Comparison and Logical Operators
Learn how to compare values.
Comparison and logical operators
We know how to use built-in methods as well as define our own methods. We also know how to take input from the user and assign it to a variable for storage (and reuse) or pass it as further input to a method. We also know how to output something on the screen for the user. But we can only write sequential programs so far, where the computer fetches and executes one line of our code after another, from the first executable line of the main method to the last, in a strictly sequential order.
This equips us with the necessary skills to solve many interesting problems, but there are still some problems that sequential programs can’t solve. Sometimes, a solution might require nonsequential execution as well:
This chapter is about learning how to write programs that can, while they’re executing, be selective about executing lines of code. Secondly, we’ll learn to write programs that can execute some lines of our code multiple times. Our project will require us to be able to code this way.
The project requirements
Let’s attend to our client’s requirements for the project once again, this time focusing on the highlighted requirements for announcing the correct score to the user.
Imagine Alex, an elementary school student, uses our app and manages to get three questions right. When Betty, another student, uses our app, she gets all five questions right. We aren’t going to write two different programs—one for Alex and one for Betty. But the same piece of code should compute that Alex’s score is 3 and Betty’s score is 5.
Of course, our app can’t know in advance what the final score of its user is eventually going to be. It’s only going to be known to our program while the code is being executed by Alex and Betty, respectively.
Note: How does the code know if the user has entered the correct answer to the question?
Till now, we’ve seen how to compute the correct expected answer, given a question; for instance, if the question was 4 * 7:
import java.util.Scanner;
public class MyJavaApp {
public static void main(String[] args) {
Scanner scan = new Scanner(System.in);
System.out.print("What is 4 * 7? ");
int userAnswer = scan.nextInt();
int correctAnswer = 4 * 7;
}
}So we have two numbers in two appropriately named variables. How would we know if userAnswer is correct? Because if we know it’s correct, we need to update the user’s score by 1. Line 9 is inviting you; what can be done there right after we have correctAnswer and userAnswer?
Well, we know what you’re thinking. You’re thinking that it’s as simple as comparing the two numbers to see if they’re equal or not!
Right, we do have a symbol for equality, we’ve known it since childhood, but we’ve been using that in Java for assigning some value to a variable. We’ve even used that in lines 7–8. But why not take a chance and use that for comparing two numbers as well to see if the’re equal?
import java.util.Scanner;
public class MyJavaApp {
public static void main(String[] args) {
Scanner scan = new Scanner(System.in);
System.out.print("What is 4 * 7? ");
int userAnswer = scan.nextInt();
int correctAnswer = 4 * 7;
userAnswer = correctAnswer;
}
}That did not work as planned, or did it? How would we know? We need to be able to test our hypotheses. How about, if we insert a print statement before and after line 10? This way, we might know what userAnswer = correctAnswer is doing.
import java.util.Scanner;
public class MyJavaApp {
public static void main(String[] args) {
Scanner scan = new Scanner(System.in);
System.out.print("What is 4 * 7? ");
int userAnswer = scan.nextInt();
int correctAnswer = 4 * 7;
System.out.println("User's answer: " + userAnswer + " Correct answer: " + correctAnswer);
userAnswer = correctAnswer;
System.out.println("User's answer: " + userAnswer + " Correct answer: " + correctAnswer);
}
}Enter “28” and it seems to work! But run it again. This time, give a wrong answer, let’s say “20,” and see what happens! Line 10 clearly ends up assigning the value in correctAnswer to userAnswer. But we wanted to use = for checking if the two numbers are equal or not.
Equality, not assignment
Since a single = is taken by Java for assignment purposes, what’s the next best option for an equality operator? How about double equals ==?
import java.util.Scanner;
public class MyJavaApp {
public static void main(String[] args) {
Scanner scan = new Scanner(System.in);
System.out.print("What is 4 * 7? ");
int userAnswer = scan.nextInt();
int correctAnswer = 4 * 7;
System.out.println("User's answer: " + userAnswer + " Correct answer: " + correctAnswer);
userAnswer == correctAnswer;
System.out.println("User's answer: " + userAnswer + " Correct answer: " + correctAnswer);
}
}However, we need to encapsulate the code in line 10 within print in the hope of figuring out what == has been up to.
import java.util.Scanner;
public class MyJavaApp {
public static void main(String[] args) {
Scanner scan = new Scanner(System.in);
System.out.print("What is 4 * 7? ");
int userAnswer = scan.nextInt();
int correctAnswer = 4 * 7;
System.out.println("User's answer: " + userAnswer + " Correct answer: " + correctAnswer);
System.out.println(userAnswer == correctAnswer);
System.out.println("User's answer: " + userAnswer + " Correct answer: " + correctAnswer);
}
}Run it for “28” and then “20.” When executed, line 10 prints True first, then False. That’s the comparison operator we wanted!
Programmers often use print to diagnose their programs.
A final word about ==
Conceptually, think of == as another built-in method that takes in two numbers and returns True or False.
Other comparison operators
We’ve seen == in its complete manifestation. There are other comparison operators as well:
Comparison Operators
Operators | Description | Example Usage |
| The two numbers are exactly equal. | 2 == 2 would return |
| The two numbers are not equal. | 2 != 2 would return |
| The first number is greater than the second. | 2 > 2 would return |
| The first number is greater than or equal to the second. | 2 >= 2 would return |
| The first number is less than the second. | 2 < 2 would return |
| The first number is less than or equal to the second. | 2 <= 2 would return |
How do we update the user’s score?
Now, we know how to compare userAnswer to correctAnswer for equality. We also know how to update the score by 1 (score = score + 1). However, our client’s project requires our code to update the score only if the user has correctly answered the question, not otherwise. How do we do that? Let’s go to the next lesson.