KotlinJava

# Practice with Loops and Algorithms

Next we’ll pause to reinforce what we’ve learned about the basic building blocks of computer science: storing data, making decisions, and repeating operations. We’ll also get some more practice with loops!

## Debugging PracticeDebugging Practice

Let’s warm up with a bit of graded debugging practice.

## `continue``continue`

Last time we discussed how we can exit a loop early using the `break` statement:

for (i in 0 until 10) {
if (i > 5) {
break
}
println(i)
}

`break` causes the enclosing loop—either `while` or `for`—to immediately exit, regardless of whether the loop condition is true or not.

`continue` is another control statement that we can use inside a loop. Let’s explore how it works together!

for (i in 0 until 8) {
println(i)
}

So while `break` causes the loop to exit, `continue` causes the code to return to the top of the loop immediately, check the loop condition, and then continue if appropriate.

### `continue` versus `if``continue` versus `if`

`continue` is used less frequently than `break`. One of the reasons is that, we can always rewrite any loop that uses `continue` to instead use an `if` statement. Let’s see how:

for (i in 0 until 8) {
println(i)
}

What’s more clear: `continue` versus an `if` statement? It really depends on the problem. If the loop body is long and complicated, it can be better to use `continue` at the beginning to avoid values that shouldn’t be considered by the rest of the loop. However, overall `continue` can make your code hard to read. So for shorter loops, it’s usually better to just put the entire loop body inside an `if` statement instead.

## Counting Array ValuesCounting Array Values

To get some additional practice, let’s work through an example together that is similar to this lesson’s homework. It also provides us a chance to practice with a very common array and loop programming pattern.

Given an array of values, how would we count the number of values that have some property? For example, let’s say that we need to count the number of values which are greater than zero.

### Solution OutlineSolution Outline

Let’s work step-by-step. First, let’s outline what we need to do.

var array = intArrayOf(-1, 42, 8, -12, 7)
// Goal: count the number of positive values in array

### `for` Loop Setup`for` Loop Setup

Next, let’s get our loop set up! It’s always a good idea to make sure that we can actually access all of the values in the array before doing anything more complicated.

var array = intArrayOf(-1, 42, 8, -12, 7)

### Testing Values Using `if`Testing Values Using `if`

As the third step, let’s review how we determine if a value is greater than zero, and integrate that into what we have so far.

var array = intArrayOf(-1, 42, 8, -12, 7)

### Identifying a Design PatternIdentifying a Design Pattern

Finally, let’s examine the design pattern that emerges from this example: a combination of iteration (our `for` loop) and selection (our `if` statement). By combining these two and using different conditions and data, we can solve a lot of different problems! We can also combine results in different ways as well.

var array = intArrayOf(-1, 42, 8, -12, 7)

## Practice: Array Sum Even Snippet

Created By: Geoffrey Challen
/ Version: 2022.1.0

Given an `IntArray` named `array` that is already declared and initialized, write a snippet of code (not a method), that prints a line with the sum of the values in `array` that are even. Don't modify `array`.

## Practice: Array Count Odd Snippet

Created By: Geoffrey Challen
/ Version: 2022.1.0

Given an `IntArray` named `array` that is previously declared and initialized, write a snippet of code (not a method), that prints a line with the count of the number of values in `array` that are odd. Don't modify `array`.

## Even and OddEven and Odd

Before we wrap up, let’s review one useful bit of Kotlin that we’ll use regularly during some of our upcoming examples: How to test whether a value is even or odd.

var even = 6
var odd = 5

## Homework: Number Sum Game Snippet

Created By: Geoffrey Challen
/ Version: 2022.8.0

Your friend invented a new game and wants your help to create the computer implementation. Of course, you're happy to help!

Here's how the game works. A set of cards with positive values are placed face down on a table. The player is allowed to turn over some number, but not all. They then guess the sum of all the cards. At that point, all the values are revealed. The players score is equal to how far off their guess was—the distance between their guess and the actual sum. (Lower is better.)

As an example, if the values were 1, 2, and 4, and the guess was 7, the score would be 0—a perfect score! However, if the values were 1, 2, and 8, and the guess was 7, the score would be 4. And if the values were 2, 2, and 5, and the guess was 124, the score would be 115.

Write a snippet of code (not a method) that computes the score of a game. The values on the cards will be stored in an existing `IntArray` named values, and the guess in an `Int` variable named `guess`. You should declare an `Int` variable `score` and set its value to the score of that game.

You will need to compute the sum of the values in the array! You may also find the `Math.abs` method helpful when completing this problem, but it is not required.

## More Practice

Need more practice? Head over to the practice page.