Variables, Expressions, and Statements Chapter 2 Flashcards
Variables
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.
A variable is a memory location used to store a value (0.6) The right side is an expression. Once the expression is evaluated, the result is placed in (assigned to)the variable on the left side (i.e., x).
x = 12.2 y = 14
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 cannot use reserved words as variable names / identifiers
Reserved Words
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 as with
Sentences or Lines
x = 2
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
Example of an Assignment Statements :
x = 3.9 * x * ( 1 - x )
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.
\+ 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
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
> > > x = 1 + 2 ** 3 / 4 * 5
print x
11
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!
This changes in Python 3.0
>>> 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
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 “”, line 1, in
TypeError: cannot concatenate
‘str’ and ‘int’ objects
»> type(eee)
> > > type(‘hello’)
> > > type(1)
> > >
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)
> > > temp = 98.6
type(temp)
> > > type(1)
> > > type(1.0)
> > >
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)
> > > f = float(i)
print f
42.0
type(f)
> > > 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)
>>> print sval + 1 Traceback (most recent call last): File "", line 1, in TypeError: cannot concatenate 'str' and 'int' >>> ival = int(sval) >>> type(ival)
>>> print ival + 1 124 >>> nsv = 'hello bob' >>> niv = int(nsv) Traceback (most recent call last): File "", line 1, in 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
Comments in Python
# 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
What is this bit of
code doing?
x1q3z9ocd = 35.0
x1q3z9afd = 12.50
x1q3p9afd = x1q3z9ocd * x1q3z9afd
print x1q3p9afd
What is this bit of
code doing?
What are these
bits of code doing?
x1q3z9ocd = 35.0
x1q3z9afd = 12.50
x1q3p9afd = x1q3z9ocd * x1q3z9afd
print x1q3p9afd
a = 35.0
b = 12.50
c = a * b
print c
hours = 35.0
rate = 12.50
pay = hours * rate
print pay
Exercise
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
- Reserved words
- Variables (mnemonic)
- Operators
- Operator precedence
- Integer Division
- Conversion between types
- User input
- Comments (#)
