06 es26 lab - selection control structure
TRANSCRIPT
Introduction to ProgrammingIntroduction to Programming
Selection Control StructureSelection Control Structure
66
Introduction to ProgrammingIntroduction to Programming
ObjectivesObjectives
After completing this lesson, you should be able to do the following:
• Understand how a selection control structure works
• Learn how to use the if-else selection control structure
Introduction to ProgrammingIntroduction to Programming
Program Control StructureProgram Control Structure
Control Flow – the order in which statements are executed in a program
3 Kinds of Control Structures• Sequence• Selection
• Repetition/Iteration
Introduction to ProgrammingIntroduction to Programming
Sequence Control StructureSequence Control Structure
• Refers mainly to the sequential execution of statements
• A statement specifies the type of operation to be done such as the declaration statement
int num;
Introduction to ProgrammingIntroduction to Programming
sum = x + y;
ave = sum / 2;
printf(“%d”,ave);
Example: Average of Two NumbersExample: Average of Two Numbers
sum = x + y
ave = sum / 2
Display value of ave
Code
Flowchart
Introduction to ProgrammingIntroduction to Programming
Compound StatementCompound Statement
• A compound statement is a group of statements enclosed by { and } that are executed sequentially
• For example
main() {
. . .
}
Introduction to ProgrammingIntroduction to Programming
Selection Control StructureSelection Control Structure
• Chooses among alternatives statements• Uses a condition to select a course of action• A condition is either true or false
3 Types of Selection Control Structure
1. if
2. if-else
3. switch
Introduction to ProgrammingIntroduction to Programming
If StatementIf Statement• Format:
if (expression) stmt;
• Example: A program that determines whether a given number is positive or not
If (number > 0) printf(“It is a positive number”);
Introduction to ProgrammingIntroduction to Programming
If StatementIf Statement
number > 0true
false
It is a positive number
Next statement
Introduction to ProgrammingIntroduction to Programming
ConditionsConditions
• Usually, the condition in an if statement is either a
1. Relational expression
2. Equality expression
3. Logical expression
Introduction to ProgrammingIntroduction to Programming
Relational ExpressionsRelational Expressions
• Relational operators:
< > <= >=• Examples: Given the declaration
int a = 5, b = 6;
if (a < b)
if (a < 3)
if (b >= 6)
true
false
true
Introduction to ProgrammingIntroduction to Programming
Equality expressionsEquality expressions
• Equality Operators
== (equal)
!= (not equal)
• Example: Given the declaration
int i = 1, j = 2;
if (i == 1)
if (i == (j+1))
if ((i+2) != j)
true
false
true
Introduction to ProgrammingIntroduction to Programming
Logical ExpressionsLogical Expressions
• Logical operator: Binary AND && • Truth table of &&
AND False True
False False False
True False True
Introduction to ProgrammingIntroduction to Programming
Logical Expressions (AND)Logical Expressions (AND)
• Examples: Given the declaration
int i = 1, j = 2, k = 3;
if ( (i < j) && ( j < k )) if ( (i && j) && k) if ( i && j)
Introduction to ProgrammingIntroduction to Programming
Logical ExpressionsLogical Expressions
• Logical operator: Unary ! (not)• Truth table of NOT
Value of expr !expr
Zero True
Non-zero False
Introduction to ProgrammingIntroduction to Programming
Logical Expressions (NOT)Logical Expressions (NOT)
• Examples: Given the declaration
int x = 7, y = 7;
if ( !x )
if ( !(x – y))
if ( !!x )
if( !(! (x + y))
Introduction to ProgrammingIntroduction to Programming
• Logical operator: Binary || (or)• Truth table
OR False True
False False True
True True True
Logical Expressions (OR)Logical Expressions (OR)
Introduction to ProgrammingIntroduction to Programming
Logical Expressions (OR)Logical Expressions (OR)
• Examples: Given the declaration
int i =1, j =2, k = 3;
if ((i<j) || (i<k))
if((i==1)||((j>1)&&(k==3)))
if (i||j)
Introduction to ProgrammingIntroduction to Programming
If-Else StatementIf-Else Statement
Syntax:
if (condition)
statement1;
else
statement2;
Semantic: If condition is true, then statement 1 is executed and statement 2 is skipped;
if condition is false, then statement 1 is skipped and statement 2 is executed.
Introduction to ProgrammingIntroduction to Programming
Example: If-Else StatementExample: If-Else Statement
if (number > 0)
printf(“A positive number. \n”);
else
printf(“Not a positive number.\n”);
Introduction to ProgrammingIntroduction to Programming
if-else Statementif-else Statement
number > 0
A positive number
truefalse
Not a positive number
Next statement
Introduction to ProgrammingIntroduction to Programming
Nested If StatementsNested If Statements
if( x > 0)printf(“x is positive”);
else{
}
if(x < 0)printf(“x is negative”);
elseprintf(“x is zero”);