An structure is a logical design that controls the order in which a set of statements execute

if Statements

The if statement is used to create a decision structure, which allows a program to have more

than one path of execution. The if statement causes one or more statements to execute only

when a Boolean expression is true.

A control structure is a logical design that controls the order in which a set of statements

execute.  sequence structure (a set of statements that execute in the order in which they

appear.)

Although the sequence structure is heavily used in programming, it cannot handle every type of

task. This is because some problems simply cannot be solved by performing a set of ordered

steps, one after the other. Programs like this require a different type of control structure: one

that can execute a set of statements only under certain circumstances. This can be

accomplished with a decision structure. (Decision structures are also known as selection

structures.)

The action is conditionally executed because it is performed only when a certain condition is

true.

single alternative decision structure: provides only one alternative path of execution.

In Python, we use the if statement to write a single alternative decision structure.

General format:

Chapter 4Control structureis a logical design that controls the order in which a set ofstatements executesDecision structure(aka selection structure) allows a program to performactions only under certain conditions.ouse to handle any part of a program that needs to test a condition andconditionally execute an action depending on the outcome of thecondition.Single alternative decision structure: only provides one alternative path ofexecutionoif condition is true(or false) we do something, otherwise we exit thestructureoif-then statementIfconditionThenstatementstatementstatementetc.End Ifocondition must be a Boolean expressionotested by relational operator’Case structure (multiple alternative decision structure): allows you to testvalue of variable or an expression and use that value to determine whichstatement or set of statements to executeoin pseudocode: useSelect CasestatementoSelecttestExpression

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relational operator, Switch statement, or operator

Presentation on theme: "The if Statement Control structure: logical design that controls order in which set of statements execute Sequence structure: set of statements that execute."— Presentation transcript:

1

2 The if Statement Control structure: logical design that controls order in which set of statements execute Sequence structure: set of statements that execute in the order they appear Decision structure: specific action(s) performed only if a condition exists Also known as selection structure

3 The if Statement (cont’d.)
In flowchart, diamond represents true/false condition that must be tested Actions can be conditionally executed Performed only when a condition is true Single alternative decision structure: provides only one alternative path of execution If condition is not true, exit the structure

4 The if Statement (cont’d.)

5 The if Statement (cont’d.)
Python syntax: if condition: Statement First line known as the if clause Includes the keyword if followed by condition The condition can be true or false When the if statement executes, the condition is tested, and if it is true the block statements are executed. Otherwise, block statements are skipped

6 Boolean Expressions and Relational Operators
Boolean expression: expression tested by if statement to determine if it is true or false Example: a > b true if a is greater than b; false otherwise Relational operator: determines whether a specific relationship exists between two values Example: greater than (>)

7 Boolean Expressions and Relational Operators (cont’d.)
>= and <= operators test more than one relationship It is enough for one of the relationships to exist for the expression to be true == operator determines whether the two operands are equal to one another Do not confuse with assignment operator (=) != operator determines whether the two operands are not equal

8 Boolean Expressions and Relational Operators (cont’d.)

9 Boolean Expressions and Relational Operators (cont’d.)
Using a Boolean expression with the > relational operator

10 Boolean Expressions and Relational Operators (cont’d.)
Any relational operator can be used in a decision block Example: if balance == 0 Example: if payment != balance It is possible to have a block inside another block Example: if statement inside a function Statements in inner block must be indented with respect to the outer block

11 Example # This program gets three test scores and displays their average. It congratulates the user if the average is high. high_score = 95 # Get the three test scores. test1 = int(input('Enter the score for test 1: ')) test2 = int(input('Enter the score for test 2: ')) test3 = int(input('Enter the score for test 3: ')) # Calculate the average test score. average = (test1 + test2 + test3) / 3 # Print the average. print('The average score is', average) # If the average is a high score, congratulate the user. if average >= high_score: print('Congratulations!') print('That is a great average!')

12 The if-else Statement Dual alternative decision structure: two possible paths of execution One is taken if the condition is true, and the other if the condition is false Syntax: if condition: statements else: other statements if clause and else clause must be aligned Statements must be consistently indented

13 The if-else Statement (cont’d.)

14 The if-else Statement (cont’d.)

15 # Variables for base hours and the overtime multiplier.
base_hours = # Base hours per week ot_multiplier = # Overtime multiplier # Get the hours worked and the hourly pay rate. hours = float(input('Enter the number of hours worked: ')) pay_rate = float(input('Enter the hourly pay rate: ')) # Calculate and display the gross pay. if hours > base_hours: # First, get the number of overtime hours worked. overtime_hours = hours - base_hours # Calculate the amount of overtime pay. overtime_pay = overtime_hours * pay_rate * ot_multiplier # Calculate the gross pay. gross_pay = base_hours * pay_rate + overtime_pay else: # Calculate the gross pay without overtime. gross_pay = hours * pay_rate # Display the gross pay. print('The gross pay is $', gross_pay)

What is the structure that causes a statement or a set of statements to execute repeatedly?

What structure can execute a set of statements only under certain circumstances?

What type of loop structure repeats the code a specific number of times?

When a function is called by its name during the execution of a program then it is?