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

10010

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 = 3 + 7

+

73

10

MemoryA

Python handles the RHS.

A = 3

A = A + 1

Memory

A3

Your Code

3

Python handles the RHS.

A = A + 1

+

1AMemory

A3

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, 5 + 2 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 = 02. 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 28th May.

The late homework will not be scored.

I will announce how to submit your homework on

next Tuesday.