Unit 2: Selection
In development
Table of Contents
Selection in programming languages makes it possible to take a certain action if a condition is met. This offers many possibilities as you can make the program do anything once something else was true or false.
If Statements
If statements are statements that check for certain conditions in the program. If that boolean condition is true, then a certain block of code is executed. Here is the general form of an if statement:
if (condition) {
// code here is executed if condition is true
}
For example, the following code prints "You can vote!".
int age = 25;
if (age >= 18) {
System.out.println("You can vote!");
}
Else
You can also make the program do something when a condition is not met by using an else statement after the if statement. In the example below, assume a Scanner object input has already been initialized.
// prompt user for their age
System.out.print("Enter your age: ");
int age = input.nextInt();
// print whether they can vote
if (age >= 18) {
System.out.println("You can vote!");
} else {
System.out.println("You can't vote.");
}
Example output 1:
Enter your age: 19 [Enter ↩]
You can vote!
Example output 2:
Enter your age: 10 [Enter ↩]
You can't vote.
Else If
You can check for multiple conditions using an else if statment after the if statement. This means that the code first checks the if statement, and if that condition is not met, then it would continue through the other else if statements below it.
// prompt user for their age
System.out.print("Enter your age: ");
int age = input.nextInt();
// print whether they can vote and/or if they're a teen
if (age >= 18) {
System.out.println("You can vote!");
} else if (age >= 13) {
System.out.println("You can't vote, but you're a teen.");
} else {
System.out.println("You can't vote and you're not a teen.");
}
Note that having the final else clause is not technically required, but is recommended to catch all other conditions you may not have thought of.
Example output 1:
Enter your age: 56 [Enter ↩]
You can vote!
Example output 2:
Enter your age: 15 [Enter ↩]
You can't vote, but you're a teen.
Example output 3:
Enter your age: 10 [Enter ↩]
You can't vote and you're not a teen.
Why We Use Braces
Technically, for if statements with only one line of code in them, you don’t need the curly braces for the code to work. That might make you tempted to omit braces entirely. However, we highly recommend that you always use braces no matter what to avoid logic errors.
Analyze the following code to avoid 4 common errors.
double radius = 5;
double area;
// common error 1: forgetting necessary braces
// wrong
if (radius >= 0) area = radius * radius * Math.PI;
// System.out.println("The area is " + area); // this line should be unindented
// correct
if (radius >= 0) {
area = radius * radius * Math.PI;
System.out.println("The area is " + area);
}
// common error 2: wrong semicolon at the if line
if (radius >= 0); // logic error: this line is equivalent to: if (radius >= 0) { }; (an empty if block)
area = radius * radius * Math.PI;
System.out.println("The area is " + area);
// common error 3: redundant testing of boolean values
boolean even = 6 % 2 == 0;
// not wrong, but could be better
if (even == true) {
System.out.println("It is even.");
}
// code below is equivalent and better than code above
if (even) {
System.out.println("It is even.");
}
// common error 4: dangling else ambiguity
int i = 1, j = 2, k = 3;
// not wrong, but ambiguous - to which if does the last else belong to?
if (i > j)
if (i > k)
System.out.println("A");
else System.out.println("B");
// equivalent to above code with correct indentation
if (i > j)
if (i > k)
System.out.println("A");
else
// else clause always matches most recent
// unmatched if clause in the same block
System.out.println("B");
Common Pitfalls
Here is some code demonstrating common pitfalls when dealing with selection structures.
// common pitfall 1: simplifying boolean variable assignment
int number = 4;
boolean even;
if (number % 2 == 0) {
even = true;
} else {
even = false;
}
// equivalent but shorter than code above
even = number % 2 == 0;
// common pitfall 2: avoiding duplicate code in different cases
boolean inState = true;
double tuition;
if (inState) {
tuition = 5000;
System.out.println("The tuition is " + tuition); // repeated
} else {
tuition = 15000;
System.out.println("The tuition is " + tuition); // repeated
}
// better
if (inState) {
tuition = 5000;
} else {
tuition = 15000;
}
System.out.println("The tuition is " + tuition);
Switch Statement
Switch statements are an easier way to execute code based on the value of a variable. Once again, assume in the example below that a Scanner object called input has been initialized.
// prompt user for their rating
System.out.print("Enter your rating: ");
int rating = input.nextInt();
switch (rating) {
case 1:
System.out.println("Poor");
break;
case 2:
System.out.println("Bad");
break;
case 3:
System.out.println("Fair");
break;
case 4:
System.out.println("Good");
break;
case 5:
System.out.println("Great");
break;
}
The switch statement above is equivalent to this if-else if chain.
// prompt user for their rating
System.out.print("Enter your rating: ");
int rating = input.nextInt();
if (rating == 1) {
System.out.println("Poor");
} else if (rating == 2) {
System.out.println("Bad");
} else if (rating == 3) {
System.out.println("Fair");
} else if (rating == 4) {
System.out.println("Good");
} else if (rating == 5) {
System.out.println("Great");
}
Example output 1:
Enter your rating: 5 [Enter ↩]
Great
Example output 2:
Enter your rating: 3 [Enter ↩]
Fair
Note that you can basically have as many cases as you want and execute basically as much code as you want for each case.
Fall Through
Notice that at the end of each case in a switch statement, there is a break statement. This break statement is important because it ends the switch statement once a case has been executed. If you don’t write a break statement, you will observe what is called fall through behavior.
What happens is that the switch checks the value of the variable against the cases from top to bottom. Once it matches a case, all of the code for that case and the ones below it are executed, until either the switch statement ends or there is a break.
For example, analyze the code below. Notice that we have removed the break statements for cases 1-3 and that rating is set to 2.
int rating = 2;
switch (rating) {
case 1:
System.out.println("Poor");
case 2:
System.out.println("Bad");
case 3:
System.out.println("Fair");
case 4:
System.out.println("Good");
break;
case 5:
System.out.println("Great");
break;
}
Output:
Bad
Fair
Good
Notice that since rating is 2, it matches case 2 and the code there is executed. Since there are no break statement in case 2 and case 3, it falls through and the code in case 3 and case 4 are executed. Once we finish the code in case 4, there is a break statement, so we exit the switch statement.
Default
There is another special optional case in a switch statement called the default case. If the value of the variable fails to match any of the explicit cases, the code in the default case will run. It’s similar to having an else clause.
int rating = 10;
switch (rating) {
case 1:
System.out.println("Poor");
break;
case 2:
System.out.println("Bad");
break;
case 3:
System.out.println("Fair");
break;
case 4:
System.out.println("Good");
break;
case 5:
System.out.println("Great");
break;
default:
System.out.println("Not a valid rating");
}
Note that there is no break at the end of the default case. You could technically have a break, but since it’s the end of the switch it doesn’t matter if you have one or not.
Output:
Not a valid rating
Ternary Operator
The final selection structure we’ll discuss is the ternary operator. It is not commonly used, but for reference we’ll include it. It’s essentially a shortened way to write an if-else statement to assign a value to a variable.
The general form of a statement using the ternary operator is:
var = (condition) ? ifValue : elseValue;
If condition is true, then var will be assigned the value of ifValue. Otherwise, it will be assigned the value of elseValue. For example, let’s use a ternary operator to decide how many days are in the month of February. (Assume all variables are declared and initialized.)
daysInMonth = (month == 2 && isLeapYear) ? 29 : 28;
The code above is equivalent to the code below.
if (month == 2 && isLeapYear) {
daysInMonth = 29;
} else {
daysInMonth = 28;
}