 Recap – Python Basics

Python  Python is an interpreter which reads and executes a python script.  A python script is a text file, which contains only text; no font, no style.  Once Python reads a file, it will execute the script line by line.  The order of execution is important to understand the behavior of the python script.

Analogy  Learning a computer program is similar to learning how to write and how to run a theatre play.

Analogy – Theatre Play  The script file is exactly the python script.

Analogy – Theatre Play  A programmer is a writer because the programmer writes a script.

Analogy – Theatre Play  The Python interpreter is a director because it interprets your script and controls the flow of execution as you intended.

Analogy – Theatre Play  Who is an actor?  There are operators and functions.

Analogy - Orchestra  You are a composer and Python is a conductor.

Analogy - Orchestra  A python script is a music score and each note is a literal (data).

Analogy - Orchestra  A music score can also have a loop though there is no counter nor conditions.

Analogy – Theatre Play  Flash Back  a device in the narrative of a motion picture, novel, etc., by which an event or scene taking place before the present time in the narrative is inserted into the chronological structure of the work.

Analogy Write a script. A director and actors practice together. The show opens in a theatre. Write a Python script. Python and operators test together. A program runs and produces an output.

Analogy  As a programmer, we are a composer or a writer.

Analogy  Our program is …

Analogy  Let’s not be fooled that we could write such a great master piece in this class or in a few days.

Analogy  The language of a theatre play is English.  The language of an orchestra is Music.  The language of a computer program is essentially Mathematics.  We have to be good at (at least) basic Mathematics.

Programming  Writing a program is a very complex activity that involves a different way of thinking.  Three things are different (at least):  Sequential order of Execution.  No automation; You should be very specific.  Different definition of operations; ( ‘=‘ for assignment, ‘==‘ for equality)

What we’ve learned  Literals: basic inputs  Numbers ( 3, 4, 3.2, 2.7 … )  String ( “hello”, ‘hi’, … )  Variable: a named storage or memory  Tuple: a composite data with an implicit form  (1,2), (“Name”, 3), …  List: a sequential container  [1,2,3], [“Hi”, “Every”, “One”], …

Statement  Direction to a computer which is interpreted by Python.  IF statement tests a condition (True or False)  While statement repeats while a condition meets.  For statement repeats for each element in a list/tuple/string

Even if we know those…  We have to know …  How to run a script?  How Python interprets your code?  What you want to do? What you want to compute?

Variable  A variable is a named storage or memory. L

Assignment Operator  ‘=‘ is used as assignment. a = 3  The assignment is an action to store a value into a variable.  The order of execution is from the right to the left.

Assignment Operator a = 1 a = a + 1

Assignment Operator  The assignment operator always move from the right to the left.  The left always has to be a destination. a + 1 = a

Python handles the RHS.  Python evaluates the expression which appear in the right hand side. A = Memory A

Python handles the RHS. A = 3 A = A + 1 Memory A 3 Your Code 3

Python handles the RHS. A = A A Memory A 3 3 4

Expression  A structure for evaluation is called ‘Expression’. + 1A 3 4

Expression  Expression is like an equation in Mathematics.  With an operator and operands (data), Python reduces an expression into a value.  For example, transforms into 7.  For example, “Hello” + “, World” changes into “Hello, World”

Note that  The result of operations is different between types! 7+8  15 “Alpha”+”bet”  ”Alphabet” 7/2  3 but 7 / 2.0  3.5

Assignment Operator  The mover can move the result only after the expression is evaluated.

Let’s see what Python doing A = 1 B = 2 A = A + 1 B = A + B A = A + B B = A + B A = A + B

Change of Flows  A Music sheet also has non-conditional loops

IF Statement  IF statement is a branch with a certain condition.  IF statement has a block of code to execute. Code to Execute With True

Conditional Expression 1. A = True 2. B = False 3. C = 3 == 4 4. D = (3+2) > 5 5. F = 3 in [1,2,3] 6. If D: X = 0

While Loop  When Python reads a WHILE statement, it will repeat a block of code within the WHILE statement. 1 I = 0 2 WHILE I < 3: 3 I = I + 1 The order of execution is 1 [ 2 3 ] [ 2 3 ] [ 2 3 ]

FOR Statement  When Python reads FOR statement, it will bring up each element in a list/tuple/string one by one. 1.A = 0 2.FOR x IN [1,2,3]: 3. A = A + x The order of execution is 1 [2 3] [2 3] [2 3]

A Program  Let’s write our own program.  Imagine as if you are the writer or the composer.  You have to know about characters very well (operators).  You have to know about what you want to convey (execute).

Practice 1  Summation  What if?

Practice 2  Test if N is a prime

Practice 2  If there is any zero in L, then N is not a prime.

Homework  Due is a week.  The homework should be submitted by 28 th May.  The late homework will not be scored.  I will announce how to submit your homework on next Tuesday.