Introduction to Programming with Python

1

Outline
Introduction to Python
Operators & Expressions
Data Types & Type Conversion
Variables: Names for data
Functions
Program Flow (Branching)
Input from the user
Iteration (Looping)
2

What is Python


Python is both an interpreted, general purpose, open source,

cross-platform,

high

level

programming language.


Python uses an interpreter. This is a software program that convert Python code to machine language. 

It is easy to jump in and experiment with
Python in an interactive fashion.

Compiling and interpreting


Many languages require you to compile (translate) your program into a form that the machine understands. compile source code
Hello.java



execute

byte code
Hello.class

output

Python is instead directly interpreted into machine instructions. interpret source code
Hello.py

output

4

Programming basics








code or source code: instructions in a program.

The

sequence

of

syntax: The set of legal structures and commands that can be used in a particular programming language. output: The messages printed to the user by a program. console: The text box onto which output is printed. 5

The Basic Pattern
Most of our programs will use the basic pattern of: 

Get some user input



Perform some algorithm on the input



Provide results as output

Identifiers in Python






Identifiers are names of various program elements in the code that uniquely identify the elements. They are the names of things like variables or functions to be performed.
In Python, identifiers


Are made of letters, digits and underscores



Must begin with a letter or an underscore



Examples: temperature, myPayrate, score2

Keywords


Keywords are reserved words that have special meaning in the Python language.
Because they are reserved, they can not be used as identifiers. Examples of keywords are if, while, class, import.

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

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

Things to note about Python
1. Python is a calculator

2. A variable is a container

3. Different types cannot be compared

4. A program is a recipe

Python - A Calculator
How is it a Calculator?
Whenever you type an expression in form of numbers below, Python compute it and provide result 5
 3+4
 44/2
 2**3
 3*4+5*6
 If precedence is unclear, use parentheses
 (72 – 32) / 9 * 5

A variable is a container


A variable has:
 A name – identifier
 A data type - int, float, str, etc.

A variable is a container in which a data value can be stored within the computer’s memory.  The stored value can then be referenced using the variable’s name. For example, x=4 z=x*2


Sentences or Lines

x=2 x=x+2 print x

Assignment Statement
Assignment with expression
Print statement

Variable Operator Constant

Reserved Word

A variable is a container (1)


A variable must be assigned an initial value before using it in a sentence – See (2) below



You can change the contents of a variable in a later statement

x = 12.2 y = 14 + x x = 100

Right side is an expression.
Once expression is evaluated, the result is placed in (assigned to) y.

x 12.2 100 y 14

(2) p = 12.2 y = 14 + x

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


Assignment Statements


Multiple variables can be initialized with a common value in a statement, for example: a=b=c=8 

Alternatively, multiple variables can be initialised with differing values in a single statement using comma separators, for example: a, b, c = 1, 2, 3

In Python, you don’t have to specify the type of data a variable may contain in his declaration.
For example, int pet = 6

Numeric Expressions
Because of the lack of mathematical symbols on computer keyboards - we use computerspeak to express the classic math operations
 Exponentiation (raise to a power) looks different from in math.
Operators Operation
+
Addition
 Asterisk is multiplication


-

Subtraction

*

Multiplication

/

Division

%

Modulus

//

Floor Division

**

Exponent

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

Operator Precedence Rules
Now solve this expression x = 1 + 2 ** 3 / 4 * 5

>>> x = 1 + 2 ** 3 / 4 * 5
>>> print x
11
>>>

Parenthesis
Power
Multiplication
Addition
Left to Right

1 + 2 ** 3 / 4 * 5
1+8/4*5
1+2*5
1 + 10
11

Another Example
An expression is evaluated from the inside out (as shown below)


How many expressions are in this Python code? an expression

(72 – 32) / 9.0 * 5

values

(72 – 32) / 9.0 * 5
(40) / 9.0 * 5
40 / 9.0 * 5
4.44 * 5
22.2

Assignment Example
Before

X

3
Execute

Y

Z

5

12

Z=X*3+Z/Y

After

X

Y

Z

3

5

11

How an assignment is executed
1.
2.

Evaluate the right-hand side to a value
Store that value in the variable x = 2 print y = x print z = x print x = 5 print print print (x)
(y)
+ 1 z State of the computer:

x: 2 x: 5 y: 2

(x)
(y)
(z)

z: 3

Printed output:

2
2
3
5
2
3

To visualize a program’s execution: http://www.pythontutor.com/visualize.html#mode=edit Invisible TAB
Key in and run the following statements a=8 b=2 print(‘Addition:\t’, a, ‘+’, b, ‘=‘, a + b) print(‘Subtraction:\t’, a, ‘-’, b, ‘=‘, a – b) print(‘Multiplication:\t’, a, ‘x’, b, ‘=‘, a * b) print(‘Division:\t’, a, ‘/’, b, ‘=‘, a / b) print(‘Floor Division:\t’, a, ‘//’, b, ‘=‘, a // b) print(‘Modulus:\t’, a, ‘%’, b, ‘=‘, a % b)
The \t escape print(‘Exponent:\t’, a, ‘2 =‘, a + b)
Output
Addition:
Subtraction:
Multiplication:
Division:
Floor Division:
Modulus:
Exponent:

8 + 2 = 10
8–2=6
8 * 2 = 16
8 / 2 = 4.0
8 // 2 = 4
8%2=0
a2 = 64

sequence shown here adds an invisible tab character to format the output Assigning values
It is important to regard the = operator to mean
“assign” rather than “equals” to avoid confusion with the ‘==‘ equality operator.
Operator

Example

Equivalent

=
+=
-=

a=b a += b a -= b

a=b a = (a + b) a = (a – b)

*=
/=
%=

a *= b a /= b a %= b

a = (a * b) a = (a / b) a = (a % b)

//

a //= b

a =(a // b

**=

a**=b

a = (a**b)

Different types cannot be compared

Data Types


Every program must deal with data



The data is usually described as a certain type







This type determines what you can do with the data and how the data is stored
In Python basic types are integers (int), floating point (float), strings and Booleans
The first two are numeric and can be used in arithmetic, the last two are not

27

Data Types
Python has many native datatypes.
1.Booleans are either True or False.
2.Numbers can be integers (1 and 2), floats
(1.1 and 1.2), or even complex numbers.
3.Strings are sequences of Unicode, characters, e. g. an html document
4.Bytes and byte arrays, e.g. a jpeg image file.
5.Lists are ordered sequences of values.
6.Tuples are ordered, immutable sequences of values.
7.Sets are unordered bags of values.
8.Dictionaries are unordered bags of key-value pairs
28

Booleans
Python has a data type named bool which has 2 values, True and False.
These are actually representations of the values 1 and
0.
bool is a subclass of int and True / False are basically values that are being printed instead of 1 and 0, this has been done by overriding the repr and str methods of the bool class.
>>> truth = True
>>> truth + 1 # Woah!
2
>>>

29

Numbers
There are 3 distinct numeric types: integers, floating point numbers, and complex numbers.
Integers - A whole number. In theory any whole number from minus infinity to plus infinity, for example, -1, 0,123456
Floating Point Numbers - Any number that has digits after the decimal point, for example, 6.3 or -1.8. It may also be in scientific notation: 1.75e26.
Complex Numbers - This is a complex number consisting of two floats. Complex literals are written as a + bj where a and b are floating-point numbers denoting the real and imaginary parts respectively.
30

Strings
A string is any piece of text that does not distinguish between a string that is a single character. Python has two different string-types.
Normal Strings
You include a string in your program by surrounding it with single quotes or double quotes : x = "And so is this."
Multi-line = 'This is a string' y = strings can be denoted using triple quotes, ''' or """.
Unicode strings
Have an internal representation which keeps text as text. 31 x = u'A unicode string'

Bytes & Bytes Array
Bytes and byte arrays, e.g. a jpeg image file.

32

Lists
A list is a changeable sequence of data. A list is contained by square brackets i.e. [1,2,3]
>>> x = ["I", "Love", "Python"]
>>> x[2] = "BDC"
>>> print(x)
['I', 'Love', 'BDC']
The above code changes element number 2 in x.

33

Tuples
Tuples are a lot like lists in that they are used to store sequences of information.
Tuples are enclosed in parentheses ( ) where items are separated by commas and cannot be updated.
They are used to write-protect data and are usually faster than list as it cannot change dynamically.
Tuples can be thought of as read-only lists.

For example − tuple = ( 'abcd', 786 , 2.23, 'john', 70.2 )
34

Sets
Set is an unordered collection of unique items.
Set is defined by values separated by comma inside braces { }. Items in a set are not ordered. For example, a
= {5,2,3,1,4}
We can perform set operations like union, intersection on two sets. Set have unique values. They eliminate duplicates. >>> a = {1,2,2,3,3,3}
>>> a
{1, 2, 3}
35

Dictionaries
Dictionary is an unordered collection of key-value pairs. Dictionaries contain a key and a value. { } enclose dictionaries. It is generally used when we have a huge amount of data.
Dictionaries are optimized for retrieving data.
We must know the key to retrieve the value.

Key and value can be of any type.
>>> d = {1:'value','key':2}
>>> d[1]
'value'
>>> d['key']
2

36

Operations behave differently on different types
3.0 + 4.0
3+4
3 + 4.0
"3" + "4"
3 + "4"
3 + True

# Error
# Insanity!

Moral: Python sometimes tells you when you do something that does not make sense.

4. A program is a recipe

What is a program?
A program is a sequence of instructions
 The computer executes one after the other, as if they had been typed to the interpreter
 Saving as a program is better than re-typing from scratch x=1 y=2 x+y print x + y print "The sum of", x, "and", y, "is", x+y


Comments


Often we want to put some documentation in our program. These are comments for explanation, but not executed by the computer. 

If we have # anywhere on a line, everything following this on the line is a comment – ignored Numerical Input


To get numerical input from the user, we use an assignment statement of the form

= input()


Here






would be replaced by a prompt for the user inside quotation marks
If there is no prompt, the parentheses are still needed Semantics




The prompt will be displayed
User enters number
Value entered is stored as the value of the variable

Example - Fahrenheit to Centigrade


We want to convert a Fahrenheit temperature to Centigrade.



The formula is

C = (F -32) x 5/9



We use type float for the temperatures.

Python Session

Exercise: Convert temperatures 




Make a temperature conversion chart:
Fahrenheit to Centrigrade, for -40, 0, 32, 68, 98.6, 212, 293, 451
Output:
-40 -40.0
0 -17.7778
32 0.0
68 20.0
98.6 37.0
212 100.0
293 145.0
451 232.778
You have created a Python program!
(It doesn’t have to be this tedious, and it won’t be.)

Print Statement


For output we use statements of the form print 



Semantics
 Value of expression is computed
 This value is displayed
Several expressions can be printed – separate them by commas

Print Statement


print : Produces text output on the console.



Syntax: print ("Message“) print (Expression)


Prints the given text message or expression value on the console, and moves the cursor down to the next line. print (Item1, Item2, ..., ItemN)





Prints several messages and/or expressions on the same line.

Examples: print ("Hello, world!“) age = 45 print ("You have", 65 - age, "years until retirement“)
Output:
Hello, world!
You have 20 years until retirement
46

Input Statement


input : Reads a number from user input.You can assign (store) the result of input into a variable.
Example:
age = input("How old are you? ") print ("Your age is“), age print ("You have“), 65 - age, "years until retirement"

Output:
How old are you? 53
Your age is 53
You have 12 years until retirement

47

Input
Exercise:


Write a Python program that prompts the user for his/her amount of money, then reports how many Nintendo Wiis the person can afford, and how much more money he/she will need to afford an additional Wii.

48

Text and File Processing

49

Strings






string: A sequence of text characters in a program.
 Strings start and end with quotation mark " or apostrophe ' characters.
 Examples:
"hello"
"This is a string"
"This, too, is a string. It can be very long!"
A string may not span across multiple lines or contain a " character.
"This is not a legal String."
"This is not a "legal" String either."
A string can represent characters by preceding them with a backslash.
 \t tab character
 \n new line character
 \" quotation mark character
 \\ backslash character


Example: "Hello\tthere\nHow are you?"

50

More expressions: strings
A string represents text
'Python'
myclass = "CSE 190p"
""

Empty string is not the same as an unbound variable
Operations:
 Length: len(myclass) 

Concatenation:
"Michael" + 'Ernst'



Containment/searching:
'0' in myclass
"O" in myclass

Indexes


Characters in a string are numbered with indexes starting at 0:


Example: name = "P. Diddy" index 1

character



0
P

.

2

3

4

5

6

7

D

i

d

d

y

Accessing an individual character of a string: variableName [ index ]


Example: print name, "starts with", name[0]
Output:
P. Diddy starts with P

52

String properties







len(string)

- number of characters in a string
(including spaces) str.lower(string) - lowercase version of a string str.upper(string) - uppercase version of a string
Example:
name = "Martin Douglas Stepp" length = len(name) big_name = str.upper(name) print (big_name, "has", length, "characters“)

Output:
MARTIN DOUGLAS STEPP has 20 characters
53

Text processing


text processing: Examining, editing, formatting text.




often uses loops that examine the characters of a string one by one

A for loop can examine each character in a string in sequence. 

Example: for c in "booyah": print c
Output:
b o o y a h 54

Strings and numbers


ord(text)








Example: ord("a") is 97, ord("b") is 98, ...
Characters map to numbers using standardized mappings such as ASCII and Unicode.

chr(number)


- converts a string into a number.

- converts a number into a string.

Example: chr(99) is "c"

Exercise: Write a program that performs a rotation cypher. 

e.g. "Attack" when rotated by 1 becomes "buubdl"
55

File processing




Many programs handle data, which often comes from files.

Reading the entire contents of a file: variableName = open("filename").read()
Example:
file_text = open("bankaccount.txt").read()
56

Line-by-line processing
Reading a file line-by-line: for line in open("filename").readlines(): statements Example: count = 0 for line in open("bankaccount.txt").readlines(): count = count + 1 print "The file contains", count, "lines."
 Exercise: Write a program to process a file of DNA text, such as:
ATGCAATTGCTCGATTAG
 Count the percent of C+G present in the DNA.


57

Repetition (loops) and Selection (if/else)

58

The for loop


for loop: Repeats a set of statements over a group of values.


Syntax: for variableName in groupOfValues: statements 





We indent the statements to be repeated with tabs or spaces. variableName gives a name to each value, so you can refer to it in the statements. groupOfValues can be a range of integers, specified with the range function.

Example: for x in range(1, 6): print x, "squared is", x * x
Output:
1 squared
2 squared
3 squared
4 squared
5 squared

is is is is is

1
4
9
16
25

59

range


The range function specifies a range of integers:




- the integers between start (inclusive) and stop (exclusive)

It can also accept a third value specifying the change between values.




range(start, stop)

range(start, stop, step) - the integers between start (inclusive) and stop (exclusive) by step

Example: for x in range(5, 0, -1): print x print "Blastoff!"
Output:
5
4
3
2
1
Blastoff!



Exercise: How would we print the "99 Bottles of Beer" song?
60

Cumulative loops


Some loops incrementally compute a value that is initialized outside the loop. This is sometimes called a cumulative sum. sum = 0 for i in range(1, 11): sum = sum + (i * i) print "sum of first 10 squares is", sum
Output:
sum of first 10 squares is 385



Exercise: Write a Python program that computes the factorial of an integer. 61

if


if statement: Executes a group of statements only if a certain condition is true. Otherwise, the statements are skipped.




Syntax: if condition: statements Example: gpa = 3.4 if gpa > 2.0: print "Your application is accepted."

62

if/else


if/else statement: Executes one block of statements if a certain condition is True, and a second block of statements if it is False.




Syntax: if condition: statements else: statements Example: gpa = 1.4 if gpa > 2.0: print "Welcome to Mars University!" else: print "Your application is denied."



Multiple conditions can be chained with elif ("else if"): if condition: statements elif condition: statements else: statements 63

while


while loop: Executes a group of statements as long as a condition is True.




good for indefinite loops (repeat an unknown number of times)

Syntax: while condition: statements 

Example: number = 1 while number < 200: print number, number = number * 2


Output:
1 2 4 8 16 32 64 128

64

Logic


Many logical expressions use relational operators:
Operator

Meaning

Example

Result

==

1 + 1 == 2

True

!=

does not equal

3.2 != 2.5

True

<

less than

10 < 5

False

>

greater than

10 > 5

True

= 5.0

True

Logical expressions can be combined with logical operators:
Operator

Result

and

9 != 6 and 2 < 3

True

or

2 == 3 or -1 < 5

True

not



Example

not 7 > 0

False

Exercise: Write code to display and count the factors of a number.
65

More expressions: Conditionals
22 > 4
22 < 4
22 == 4 x = 100
# Assignment, not conditional!
22 = 4
# Error! x >= 5 x >= 100 x >= 200 not True
Numeric operators: +, *, ** not (x >= 200)
Boolean operators: not, and, or
3 3): print(“You are a Car!”) else: print(“That's a LOT of wheels!”)
What's wrong with testing using the greater-than operator? 91

Input Example – possible errors from the input() function userName = input(“What is your name?”)
 userAge = int( input(“How old are you?”)
)
 birthYear = 2007 - userAge


print(“Nice to meet you, “ + userName)
 print(“You were born in: “, birthYear)


input() is guaranteed to give us a string, no matter WHAT the user enters.
But what happens if the user enters “ten” for their age instead of 10?

92

Input Example – possible errors from the input() function 












userName = raw_input(“What is your name?”) userAge = input(“How old are you?”) try: userAgeInt = int(userAge) except: userAgeInt = 0 birthYear = 2010 - userAgeInt print(“Nice to meet you, “ + userName) if userAgeInt != 0: print(“You were born in: “, birthYear )

The try/except statements protects us if the user enters something other than a number. If the int() function is unable to convert whatever string the user entered, the except clause
93
will set the userIntAge variable to zero.

Repetition can be useful!
Sometimes you want to do the same thing several times.
Or do something very similar many times.
One way to do this is with repetition: print 1 print 2 print 3 print 4 print 5 print 6 print 7 print 8 print 9 print 10
94

Looping, a better form of repetition.
Repetition is OK for small numbers, but when you have to do something many, many times, it takes a very long time to type all those commands.
We can use a loop to make the computer do the work for us.
One type of loop is the “while” loop. The while loop repeats a block of code until a boolean expression is no longer true.
Syntax:
while boolean expression :
STATEMENT
STATEMENT
95
STATEMENT

How to STOP looping!
It is very easy to loop forever: while True : print( “again, and again, and again”)

The hard part is to stop the loop!
Two ways to do that is by using a loop counter, or a termination test.
A loop counter is a variable that keeps track of how many times you have gone through the loop, and the boolean expression is designed to stop the loop when a specific number of times have gone bye.
A termination test checks for a specific condition, and when it happens, ends the loop. (But does not guarantee that the loop will end.)
96

Loop Counter






timesThroughLoop = 0

while (timesThroughLoop < 10): print(“This is time”, timesThroughLoop, the loop.”) timesThroughLoop = timesThroughLoop + 1

“in

Notice that we:

Initialize the loop counter (to zero)
Test the loop counter in the boolean expression (is it smaller than 10, if yes, keep looping)
Increment the loop counter (add one to it) every time we go through the loop

If we miss any of the three, the loop will NEVER stop! 97

While loop example, with a termination test

Keeps asking the user for their name, until the user types “quit”. keepGoing = True while ( keepGoing): userName = input(“Enter your name! (or quit to exit)” ) if userName == “quit”: keepGoing = False else: print(“Nice to meet you, “ + userName) print(“Goodbye!”) 98