Python

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Introduction to Programming
with Python
Created by : Jagpreet Kaur
Asst prof. in CS

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 Some influential ones:
 FORTRAN
 science / engineering
 COBOL
 business data
 LISP
 logic and AI
 BASIC
 a simple language
Languages

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Compiling and interpreting
 Many languages require you to compile (translate) your program
into a form that the machine understands.
 Python is instead directly interpreted into machine instructions.
compile execute
outputsource code
Hello.java
byte code
Hello.class
interpret
outputsource code
Hello.py

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Expressions
 expression: A data value or set of operations to compute a value.
Examples: 1 + 4 * 3
42
 Arithmetic operators we will use:
 + - * / addition, subtraction/negation, multiplication, division
 % modulus, a.k.a. remainder
 ** exponentiation
 precedence: Order in which operations are computed.
 * / % ** have a higher precedence than + -
1 + 3 * 4 is 13
 Parentheses can be used to force a certain order of evaluation.
(1 + 3) * 4 is 16

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Variables
 variable: A named piece of memory that can store a value.
 Usage:
 Compute an expression's result,
 store that result into a variable,
 and use that variable later in the program.
 assignment statement: Stores a value into a variable.
 Syntax:
name = value
 Examples: x = 5
gpa = 3.14
x 5 gpa 3.14
 A variable that has been given a value can be used in expressions.
x + 4 is 9
 Exercise: Evaluate the quadratic equation for a given a, b, and c.

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 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
print

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 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
 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.
input

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Repetition (loops)
and Selection (if/else)

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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 is 1
2 squared is 4
3 squared is 9
4 squared is 16
5 squared is 25

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range
 The range function specifies a range of integers:
 range(start, stop) - the integers between start (inclusive)
and stop (exclusive)
 It can also accept a third value specifying the change between values.
 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?

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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."

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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

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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

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Logic
 Many logical expressions use relational operators:
 Logical expressions can be combined with logical operators:
 Exercise: Write code to display and count the factors of a number.
Operator Example Result
and 9 != 6 and 2 < 3 True
or 2 == 3 or -1 < 5 True
not not 7 > 0 False
Operator Meaning Example Result
== equals 1 + 1 == 2 True
!= does not equal 3.2 != 2.5 True
< less than 10 < 5 False
> greater than 10 > 5 True
<= less than or equal to 126 <= 100 False
>= greater than or equal to 5.0 >= 5.0 True

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 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: "HellottherenHow are you?"
Strings

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Indexes
 Characters in a string are numbered with indexes starting at 0:
 Example:
name = "P. Diddy"
 Accessing an individual character of a string:
variableName [ index ]
 Example:
print name, "starts with", name[0]
Output:
P. Diddy starts with P
index 0 1 2 3 4 5 6 7
character P . D i d d y

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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

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 raw_input : Reads a string of text from user input.
 Example:
name = raw_input("Howdy, pardner. What's yer name? ")
print name, "... what a silly name!"
Output:
Howdy, pardner. What's yer name? Paris Hilton
Paris Hilton ... what a silly name!
raw_input

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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

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Strings and numbers
 ord(text) - converts a string into a number.
 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 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"

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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()

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