Lesson 05: Branches
C/C++ language executes program statements in a sequence. Sometimes we need to alter the flow of the sequence of statements. This is possible using branching statements offered by the C/C++ language. The programmer can jump from one part of the program to another with the help of such statements.
In C/C++, branching statements are classified into two types:
- Conditional branching
- Unconditional branching
Conditional Branching
It includes the statements which work on checking the condition of giving expression to execute the code.
- if statement
- if-else statement
- Nested if statement
- else-if ladder
- switch statement
Unconditional Branching
These statements don't check any condition to execute the code. These statements are pretty much useful to transfer the control from one block to another block over the program.
- break statement
- continue statement
- goto statement
- return statement
5.1 Conditional Branching
In real life, sometimes we need to make some decisions, and based on these decisions, we decide what should we do next step. Similar situations arise in the programming also where we need to make some decisions and based on these decisions we will execute the next block of code.
if and if-else Statement
if (condition) {
// Statements inside the body of 'if', will be executed if the condition is true
}
If the condition is evaluated as true, the statements inside the body of if are executed. Otherwise, the statements inside the body of if are skipped.
if (condition) {
// Statements inside the body of 'if', will be executed if the condition is true
} else {
// Statements inside the body of 'else', will be executed if the condition is false
}
If the condition is evaluated as true, the statements inside the body of if are executed. Otherwise, the statements inside the body of else are executed.
The conditional operator was invented because a particular construction occurs often in C/C++ programs and it is nice to shorten it. Here an example of the lengthy version of the code:
if (alpha < beta)
min = alpha;
else
min = beta;
The program set min to a smaller value from alpha and beta variables. This if-else statement requires four lines of code. However, using a conditional operator, the code can be shorted it to :
min = (alpha < beta) ? alpha : beta ;
Not all if-else statements can be rewritten using a conditional operator.
The simple way to make a decision in C/C++ is with the if statement. Here is an example of an if statement at work
Nested if-else Statement
When an if-else statement is present inside the body of another if or/and else, then this is called nested if-else.
if (condition1) {
// Nested if-else inside the body of "if"
if (condition2) {
// Statements inside the body of nested "if"
} else {
// Statements inside the body of nested "else"
}
} else {
// Statements inside the body of "else"
}
if (condition1) {
// Statements inside the body of "if"
} else {
// Nested if-else inside the body of "else"
if (condition2) {
// Statements inside the body of nested "if"
} else {
// Statements inside the body of nested "else"
}
}
else-if Ladder
The else-if ladder statement is useful when you need to check multiple conditions within the program, nesting of if-else blocks can be avoided using else-if ladder statement.
if (condition1) {
// These statements would execute if the condition1 is true
} else if (condition2) {
// These statements would execute if the condition2 is true
} else if (condition3) {
// These statements would execute if the condition3 is true
} …
} else {
// These statements would execute if all the conditions return false
}
switch Statement
The switch statement is C/C++'s multiway branch statement. It is used to route execution one of several different ways. In fact, the switch statement is a special else-if ladder statement, which the conditions are used to compare the same expression with different values. The general form of the statement is:
switch (expression){
case constant1:
// Statements sequence 1
break;
case constant2:
// Statements sequence 2
break;
•
•
•
case constantN:
// Statements sequence N
break;
default:
// Default statements
}
Each statement sequence may be from one to several statements long. The default portion is optional. Both expression and the case constants must be integer types.
The switch works by checking the expression aginst the constants.
- If a match is found, that sequence of statements is executed.
- If the statement sequence associated with the matching case does not contain a break, execution will continue on into the next case. Put differently, from the point of the match, execution will continue until either a break statement is found or the switch ends.
- If no match is found and a default case is existent, its statement sequence is executed. Otherwise, no action takes place.
Summary
- else and else if are optional statements, a program having only if statement would run fine.
- else and else if cannot be used without the if.
- There can be any number of else if statement in an "if else-if" block.
- If none of the conditions are met, then the statements in the else block get executed.
Exercises
- Which of the following tests expression(s) for an if statement that determines whether a character ch is a digit; that is, if it is in the rage of character from '0' to '9'.
- if (ch >= 0 && ch <= 9)
- if (ch > 0) else (ch < 9)
- if (ch < '0' || ch > '9')
- if (ch > -1 && ch < 10)
- if (ch >= '0' && ch <= '9')
- if (ch >= 0 && ch <= 9)
- Suppose you want to display "weekday" if a variable day is between 1 (Monday) and 5 (Friday), but you want to display "Saturday" if day is 6 and "Sunday" if day is 7. Which of the following fragment might reasonably be part of the code for this task? Assume day is always in the range of 1 to 7.
- if else ( day == 7) cout << "Sunday";
- else if (day == 6) cout << "Saturday"; else cout << "Sunday";
- else if (day < 6) cout << "Weekday"; else if (day == 6);
- else cout << "Weekday";
- if (dya < 6 ) cout << "Weekday"; else cout << "Saturday";
- If you want to display "Too slow" if speed is less than 40, "Too fast" if speed is greater than 65, and nothing otherwise, it would be appropriate to use:
- a switch statement.
- a series of if statements.
- nested if-else statements.
- an else-if ladder.
- a conditional operator.
5.2 Unconditional Branching
break statement
The break is used to exit from a do, for, or while loop, bypassing the normal loop condition, as shown in Figure 2.1. It is also used to exit from a switch statement.
Figure 2.1: Operation of the break Statement
The break statement is often used to handle unexpected or nonstandard situations that arise within a loop. An example of break in a loop is shown below:
do {
x = getx();
if ( x < 0 ) break; // terminate if negative
process(x);
} while (!done);
Here, if x is negative, the loop is terminated.
In a switch statement, break keeps program execution from "falling through" to the next case. (Refer to the switch statement for details.)
A break terminates only the for, do, while, or switch that contains it. It will not break out of any nested loops or switch statements.
continue statement
The continue statement is similar to the break statement in that it is usually activated by an unexpected condition in a loop. However, it returns control to the top of the loop — causing the loop to continue — rather than causing an exit from the loop. Figure 2.2 shows how this looks.
Figure 2.2: Operation of the continue Statement
Whereas the break statement causes an exit from a loop, the continue statement causes part of the loop to be "short-circuited" or bypassed while the loop keeps running. That is, following a continue, control goes back to the top of the loop. For example, the following while loop reads characters from the keyboard until an s is typed:
while (ch = getchar()) {
if (ch != 's')
continue; // read another char
process(ch);
}
The call to process() will not occur until ch contains the character s.
goto Statement
The goto keyword causes program execution to jump to the label specified in the goto statement. The general form of goto is:
goto label;
label:
All labels must end in a colon (:) and must not conflict with keywords or function names. Furthermore, a goto can branch only within the current function — not from one function to another.
return statement
The return statement forces a return from a function and can be used to transfer one value back to the calling routine. It has these two forms:
return;
return value;
In C99 and C++, the form of return that does not specify a value must be used only in void functions.
Keep in mind that as soon as a return is encountered, the function will return, skipping any other code that may be in the function.
Also, a function can contain more than one return statement.
Questions
- Write a console application to check whether a given number by the user is EVEN or ODD.
- Write a console application to ask users to input three integer numbers, then display the largest number.
- Write a console application to ask users to input three values for each side of a triangle, then check whether the triangle is valid or not if sides are given?
- The following table is the salary lists of the ABC company. Write a console application, when the user inputs the salary value, the system will display their position.
Salary above $60,001 $50,000 ~ $60,000 $40,001 ~ $50,000 $30,001 ~ $40,000 below $30,000 Position Manager Assitant Director Office Clerk Part Time - Write a temperature-conversion program that gives the user the option of converting Fahrenheit to Celsius, or Celsius to Fahrenheit. Then carry out the conversion. Use floating-point numbers. Interaction with the program might look like this:
Type 1 to convert Fahrenheit to Celsius,
2 to convert Celsius to Fahrenheit: 1
Enter temperature in Fahrenheit: 70
In Celsius, that's 21.111111 - Write a program to calculate the final price after the discount. The discount rates are as follows.
Customers can enjoy a 10% discount for the total price ≥ $50 US dollars; 15% discount for the total price ≥ $100 US dollars; 20% discount for the total price ≥ $200 US dollars.
The cashier enters the total price of the customer's purchase, and the system displays the payment amount after the discount on the screen.