Arrays - Iteration

Hodor. -Hodor

Overview

In this lesson, we'll cover how to iterate, or perform the same action multiple times, over an array.

Learning Objectives

• Use looping to iterate over an array
• Use conditional statements within loops
• Add loops to functions
• Create a for-in loop to iterate over an Array

Improving Repetitive Tasks

Creating arrays can allow us to manipulate a collection of values. You can also print individual items in that collection, so if you wanted to print a list of your three friends you could do the following:

let friends = ["Chris", "Billy", "Jessica"]
print(friends[0])
// "Chris"
print(friends[1])
// "Billy"
print(friends[2])
// "Jessica"

But what if you had 100 friends? Printing values like this will become tedious quickly and if you added more friends to the list, you would have to add more print commands to handle them.

Programming languages are very good at repetitive tasks, in fact they are better than humans, so in this lesson you will learn how to perform the same action(s) on values in an array multiple times. This is called iterating over an array and introduces a new concept, a loop.

Loops

let friends = ["Chris", "Billy", "Jessica"]

for friend in friends {
    print("Hello \(friend)")
}

// Hello Chris
// Hello Billy
// Hello Jessica

This example starts with a familiar array initialization, but then instead of using the print command for each value in the array, it says that for each value in the friends array, process the instructions in-between the {} symbols, which are called 'curly brackets'. In this example, these instructions print the current value of friend, or the value of the current index in the friends array, in the order they appear in the array.

As friends is an array of type String, the type of friend will also be a String, and you can confirm this by typing the example above into a playground and option + clicking friend.

Loops and Calculations

You typically use loops to undertake more complex repetitive processing than printing the value of a variable, such as calculations.

Say you have a list of numbers and want to know what the square of each one and add that value to a new list. You create one array containing the original list of numbers and a placeholder array for the list of squared numbers. Then iterate through the array containing the list of numbers, perform the necessary calculations to find its square, and add that value to the second array. Let's look at some code!

let numbers = [1, 2, 3, 4, 5]
var squaredNumbers: [Int] = []

for number in numbers {
    let squaredNumber = number * number
    squaredNumbers.append(squaredNumber)
}

print(squaredNumbers)
// prints "[1, 4, 9, 16, 25]"

For the first numbers array, you use the let keyword as you don't want to change the values in the array. As you don't know what the values of the second squaredNumbers array will be yet, you initialize it first with no values. Inside the for in loop, you generate the square of the value at the current index of the array and assign it to another unchanging variable. The next line introduces something called a method, which you can think of as a pre-written convenient way of performing actions on something. In this particular case .append adds a new value to the end of the squaredNumbers array.

In the first iteration of the for in loop, number, which is of type Int, will have a value of 1. The value of number is then squared (1 1), assigned to the squaredNumber constant, and appended to the squaredNumbers array. During the second iteration, number will have a value of 2 and the squared value will be 4 (2 2). This value is then assigned to a squaredNumber constant and appended to the squaredNumbers array. The same process continues until the array is fully iterated.

This last example introduced a couple of new concepts, if you want to understand better what a line of code is doing then feel free to add print and if statements to inspect values (this is called 'debugging'). For example, to see the value of squaredNumber you could add print (squaredNumber) inside the loop.

Maybe one of your friends deserves a special mention for being so awesome, so you need to find them in the friends list, and when you do, print a message for them:

let friends = ["Chris", "Billy", "Jessica"]

for friend in friends {
    print("We are inside the for loop, friend: \(friend)")

    if friend == "Billy" {
        print("Hello \(friend), you are awesome!")
    }
}

// We are inside the for loop, friend: Chris
// We are inside the for loop, friend: Billy
// Hello Billy, you are awesome!
// We are inside the for loop, friend: Jessica

In the above example, you iterate the friends list printing the value of each index in the array. If the current value is equal to "Billy", then inside an if statement you print them a special greeting.

Loops and Functions

Functions are a convenient way to package functionality into reusable components. You can add any code into a function, including loops.

Say for example you want to know how many of your friends got over 70 in their recent exam.

You first create an array of their grades and pass that array to a function. That function iterates through the array and maintains a count of how many grades were above 70, and returns that value.

func numberOfFriendsAbove70(grades: [Int]) -> Int {
    var studentsAbove70 = 0

    for grade in grades {
        if grade >= 70 {
            studentsAbove70 += 1
        }
    }

    return studentsAbove70
}

let gradesFromExam = [80, 45, 79, 32, 84, 100, 53, 60]

print(numberOfFriendsAbove70(grades: gradesFromExam))
// 4

Again, there's a lot happening in this example, so let's break everything down by starting at the bottom of the code. First you create an array called gradesFromExam of your friends' grades that can't change and then pass that array to the numberOfFriendsAbove70 function and print the returned result.

At the declaration of the numberOfFriendsAbove70 function you set the incoming parameter (accessed by the grades variable) as an array of numbers and the return value as a single number. Inside the function you first initialize to 0 a variable to count the number of grades over 70.

Next, iterate through each value in the grades array and if the value at the current index is greater than 70 increase the value of studentsAbove70 by 1. Once the array has been fully iterated, return the total value or studentsAbove70.

Have a good look at the code above as it introduces a lot of new, but essential concepts. Try this code in a playground and add some print commands to see if your understanding of it matches the practical application.

View this lesson on Learn.co

View Iterating Over an Array on Learn.co and start learning to code for free.