Difference between revisions of "The If-Statement"

Latest revision as of 13:39, 7 December 2011

COMP 1010 Home > Control Stuctures

Introduction

An if statement is an expression that contains one or more Boolean expressions whose result will determine whether to execute some segment of code. They are one of the simplest and most widely used control structures. This section will teach you how to write them. It will also teach you when and why to use them.

The If-Statement

Now that you have a firm grasp of variables, you may have wondered what is the point of having Boolean as a variable?

You probably already know that they can store either true or false, but what is the importance in storing this information if all you know how to do is simply print its contents out to the screen?

In this section you will be able to see why Booleans are important. So far the programs you have written have read every line of code from the top to the bottom, but sometimes you may not want your program to read every line of code. Now you are probably asking yourself “Why wouldn't I want my program to read every line of code every time?” This example will help explain why sometimes this is needed.

First Example

Let us say we want the absolute value of a number. Remember an absolute number is just the positive value of any number. For example, the absolute value of -4 would be 4. To get the absolute value of a number you take a number and square it. Next you take the square root of that answer.

So let's program this.

 public static void main(String[] args)
 {
     double number;
     double absNumber;
     
     String input = JOptionPane.showInputDialog(null,"Enter a number");
     Scanner s = new Scanner(input);
     number = s.nextDouble();
     
     //compute the absolute value
     absNumber = number * number;
     absNumber = (double)Math.sqrt(absNumber);
     
     System.out.println("The absolute value of " + number + " is " + absNumber);
 }//main 

This code will definitely work, but is it the most elegant way to solve this kind of problem? Is it necessary to square and square root in this case? The answer is no.

When the number is positive, the absolute value is already calculated, so it can just be printed. The only problem is when the number is negative. What if we could check if the number was negative and then do some special code only if it was negative? Turns out we can do exactly that by using something called an if statement.

If-Statement

An if statement has 2 parts:
(1) The Condition
(2) The Body

The keyword if is used to tell the computer to make a decision based on a condition. If the condition returns a value of true then the lines of code contained inside the body is executed. If the condition returns false the code inside the body is skipped entirely. Below is a general example of an if statement so you may better understand how it works.

 if(condition) 
 { 
    //Body - This will get executed only if the condition is satisfied. 
 } 

 //if the condition is not satisfied, the code in the curly braces will be skipped 

Now let's get back to our absolute value question. In order to make this program more efficient, we have to do two things:
(1) Check to see if the number is negative
(2) If the number is negative then we need to multiply it by -1, making it positive

So now let's program this:

 public static void main(String[] args)
 {
     int number;
     boolean isNumberNegative;

     number = Integer.parseInt(JOptionPane.showInputDialog(null,"Enter a number"));

     //Check to see if the number is negative
     isNumberNegative = number < 0;

     //use an if to execute the code in the braces only if the number is negative
     if(isNumberNegative)
     {
        //we remove the negative sign
        number = number * -1;
     }

     System.out.println("The number" + number);

 }//main 

This program can also be written a little differently. A Boolean variable isn't necessary here. It was only used to make the code more readable. You may choose to get rid of the variable. If you did the if-statement would look like this if(number < 0). The reason why Boolean variables are invaluable is because they give the ability to break long complicated expressions into a bunch of shorter ones.

Another Example

In this example the user is asked for a number and the program tells the user if the number is even.

 public static void main(String[] args)
 {
     int number;
     number = Integer.parseInt(JOptionPane.showInputDialog(null,"Enter a number"));
 
     //if the number is even, print a message saying "The number is even"
     if(number%2 == 0)
     {
         System.out.println("The number is even");
     }

 }//main 

In this example the condition is number%2 == 0. You can always identify the condition of an if-statement by looking at what is contained in the parentheses after the word if. The condition must always return either true or false. Any other values (such as a number, string, or character) will result in an error when you try to compile your program.

The body of the if-statement is everything contained within the curly braces. In this case the body contains one line: System.out.println("The number is even");.

Together the keyword if, the condition, and the body form what is called an if block or if statement.

Review: Anatomy of the If-Statement

As described in the previous example an if-statement is made up of several parts:

• the keyword if
• the condition
• the body

The general form for the if-statement is:

 if (condition)    //"if" and condition on same line, condition is contained inside parentheses
{
    body          //all statements that should be executed if and only if the condition returns a value of true
}//if 

Normally the body is contained inside the curly braces (the {...} characters). However, if the body is only one statement these braces may be omitted. The following two if statements are equivalent:

 if(condition)
{
    System.out.println("The condition was true");
} 

 if(condition)
    System.out.println("The condition was true"); 

If the body has more than one statement it is very important to put curly braces around it. If not then the if statement will only control the line of code right after the if(condition) line and all others would be executed regardless. For example, the output would be "Congratulations on your A+! Your grade is: 59" when the programmer was expecting just "Your grade is: 59".

 int grade = 59;

if(grade >= 90)
     System.out.print("Excellent work!");
     System.out.print("Congratulations on your A+! ");

System.out.println("Your grade is: " + grade); 

As you can see, bad things can happen if you forget your curly braces. It is good practice to always put curly braces, even if you only have one line in your body, that way you won't forget to put them if you add to the body of the if statement later.

Summary

If statements are very useful for executing code only at certain times. The if-statement has two main parts: the condition and the body. The condition is a statement that can result in true or false depending on variables. The code inside the if-statement is called the body. The body is only evaluated if the condition is true. If the condition is false, the body will not be executed.

If there is no curly braces directly following the if(condition) part of the if-statement, the next line will be considered part of the if-statement. Any lines following this will get executed regardless of if the condition is true or not.