with a minus sign. The minus sign is called a "unary" operator. That is, 
it produces its result from only one number.

Since a unary minus sign negates the number it precedes, 25 + =15 adds a 
negative 15 to 25, which is the same as subtracting 15 from 25.