http://www.cs.indiana.edu/classes/a201

Let's examine Python. 

We start IDLE and we get a prompt. 

Type numbers: integers, floating point numbers, negative numbers. 

We are in fact typing expressions. 

Python evaluates them and writes back their values. 

Write some expressions with numbers in Python. 

Expressions are composed of operands and operators.

For example: 3 + 5

3 and 5 are operands and + is an operator. 

List some operators for numeric expressions: 

+ addition 
- subtraction 
 (use: 3 - 5 this is a binary operator
   or: -2 shows the sign of a number unary operator

* multiplication 
/ division

Use parentheses to force an order for operations: 

( 3 + 5 ) - 2 evaluates to 8 - 2 which evaluates to 6

3 + ( 5 - 2) evaluates to 3 + 3 which evaluates to 6

Try to predict the values of these expressions: 


a)  3 - 5 + 2 This evaluates to 0 because + and - 
have the same priority as operators and without parens
we evaluate operators with the same priority in order 
left to right. 

    3 + 5 - 2 

    3 - 5 + 2 

    3 - 5 * 2 This evaluates to -7 because * has higher
priority than -. If it were ( 3 - 5 ) * 2 then the parens
would force a different order of evaluations and the answer
is -2. / and * are at the same level, just above + and -.

       
b)  3 - ( 5 + 2 ) evaluates to 3 - 7 which evaluates to -4.


c)  7 / 2 this is division. Integer division. The answer
is the quotient of the division. There are three 2's in 7
so the answer is 3. What is left unaccounted for? The 
remainder. The % operator calculates the remainder. So
we had some examples: 

7 % 2 evaluates to 1
7 / 2 evaluates to 3

*, / and % have the same priority
+, - come right under them

d)  1 / 2 evaluates to 0 but if at least one of the 
operands are floating-point the result is a floating-point
number. Floating-point number = number with a fractional part.

e)  6 * 2 / 3 evaluates to 12 / 3 which is 4

f)  2 / 3 * 6 evaluates to 0 * 6 which is 0 
But as we have noticed 2.0 / 3 * 6 is 4.0 
Yet 2 / 3 * 6.0 is 0.0 because the floating-point number
appears too late in the expression. 


        dgerman@indiana.edu

>>> 
>>> 
>>> 
>>> 
>>> 
>>> 
>>> nothing

Traceback (most recent call last):
  File "<pyshell#8>", line 1, in <module>
    nothing
NameError: name 'nothing' is not defined
>>> 
>>> 
>>> 
>>> 3
3
>>> 5
5
>>> 12
12
>>> 109
109
>>> -3
-3
>>> 2.5
2.5
>>> 1 + 2
3
>>> 3 * 4
12
>>> 1 - 2
-1
>>> 5 + 3 - 2
6
>>> 5 + ( 3 - 2 )
6
>>> (5 + 3) - 2
6
>>> 3 - 5 + 2
0
>>> 3 - 5 + 2
0
>>> 3 + 5 - 2
6
>>> 3 - 5 + 2
0
>>> 3 - 5 * 2
-7
>>> (3 - 5) * 2
-4
>>> 3 / 2
1
>>> 13 / 4
3
>>> 13 % 4
1
>>> 3 % 2
1
>>> 13 % 10
3
>>> 65 % 25
15
>>> 65 / 25
2
>>> 6 / 3
2
>>> 6 % 3
0
>>> 3 / 4
0
>>> 3 % 4
3
>>> 2 % 5
2
>>> 4 / 3
1
>>> 4.0 / 3.0
1.3333333333333333
>>> 1 / 3
0
>>> 1.0 / 3.0
0.33333333333333331
>>> 4.35 * 100
434.99999999999994
>>> 2 / 3 * 6
0
>>> 2.0 / 3 * 6
4.0
>>> 2 / 3 * 6.0
0.0
>>> 3 + 4 - 1
6
>>> nothing

Traceback (most recent call last):
  File "<pyshell#51>", line 1, in <module>
    nothing
NameError: name 'nothing' is not defined
>>> "nothing"
'nothing'
>>> "blue" + "berry"
'blueberry'
>>> "blue" + 3

Traceback (most recent call last):
  File "<pyshell#54>", line 1, in <module>
    "blue" + 3
TypeError: cannot concatenate 'str' and 'int' objects
>>> "blue" * 3
'blueblueblue'
>>>