PROBLEM SOLVING AND PYTHON PROGRAMMING UNIT 2

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Python Programming
Using Problem Solving Approach
Reema Thareja
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© OXFORD UNIVERSITY PRESS 2017.ALL RIGHTS RESERVED.

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CHAPTER 4
Decision Control Statements

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Control Statements
A control statement is a statement that determines the control flow of a set of
instructions, i.e., it decides the sequence in which the instructions in a program
are to be executed.
Types of Control Statements —
• Sequential Control: A Python program is executed sequentially from the first
line of the program to its last line.
• Selection Control: To execute only a selected set of statements.
• Iterative Control: To execute a set of statements repeatedly.

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If Statement
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© OXFORD UNIVERSITY PRESS 2017.ALL RIGHTS RESERVED. 5
# Get user input for a number
number = float(input("Please enter a
number: "))
# Check if the number is positive
if number > 0:
print("The number is positive.")

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If-Else Statement

# Input: Get the user's age
age = int(input("Enter your age: "))
# Check for voting eligibility
if age >= 18:
print("You are eligible to vote.")
else:
print("You are not eligible to vote.")

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Nested if Statements
A statement that contains other statements is called a
compound statement. To perform more complex checks, if
statements can be nested, that is, can be placed one inside
the other. In such a case, the inner if statement is the
statement part of the outer one. Nested if statements are
used to check if more than one conditions are satisfied.
Example:

# Input: Get age and student status from the user
age = int(input("Enter your age: "))
is_student = input("Are you a student? (yes/no): ")
# Check for discounts
if age < 18:
print("You qualify for a child discount.")
if age >= 18 and age < 65:
if is_student == "yes":
print("You qualify for a student discount.")
else:
print("You do not qualify for a discount.")
if age >= 65:
print("You qualify for a senior discount.")

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If-elif-else Statement
Python supports if-elif-else statements to test additional
conditions apart from the initial test expression.The if-elif-
else construct works in the same way as a usual if-else
statement. If-elif-else construct is also known as nested-if
construct.

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While Loop
Example:

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For Loop
For loop provides a mechanism to repeat a task until a particular condition isTrue. It is usually known as a
determinate or definite loop because the programmer knows exactly how many times the loop will repeat.
The for...in statement is a looping statement used in Python to iterate over a sequence of objects.

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For Loop and Range() Function
The range() function is a built-in function in Python that is used to iterate over a sequence of numbers.The
syntax of range() is range(beg, end, [step])
The range() produces a sequence of numbers starting with beg (inclusive) and ending with one less than the
number end.The step argument is option (that is why it is placed in brackets). By default, every number in
the range is incremented by 1 but we can specify a different increment using step. It can be both negative and
positive, but not zero.
Examples:

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Range() Function
If range() function is given a single argument, it produces an object with values from 0 to argument-1. For
example: range(10) is equal to writing range(0, 10).
• If range() is called with two arguments, it produces values from the first to the second. For example,
range(0,10).
• If range() has three arguments then the third argument specifies the interval of the sequence produced. In
this case, the third argument must be an integer. For example, range(1,20,3).
Examples:

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Condition-controlled and Counter-controlled Loops

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Nested Loops
Python allows its users to have nested loops, that is, loops that can be placed inside other loops.Although this
feature will work with any loop like while loop as well as for loop.
A for loop can be used to control the number of times a particular set of statements will be executed.
Another outer loop could be used to control the number of times that a whole loop is repeated.
Loops should be properly indented to identify which statements are contained within each for statement.
Example:

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The Break Statement
The break statement is used to terminate the execution of the nearest enclosing loop in which it appears.
The break statement is widely used with for loop and while loop. When compiler encounters a break
statement, the control passes to the statement that follows the loop in which the break statement appears.
Example:

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The Continue Statement
Like the break statement, the continue statement can only appear in the body of a loop.When the compiler
encounters a continue statement then the rest of the statements in the loop are skipped and the control is
unconditionally transferred to the loop-continuation portion of the nearest enclosing loop.
Example:

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The Pass Statement
Pass statement is used when a statement is required syntactically but no command or code has to be
executed. It specified a null operation or simply No Operation (NOP) statement. Nothing happens when the
pass statement is executed.
Difference between comment and pass statements In Python programming, pass is a null statement. The
difference between a comment and pass statement is that while the interpreter ignores a comment
entirely, pass is not ignored. Comment is not executed but pass statement is executed but nothing happens.
Example:

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The Else Statement Used With Loops
Unlike C and C++, in Python you can have the else statement associated with a loop statements. If the else
statement is used with a for loop, the else statement is executed when the loop has completed iterating. But
when used with the while loop, the else statement is executed when the condition becomes false.
Examples:

PROBLEM SOLVING AND PYTHON PROGRAMMING UNIT 2

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