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Introduction to
Programming
Languages
Welcome Python Programming 1

Programming language
•A programming language is a system of
notation for writing computer programs
• Programming is used to automate, maintain, assemble,
measure, and interpret the processing of the data and
information
Welcome Python Programming 2

Programs
•Programs are written in programming languages
•Pieces of the same program can be written in
different programming languages
•A set of rules and symbols used to construct a
computer program
•A language used to interact with the computer

The Programming Cycle for
Python
Jan-24 Programming 5

Write/Edit
Run
with some input
OK?
YES
More
Inputs?
YES
NO
NO

Filename, preferred extension is py
User Program

Python Shell is Interactive
IN[1]:
IN[2]:
IN[4]:
IN[3]:
Python Shell Prompt
User Commands
(Statements)
Outputs
( )

Elements of Python
• A Python program is a sequence of definitions and commands
(statements)
• Commands manipulate objects
• Each object is associated with a Type
• Type:
• A set of values
• A set of operations on these values
• Expressions: An operation (combination of objects and operators)

Types in Python
• int
• Bounded integers, e.g. 732 or -5
• float
• Real numbers, e.g. 3.14 or 2.0
• long
• Long integers with unlimited precision
• str
• Strings, e.g. ‘hello’ or ‘C’
• bool
• Boolean, true or false

Example of Types

Type Conversion (Type Cast)
• Conversion of value of one type to other
• We are used to int ↔ float conversion in Math
• Integer 3 is treated as float 3.0 when a real number is expected
• Float 3.6 is truncated as 3, or rounded off as 4 for integer contexts
• Type names are used as type converter functions

Type Conversion Examples
Note that float to int conversion
is truncation, not rounding off

Type Conversion and Input

Operators
• Arithmetic
• Comparison
+ - * // / % **
== != > < >= <=

Variables
• A name associated with an object
• Assignment used for binding
m = 64;
c = ‘Acads’;
f = 3.1416;
• Variables can change their bindings
f = 2.7183;
64
Acads
3.1416
2.7183
m
c
f

Assignment Statement
• A simple assignment statement
Variable = Expression;
• Computes the value (object) of the expression on the right hand side
expression (RHS)
• Associates the name (variable) on the left hand side (LHS) with the
RHS value
• = is known as the assignment operator.

Programming using Python
Operators and Expressions
1/22/2024 Programming 18

Binary Operations
Op Meaning Example Remarks
+ Addition 9+2 is 11
9.1+2.0 is 11.1
- Subtraction 9-2 is 7
9.1-2.0 is 7.1
* Multiplication 9*2 is 18
9.1*2.0 is 18.2
/ Division 9/2 is 4.50 In Python3
9.1/2.0 is 4.55 Real div.
// Integer Division 9//2 is 4
% Remainder 9%2 is 1

The // operator
• Also referred to as “integer division”
• Result is a whole integer (floor of real division)
• But the type need not be int
• the integral part of the real division
• rounded towards minus infinity (−∞)
• Examples
9//4 is 2 (-1)//2 is -1 (-1)//(-2) is 0
1//2 is 0 1//(-2) is -1 9//4.5 is 2.0

The % operator
•The remainder operator % returns the
remainder of the result of dividing its
first operand by its second.
9%4 is 1 (-1)%2 is 1 (-1)//(-2) is 0
9%4.5 is 0.0 1%(-2) is 1 1%0.6 is 0.4
Ideally: x == (x//y)*y + x %y

Conditional Statements
•In daily routine
•If it is very hot, I will skip exercise.
•If there is a quiz tomorrow, I will
first study and then sleep.
Otherwise I will sleep now.
•If I have to buy coffee, I will
go left. Else I will go
straight.

if-else statement
• Compare two integers and print the min.
1. Check if x is less
than y.
2. If so, print x
3. Otherwise, print y.
if x < y:
print (x)
else:
print (y)
print (‘is the minimum’)

x,y = 6,10
if x < y:
print (x)
else:
print (y)
print (‘is the min’)
x y
6 10
Run the program
Output
6
Indentation
•Indentation is important in Python
• grouping of statement (block of statements)
• no explicit brackets, e.g. { }, to group statements

if statement (no else!)
• General form of the if statement
• Execution of if statement
• First the expression is evaluated.
• If it evaluates to a true value, then S1 is executed and then control moves to
the S2.
• If expression evaluates to false, then control moves to the S2 directly.
if boolean-expr :
S1
S2
S1
S2

if-else statement
• General form of the if-else statement
• Execution of if-else statement
• First the expression is evaluated.
• If it evaluates to a true value, then S1 is executed and
then control moves to S3.
• If expression evaluates to false, then S2 is executed and
then control moves to S3.
• S1/S2 can be blocks of statements!
if boolean-expr :
S1
else:
S2
S3
S2
S1
S3

Nested if, if-else
if a <= b:
if a <= c:
…
else:
…
else:
if b <= c) :
…
else:
…

Elif
• A special kind of nesting is the chain of if-else-if-else-… statements
• Can be written elegantly using if-elif-..-else
if cond1:
s1
elif cond2:
s2
elif cond3:
s3
elif …
else
last-block-of-stmt
if cond1:
s1
else:
if cond2:
s2
else:
if cond3:
s3
else:
…

Summary of if, if-else
•if-else, nested if's, elif.
•Multiple ways to solve a problem
•issues of readability,
maintainability
•and efficiency

Class Quiz
• What is the value of expression:
a) Run time crash/error
b) I don’t know / I don’t care
c) False
d) True
(5<2) and (3/0 > 1)
The correct answer is
False

Programming using Python
Loops
Jan-24 Python Programming 31

Program…
n = int(input('Enter a number: '))
print (n, 'X', 1, '=', n*1)
print (n, 'X', 2, '=', n*2)
print (n, 'X', 3, '=', n*3)
print (n, 'X', 4, '=', n*4)
print (n, 'X', 5, '=', n*5)
print (n, 'X', 6, '=', n*6)
….
Too much
repetition!
Can I avoid
it?

While Statement
1. Evaluate expression
2. If TRUE then
a) execute statement1
b) goto step 1.
3. If FALSE then execute statement2.
while (expression):
S1
S2
FALSE
TRUE
S1
expression
S2

For loop in Python
• General form
for variable in sequence:
stmt

For Loop
The range() Function: To loop through a set
of code a specified number of times, we
can use the range() function
 for x in range(6):
print(x)

For Loop
The range() function defaults to
0 as a starting value, it is
possible to specify the starting
value by adding a parameter:
range(2, 6), which means values
from 2 to 6 (but not including
6):
 for x in range(2, 6):
print(x)

# print all odd numbers < 10
i = 1
while i < 10:
print(i)
i += 2
Quiz

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