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Chapter No#
Decision Making Statements in C++
Decision making structures require that the programmer specify one or more conditions to be evaluated or tested by the program, along with a statement or statements to be executed if the condition is determined to be true, and optionally, other statements to be executed if the condition is determined to be false. Following is the general form of a typical decision making structure found in most of the programming languages − C++ programming language provides following types of decision making statements. Sr.No Statement & Description 1 if statement An ‘if’ statement consists of a boolean expression followed by one or more statements. 2 if...else statement An ‘if’ statement can be followed by an optional ‘else’ statement, which executes when the boolean expression is false. 3 switch statement
A ‘switch’ statement allows a variable to be tested for equality against a list of values. 4 nested if statements You can use one ‘if’ or ‘else if’ statement inside another ‘if’ or ‘else if’ statement(s). 5 nested switch statements You can use one ‘switch’ statement inside another ‘switch’ statement(s).
if statement
An if statement consists of a boolean expression followed by one or more statements.
Syntax
The syntax of an if statement in C++ is − if(boolean_expression) { // statement(s) will execute if the boolean expression is true } If the boolean expression evaluates to true , then the block of code inside the if statement will be executed. If boolean expression evaluates to false , then the first set of code after the end of the if statement (after the closing curly brace) will be executed.
Flow Diagram
Flow Diagram
Example
#include using namespace std; int main () { // local variable declaration: int a = 100 ; // check the boolean condition if( a < 20 ) { // if condition is true then print the following cout << "a is less than 20;" << endl; } else { // if condition is false then print the following cout << "a is not less than 20;" << endl; } cout << "value of a is : " << a << endl; return 0 ; } When the above code is compiled and executed, it produces the following result − a is not less than 20; value of a is : 100
if...else if...else Statement
An if statement can be followed by an optional else if...else statement, which is very usefull to test various conditions using single if...else if statement. When using if , else if , else statements there are few points to keep in mind. An if can have zero or one else's and it must come after any else if's. An if can have zero to many else if's and they must come before the else. Once an else if succeeds, none of he remaining else if's or else's will be tested.
Syntax
The syntax of an if...else if...else statement in C++ is − if(boolean_expression 1) { // Executes when the boolean expression 1 is true } else if( boolean_expression 2) { // Executes when the boolean expression 2 is true } else if( boolean_expression 3) { // Executes when the boolean expression 3 is true } else { // executes when the none of the above condition is true. }
Example
#include using namespace std; int main () { // local variable declaration: int a = 100 ; // check the boolean condition if( a == 10 ) { // if condition is true then print the following cout << "Value of a is 10" << endl; } else if( a == 20 ) { // if else if condition is true cout << "Value of a is 20" << endl; } else if( a == 30 ) { // if else if condition is true cout << "Value of a is 30" << endl; } else { // if none of the conditions is true cout << "Value of a is not matching" << endl; }
A switch statement can have an optional default case, which must appear at the end of the switch. The default case can be used for performing a task when none of the cases is true. No break is needed in the default case.
Flow Diagram
Example
#include using namespace std; int main () { // local variable declaration: char grade = 'D'; switch(grade) { case 'A' : cout << "Excellent!" << endl; break; case 'B' : case 'C' : cout << "Well done" << endl; break;
case 'D' : cout << "You passed" << endl; break; case 'F' : cout << "Better try again" << endl; break; default : cout << "Invalid grade" << endl; } cout << "Your grade is " << grade << endl; return 0 ; } This would produce the following result − You passed Your grade is D
Nested if Statements
It is always legal to nest if-else statements, which means you can use one if or else if statement inside another if or else if statement(s).
Syntax
The syntax for a nested if statement is as follows − if( boolean_expression 1) { // Executes when the boolean expression 1 is true if(boolean_expression 2) { // Executes when the boolean expression 2 is true } } You can nest else if...else in the similar way as you have nested if statement.
Example
#include using namespace std; int main () { // local variable declaration: int a = 100 ; int b = 200 ; // check the boolean condition if( a == 100 ) { // if condition is true then check the following
switch(a) { case 100 : cout << "This is part of outer switch" << endl; switch(b) { case 200 : cout << "This is part of inner switch" << endl; } } cout << "Exact value of a is : " << a << endl; cout << "Exact value of b is : " << b << endl; return 0 ; } This would produce the following result − This is part of outer switch This is part of inner switch Exact value of a is : 100 Exact value of b is : 200
The? : Operator
We have covered conditional operator “? :” in previous chapter which can be used to replace if...else statements. It has the following general form − Exp1? Exp2 : Exp3; Exp1, Exp2, and Exp3 are expressions. Notice the use and placement of the colon. The value of a ‘?’ expression is determined like this: Exp1 is evaluated. If it is true, then Exp2 is evaluated and becomes the value of the entire ‘?’ expression. If Exp1 is false, then Exp3 is evaluated and its value becomes the value of the expression.
Conditional? : Operator
Exp1? Exp2 : Exp3; where Exp1, Exp2, and Exp3 are expressions. Notice the use and placement of the colon. The value of a? expression is determined like this: Exp1 is evaluated. If it is true, then Exp2 is evaluated and becomes the value of the entire? expression. If Exp is false, then Exp3 is evaluated and its value becomes the value of the expression. The? is called a ternary operator because it requires three operands and can be used to replace if-else statements, which have the following form − if(condition) { var = X; } else { var = Y; } For example, consider the following code −
if(y < 10) { var = 30; } else { var = 40; } Above code can be rewritten like this − var = (y < 10)? 30 : 40; Here, x is assigned the value of 30 if y is less than 10 and 40 if it is not. You can the try following example − #include using namespace std; int main () { // Local variable declaration: int x, y = 10 ; x = (y < 10 )? 30 : 40 ; cout << "value of x: " << x << endl; return 0 ; } When the above code is compiled and executed, it produces the following result − value of x: 40
