Operator Precedence

Date: 08/13/2003 at 15:47:44
From: Felix
Subject: Operators and operands

These questions are from the following web site:

   Variables, expressions and statements
   http://www.ibiblio.org/obp/thinkCSpy/chap02.htm 

It is from:
How to Think Like a Computer Scientist
Learning with Python
by Allen B. Downey, Jeffrey Elkner and Chris Meyers
Printed copies now available from Green Tea Press.

I am attempting to understand Python and I have not ever programmed.

" . . . order of operations: 
·
Parentheses have the highest precedence and can be used to 
force an expression to evaluate in the order you want. Since 
expressions in parentheses are evaluated first, 2 * (3-1) is 4, and 
(1+1)**(5-2) is 8. You can also use parentheses to make an expression 
easier to read, as in (minute * 100) / 60, even though it doesn't 
change the result. 
·
Exponentiation has the next highest precedence, so 2**1+1 is 
3 and not 4, and 3*1**3 is 3 and not 27... "

Since the following statement is true:  (1+1)**(5-2) is 8.
Why is the following statement true and not false:  2**1+1 is 3 and 
not 4, and 3*1**3 is 3 and not 27 ?

Date: 08/13/2003 at 16:59:05
From: Doctor Peterson
Subject: Re: Operators and operands

Hi, Felix.

This is essentially the same idea as what we call "order of 
operations" in math:

   Order of operations
   http://mathforum.org/dr.math/faq/faq.order.operations.html 

"Operator precedence" is a more sophisticated term for it.

The main difference between what you see in a programming language 
and what we do in writing math is that exponents are indicated by the 
operator "**" (or by "^" in some other languages) rather than by 
writing a superscript, and that makes it a little less obvious what 
order should be used. The rule you have been given is that the 
exponentiation operator has the highest precedence; that means it 
must be done before other operations. So when we see

  2**1+1

we have to evaluate it this way:

  2**1+1
  \__/
    2 +1
    \__/
      3

If we meant to do the addition first, we would have to use 
parentheses:

  2**(1+1)
     \___/
  2**  2
  \____/
     4

If we were writing this in the usual mathematical way, the two 
expressions would look like

   1          1+1
  2 +1  and  2

and parentheses would not be needed to make the difference clear, 
since the location of the numbers shows how they are to be connected. 
The rule of precedence is still needed, though, for expressions like 
this:

     3
  1+2

which means

        3
  1 + (2 )

rather than

         3
  (1 + 2)

In Python, these are

  1+2**3    1+(2**3)   (1+2)**3

and the first two, again, mean the same thing.

If you have any further questions, feel free to write back.

- Doctor Peterson, The Math Forum
  http://mathforum.org/dr.math/