Variables, Expressions, and
Statements!
Chapter 2!
Python for Informatics: Exploring Information!
www.pythonlearn.com!
Unless otherwise noted, the content of this course material is licensed under a Creative
Commons Attribution 3.0 License.
http://creativecommons.org/licenses/by/3.0/.
Copyright 2010- Charles R. Severance
Constants!
• Fixed values such as numbers, letters, and strings are called
constants - because their value does not change!
• Numeric constants are as you expect!
• String constants use single-quotes (')"
or double-quotes (")"
!
>>> print 123!
123!
>>> print 98.6!
98.6!
>>> print 'Hello world'!
Hello world!
Variables!
• A variable is a named place in the memory where a programmer can
store data and later retrieve the data using the variable name !
• Programmers get to choose the names of the variables!
• You can change the contents of a variable in a later statement!
12.2!
x!
14 !
y!
x = 12.2!
y = 14!
!
100!
x = 100!
Python Variable Name Rules!
• Must start with a letter or underscore _ !
• Must consist of letters and numbers and underscores!
• Case Sensitive!
• Good: spam eggs spam23 _speed!
• Bad: 23spam #sign var.12!
• Different: spam Spam SPAM!
Reserved Words!
• You can not use reserved words as variable names / identifiers!
and del for is raise !
assert elif from lambda return !
break else global not try !
class except if or while !
continue exec import pass yield !
def finally in print !
Sentences or Lines!
x = 2!
x = x + 2!
print x!
Variable! Operator!
Constant! Reserved Word!
Assignment Statement!
Assignment with expression!
Print statement!
Assignment Statements!
• We assign a value to a variable using the assignment statement (=)!
• An assignment statement consists of an expression on the right hand
side and a variable to store the result!
x = 3.9 * x * ( 1 - x )!
x = 3.9 * x * ( 1 - x )!
0.6!
x!
Right side is an expression. Once
expression is evaluated, the result
is placed in (assigned to) x.!
0.6!
0.6!
0.4!
0.93!
A variable is a memory location
used to store a value (0.6).!
x = 3.9 * x * ( 1 - x )!
0.6 0.93!
x!
Right side is an expression. Once
expression is evaluated, the result
is placed in (assigned to) the
variable on the left side (i.e. x).!
0.93!
A variable is a memory location
used to store a value. The value
stored in a variable can be updated
by replacing the old value (0.6)
with a new value (0.93).!
Numeric Expressions!
• Because of the lack of mathematical
symbols on computer keyboards - we
use computer-speak to express the
classic math operations!
• Asterisk is multiplication!
• Exponentiation (raise to a power) looks
different from in math.!
Operator! Operation!
+! Addition!
-! Subtraction!
*! Multiplication!
/! Division!
**! Power!
%! Remainder!
Numeric Expressions!
>>> xx = 2!
>>> xx = xx + 2!
>>> print xx!
4!
>>> yy = 440 * 12!
>>> print yy!
5280!
>>> zz = yy / 1000!
>>> print zz!
5!
>>> jj = 23!
>>> kk = jj % 5!
>>> print kk!
3!
>>> print 4 ** 3!
64!
Operator! Operation!
+! Addition!
-! Subtraction!
*! Multiplication!
/! Division!
**! Power!
%! Remainder!
5! 23!
4 R 3!
20!
3!
Order of Evaluation!
• When we string operators together - Python must know which one
to do first!
• This is called operator precedence !
• Which operator takes precedence over the others!
x = 1 + 2 * 3 - 4 / 5 ** 6!
Operator Precedence Rules!
• Highest precedence rule to lowest precedence rule!
• Parenthesis are always respected!
• Exponentiation (raise to a power)!
• Multiplication, Division, and Remainder!
• Addition and Subtraction!
• Left to right!
Parenthesis!
Power!
Multiplication!
Addition!
Left to Right!
Parenthesis!
Power!
Multiplication!
Addition!
Left to Right!
1 + 2 ** 3 / 4 * 5!
1 + 8 / 4 * 5!
1 + 2 * 5!
1 + 10!
11!
>>> x = 1 + 2 ** 3 / 4 * 5!
>>> print x!
11!
>>> !
Parenthesis!
Power!
Multiplication!
Addition!
Left to Right!
>>> x = 1 + 2 ** 3 / 4 * 5!
>>> print x!
11!
>>> !
1 + 2 ** 3 / 4 * 5!
1 + 8 / 4 * 5!
1 + 2 * 5!
1 + 10!
11!
Note 8/4 goes before 4*5
because of the left-right
rule.!
Operator Precedence!
• Remember the rules top to bottom!
• When writing code - use parenthesis!
• When writing code - keep mathematical expressions simple enough
that they are easy to understand!
• Break long series of mathematical operations up to make them more
clear!
Parenthesis!
Power!
Multiplication!
Addition!
Left to Right!
Exam Question: x = 1 + 2 * 3 - 4 / 5!
Python Integer Division is Weird!!
• Integer division truncates!
• Floating point division produces
floating point numbers!
>>> print 10 / 2!
5!
>>> print 9 / 2!
4!
>>> print 99 / 100!
0!
>>> print 10.0 / 2.0!
5.0!
>>> print 99.0 / 100.0!
0.99!
This changes in Python 3.0!
Mixing Integer and Floating!
• When you perform an
operation where one
operand is an integer and the
other operand is a floating
point the result is a floating
point!
• The integer is converted to a
floating point before the
operation!
>>> print 99 / 100!
0!
>>> print 99 / 100.0!
0.99!
>>> print 99.0 / 100!
0.99!
>>> print 1 + 2 * 3 / 4.0 - 5!
-2.5!
>>> !
What does Type Mean?!
• In Python variables, literals, and
constants have a type !
• Python knows the difference
between an integer number and a
string!
• For example + means addition
if something is a number and
concatenate if something is a
string !
>>> ddd = 1 + 4!
>>> print ddd!
5!
>>> eee = 'hello ' + 'there'!
>>> print eee!
hello there!
concatenate = put together!
Type Matters!
• Python knows what type
everything is !
• Some operations are prohibited!
• You cannot add 1 to a string!
• We can ask Python what type
something is by using the type()
function.!
>>> eee = 'hello ' + 'there'!
>>> eee = eee + 1!
Traceback (most recent call last):!
File "<stdin>", line 1, in <module>!
TypeError: cannot concatenate 'str'
and 'int' objects!
>>> type(eee)!
<type 'str'>!
>>> type('hello')!
<type 'str'>!
>>> type(1)!
<type 'int'>!
>>> !
Several Types of Numbers!
• Numbers have two main types!
• Integers are whole numbers: -14, -2, 0,
1, 100, 401233!
• Floating Point Numbers have decimal
parts: -2.5 , 0.0, 98.6, 14.0!
• There are other number types - they
are variations on float and integer!
>>> xx = 1!
>>> type (xx)!
<type 'int'>!
>>> temp = 98.6!
>>> type(temp)!
<type 'float'>!
>>> type(1)!
<type 'int'>!
>>> type(1.0)!
<type 'float'>!
>>> !
Type Conversions!
• When you put an integer and
floating point in an expression
the integer is implicitly
converted to a float!
• You can control this with the
built in functions int() and float()!
>>> print float(99) / 100!
0.99!
>>> i = 42!
>>> type(i)!
<type 'int'>!
>>> f = float(i)!
>>> print f!
42.0!
>>> type(f)!
<type 'float'>!
>>> print 1 + 2 * float(3) / 4 - 5!
-2.5!
>>> !
String
Conversions!
• You can also use int() and
float() to convert between
strings and integers!
• You will get an error if the
string does not contain
numeric characters!
>>> sval = '123'!
>>> type(sval)!
<type 'str'>!
>>> print sval + 1!
Traceback (most recent call last):!
File "<stdin>", line 1, in <module>!
TypeError: cannot concatenate 'str' and 'int'!
>>> ival = int(sval)!
>>> type(ival)!
<type 'int'>!
>>> print ival + 1!
124!
>>> nsv = 'hello bob'!
>>> niv = int(nsv)!
Traceback (most recent call last):!
File "<stdin>", line 1, in <module>!
ValueError: invalid literal for int() !
User Input!
• We can instruct Python
to pause and read data
from the user using the
raw_input function!
• The raw_input function
returns a string!
nam = raw_input( Who are you? )!
print 'Welcome', nam!
Who are you? Chuck!
Welcome Chuck!
Converting User Input!
• If we want to read a
number from the user, we
must convert it from a
string to a number using a
type conversion function!
• Later we will deal with
bad input data!
inp = raw_input( Europe floor? )!
usf = int(inp) + 1!
print 'US floor', usf!
Europe floor? 0!
US floor 1!
Comments in Python!
• Anything after a # is ignored by Python!
• Why comment?!
• Describe what is going to happen in a sequence of code!
• Document who wrote the code or other ancillary information!
• Turn off a line of code - perhaps temporarily!
# Get the name of the file and open it!
name = raw_input('Enter file:')!
handle = open(name, 'r')!
text = handle.read()!
words = text.split()!
!
# Count word frequency!
counts = dict()!
for word in words:!
counts[word] = counts.get(word,0) + 1!
!
# Find the most common word!
bigcount = None!
bigword = None!
for word,count in counts.items():!
if bigcount is None or count > bigcount:!
bigword = word!
bigcount = count!
!
# All done!
print bigword, bigcount!
String Operations!
• Some operators apply to strings!
• + implies concatenation !
• * implies multiple concatenation !
• Python knows when it is dealing
with a string or a number and
behaves appropriately!
>>> print 'abc' + '123’!
Abc123!
>>> print 'Hi' * 5!
HiHiHiHiHi!
>>> !
Mnemonic Variable Names!
• Since we programmers are given a choice in how we choose
our variable names, there is a bit of best practice !
• We name variables to help us remember what we intend to
store in them ( mnemonic = memory aid )!
• This can confuse beginning students because well named
variables often sound so good that they must be keywords!
http://en.wikipedia.org/wiki/Mnemonic !
x1q3z9ocd = 35.0 !
x1q3z9afd = 12.50!
x1q3p9afd = x1q3z9ocd * x1q3z9afd!
print x1q3p9afd!
hours = 35.0 !
rate = 12.50 !
pay = hours * rate !
print pay!
a = 35.0 !
b = 12.50 !
c = a * b !
print c!
What is this
code doing?!
Exercise!
Write a program to prompt the user for hours and
rate per hour to compute gross pay.!
Enter Hours: 35 !
Enter Rate: 2.75 !
Pay: 96.25!
Summary!
• Type!
• Resrved words!
• Variables (mnemonic)!
• Operators!
• Operator precedence!
• Integer Division!
• Conversion between types!
• User input!
• Comments (#)!
