Control Structures
Dr. Sumi Suresh M. S.
Assistant Professor
Department of Computer Science and Engineering

Flow of Control
• Control flow is the order that instructions are executed in a program.
• A control statement is a statement that determines the control flow of
a set of instructions.
• There are three fundamental forms of control that programming
languages provide:
➢ Sequential control - default mode
➢ Selection control - used for decisions and branching(if, if else, if
elif, nested if)
➢ Iterative control- used for looping, i.e., repeating a piece of code
multiple times ( for, while)
➢Transfer /Jump- (break, continue, return)

Control Structure

Sequential Control
• Sequential statements are a set of statements whose execution process
happens in a sequence.
• The problem with sequential statements is that if the logic has broken in
any one of the lines, then the complete source code execution will break.

Selection Control
• Provides selective execution of instructions.
• The selection statement allows a program to test several conditions and execute instructions
based on which condition is true.
• Python have the following types of selection statements
1. if statement
2. if else statement
3. if-elif-else
4. Nested if statement

if Statement
• If statements are control flow statements that help us to run a particular code,
but only when a certain condition is met or satisfied
• A simple if only has one condition to check.

if statement

Indentation
• Python uses indentation to indicate a block of code.
• Python indentation refers to adding white space
before a statement to a particular block of code.
• If statement, without indentation (will raise an error):

if else Statement
• Two alternative paths and the control condition determines which
path gets executed.
• The syntax for if else statement is as follows.

If else Statement
Q1. Display the number is odd or even

if else Statement
Q1. Display the positive difference of two numbers num1 and num2

if else Statement
1. Write a program to print if the person is eligible for voting or not (accept age as
input.
2. Take two different integer values from user and print greatest among them
using if else statements.
3. Take values of length and breadth of a rectangle from user and check if it is
square or not using if else statements.

If- elif Statement
• elif means else..if
• The elif keyword is pythons way of saying "if the previous conditions were not
true, then try this condition".
• At the end else block is added which is performed if none of the above if-elif
statement is true

If- elif Statement

If- elif Statement

If- elif Statement

Nested if statement
• if statements can be nested, resulting in multi-way selection

Nested if statement
• if statements can be nested, resulting in multi-way selection

Nested if statement

Questions
1. Check whether a number is positive, negative, or zero.
2. Write a program to find largest among three numbers.
3. A school has following rules for grading system:
a. Below 25 - F
b. 25 to 45 - E
c. 45 to 50 - D
d. 50 to 60 - C
e. 60 to 80 - B
f. Above 80 - A
Ask user to enter marks and print the corresponding grade.
4. Display the appropriate message as per the colour of signal at the road
crossing.
5. Take input of age of 3 people by user and determine oldest and
youngest among them.

Questions
6. Write a program to prompt a user to enter a day of the week. If the
entered day of the week is between 1 and 7 then display the
respective name of the day.
7. Write a program that prompts a user to enter two different
numbers. Perform basic arithmetic operations based on the choices.
1. Addition
2. Subtraction
3. Multiplication
4. Division

Nested if statement

Iterative Control
• Loop control statements
• Provides the repeated execution of a set of instructions.
• Iterative control structures are commonly referred to as “loops.”
while Loop
for Loop
• A definite loop is a program loop in which the number of times the
loop will iterate can be determined before the loop is executed.
• An indefinite loop is a program loop in which the number of times that
the loop will iterate cannot be determined before the loop is executed.

While Statement
• Iterative control statement that repeatedly executes a set of statements
based on a provided Boolean expression (condition).
• If the condition is true the statements will be executed, otherwise the
control will jump to immediate statement outside the while loop block.

While Statement

While Statement
• Infinite loops

Questions
• Write a program to print first 10 numbers using while loop.
• Write a program to calculate sum and average of first 10 numbers.
• Write a program to calculate the sum of numbers from m to n.
• Write a program to find the given number is an amstrong number.
(eg: 371 = 33+73+13)
• Write a program to print the reverse of a number.

For Loop
• A for statement is an iterative control statement that iterates once for each
element in a specified sequence of elements.
• Usually known as a definite loop because the programmer knows the
number of times the loop will repeat.
Using while loop :

For Loop

For Loop
1. 4 2. Apple 3. 4 4. A
3 Banana 2 p
2 Pear 3 p
1 1 l
e

The range() function
• Generate a list of integers.
• The range function has one, two or three parameters.
• The last two parameters in range() are optional.
• The general form of the range function is:
range(begin, end, step)

The range() function

Iterating Over List Elements vs. List Index Values
• Try
print(sum, end='**’)
• end – print at the end

Questions
• Write a program using for loop to print numbers from 1 to 10.
• Write a program using for loop to print even numbers from 1 to 10.
• Write a program using for loop to print odd numbers from 1 to 10.
• Write a program to display capital letters from A to Z (A-65, Z-91).

H.W Questions
• Use for loop to print numbers from 1 to 10 in reverse order.
• Write a program to calculate the sum of numbers from 1 to 20 which
are not divisible 2, 3 or 5.
• Write a program that prompts a user to enter four numbers and find the
greatest number among the four numbers entered.

Nested loops
• Loops within the loops.
• One loop is inserted completely within another loop
for i in range(1,4,1):
print(i)
Output
1
2
3
for i in range(1,4,1):
for j in range(1,4,1):
print(i)
Output
1
1
1
2
2
2
3
3
3

Questions
• Write a program to print
(a) 1 2 3 4 5 (b) pass1- 1 2 3 4 5
1 2 3 4 5 pass2- 1 2 3 4 5
1 2 3 4 5 pass3- 1 2 3 4 5
for i in range(1,6,1):
print("pass",i,"- ",end='')
for j in range(1,6,1):
print(j,end=' ')
print('n')

Questions
• Write a program to print pattern
*
**
***
****
• Write a program to print multiplication table from 1 to 5

Break Statement
• The keyword break allows a programmer to terminate a loop.
• The break statement is encountered inside a loop, the loop is
immediately terminated and the program control automatically goes to
the first statement following the loop.

Break Statement

Break Statement

Continue Statement
• Exactly opposite of the break statement.
• When continue is encountered within a loop, the remaining statements
within the body are skipped but the loop condition is checked to see if
the loop should continue or exit.

Continue Statement

Continue Statement

Continue Statement

pass Statement
• No command or code is executed
• Can be used as a placeholder.
E.g. If we have a loop that is not implemented yet. But we wish to
write some code in the future.
Cannot have an empty loop, so we can enter pass

The else Statement used with loops
• Can be used with for loop and while loop.
• When used with for loop, the else statement is executed when the loop
has completed iterating.
• When used with while loop, the else statement is executed when the
condition becomes false.