CS2 Lesson 3: Conditionals and Loops

  1. Conditionals and Logic (Chapter 5)
    1. Using hasNextInt
    2. String equals
    3. Exercise
    4. Reminder
  2. Loops (Chapter 6)
    1. Nested loops
    2. Exercise
    3. Exercise
  3. Preview: Arrays / References (Chapter 7)
  4. Quiz

Conditionals and Logic (Chapter 5)

Please read through Chapter 5 on conditionals and logic. I hope you have seen “if” statements before, as well as the comparison operators and the logical operators.

Consider the following two code snippets:

System.out.println("Enter a number from 1 to 10: ");
x = sc.nextInt();
if (x < 1 || x > 10) {
  System.out.println("I said 1 to 10!");
}

System.out.println("Enter a number from 1 to 10: ");
x = sc.nextInt();
if (x < 1 && x > 10) {
  System.out.println("I said 1 to 10!");
}

Which one of these correctly determines if the input is between 1 and 10 (inclusive)? What happens in the other case?

Using hasNextInt

In this video I go through using the hasNextInt method to validate input. Notice the syntax:

if (sc.hasNextInt()) {
  input = sc.nextInt();
}

Inside the if statement, I can use sc.hasNextInt() by itself (without comparing it to anything), because the hasNextInt method returns a boolean value (true or false). You can put anything inside an if statement as long as it evaluates to true or false. That’s why something like “x > 0” works, if x is an integer, since “x > 0” is true if x is positive and false if x is 0 or negative. But “if (x)” by itself does not work, if x is an integer.

String equals

I do wish to point out, here, that we do not use the double equals sign to compare Strings. Try this out:

System.out.println("Type in yes or no");
Scanner sc = new Scanner(System.in);
String word = sc.nextLine();
if (word == "yes") {
  System.out.println("Cool!");
} else {
  System.out.println("Boo!");
}

What does this output if you type in “yes”? What if you type in “no”?

To get this to work properly, we would use the equals method:

System.out.println("Type in yes or no");
Scanner sc = new Scanner(System.in);
String word = sc.nextLine();
if (word.equals("yes")) {
  System.out.println("Cool!");
} else {
  System.out.println("Boo!");
}

Try this out and see what happens when you input yes. Why does == not work for Strings? We’ll see this a bit more in the next couple of chapters.

Exercise

Write a program which asks the user to input an integer from 1 to 10 (inclusive). If they input something that is not an integer, you should output an angry message and end the program (hint: look at this Example Program from the text). If they input an integer that is not in that range, you should remind them that you asked for the number to be between 1 and 10. If they do input a number in that range, you should congratulate them on following directions.

This exercise will really help you get ready for the first project, so please be sure to do this!

Reminder

Loops (Chapter 6)

Now read through Chapter 6 on for and while loops.

Consider the following code snippet:

Scanner sc = new Scanner(System.in);
int x = sc.nextInt();
int i = 0;
while (x / 2 > 0) {
  i++;
  x /= 2;
}
System.out.println(i);

What does this program output if we input 10? What if we input 20? What if we input 100? 200? 1000? What mathematical function is being computed here?

Another code snippet to look at:

Scanner sc = new Scanner(System.in);
int input = sc.nextInt();
if (input >= 0 && input <= 10) {
  for (int i = 0; i < input; i++) {
    System.out.println(i * i);
  }
}

What does this output if our input is 1000? What if our input is 10?

Nested loops

What does the following output if input is 5? What if input is 10?

for (int i = 0; i < input; i++) {
  System.out.print("i=" + i + ": ");
  for (int j = 0; j < i; j++) {
    System.out.print("(" + i + ", " + j+") ");
  }
  System.out.println();
}

Exercise

Write a program which asks the user to input a positive integer $N$. For the purposes of this, don’t worry so much about checking the input, just assume the user puts in a positive integer. Then ask the user to input in $N$ more integers, and then output the sum of all of those numbers. For example, if the user first types in 3, then ask the user to type in 3 more numbers. If they then input 1, 5, and -23, then output -17.

Exercise

Write a program which asks the user to input a positive integer. Check that the input is valid (using hasNextInt) and that it is positive. If the number is positive, determine if it is a perfect square.

Do this without using the built-in Math libraries. Think for a bit first about how you might try to figure this one out. Describe your method on paper, in words or in pseudocode first, before you write the code.

Some problem solving tips:

  1. Examples: Start with some simple examples. Think of different kinds of example inputs for each possible output: one where the output should be “not a number / not valid”, another where the output should be “No, it’s not a perfect square”, and another where the output should be “Yes, it is a perfect square.”
  2. Steps: For each of those examples, think about the steps you need to take to determine that the given number is or is not a perfect square. For example, what steps do you need to do to figure out that 16 is a perfect square? What about 326? How would you figure out that 326 is a perfect square, without resorting to a calculator?
  3. Algorithm: Turn your steps into an algorithm. Write down those steps and try to be as explicit as possible. Use common programming constructs in your algorithm: loops, variables, if/else statements, etc.
  4. Code: Once you have an algorithm written down (on paper), start writing code. The algorithm should not be too hard to translate to code. Notice that I don’t necessarily recommend going straight to code: some people can think like that, but often you are stuck staring at a blank screen if you don’t have a plan ahead of time.
  5. Testing: Try out your code using several different inputs. See if it works if the input is valid, not valid, if the input is a perfect square, not a perfect square, etc. More generally: try to test at least two inputs for each possible output that you expect.

Preview: Arrays / References (Chapter 7)

Skim through Chapter 7 on arrays This section can be tricky, so please take some time to look at it first, so that we can discuss it more on Thursday.

Arrays are references to data. What happens when you do the following:

int[] x = new int[10];
System.out.println(x);

What about this:

int[] x = new int[1];
x[0] = 10;
int[] y = new int[1];
y[0] = x[0];
if (x == y) {
  System.out.println("Equal.");
} else {
  System.out.println("What?");
}

See if you can figure out why these are not outputting what you might expect. This has something to do with the fact that arrays are references to data, and not data themselves. Read up a bit about this in the text, and we will talk more on Thursday.

Quiz

The first quiz is on BrightSpace. You have 20 minutes to work on it once you start.