For-In Loops

Learning Goals

• Explain the concept of iteration
• Write a for-in loop to accomplish repetitive code
• Use for-in loops to iterate over arrays and dictionaries

Technical Vocabulary

• Array
• Dictionary
• Element
• Index
• Iterate
• Loop

Warm Up

Iteration (or looping for a specified number of times) is a process of repeating a task for each object in a collection. For each of the scenarios below, we’ll walk through using examples we are familiar with to demonstrate the concept.

These are examples that would be programmatically challenging because they would require several steps, but are things that we do every day.

We can also iterate in programming. By “in programming,” we mean things that could be done, with relative ease, by a computer like crunching numbers.

What is a for-in loop?

A for-in loop is a block that will run code a specific number of times. It can iterate through a collection (array or dictionary) and do something with each element. Loops are very common and necessary parts of a programming language; every programming language has its own version of getting this done.

The benefit of loops is they save us time. When we want to do to the same thing for a lot of items, we only have to write that command once. For example, if we wanted to do something with each string in the trending array, our code would look like this:

var trending = ["@cosette", "@avani", "@lorengray", "@jamescharles"]

print("Have you seen that TikTok from /(trending[0]) yet?!");
print("Have you seen that TikTok from /(trending[1]) yet?!");
print("Have you seen that TikTok from /(trending[2]) yet?!");
print("Have you seen that TikTok from /(trending[3]) yet?!");

Right now, this doesn’t seem so bad. What if we had 100 creators on our list? 1000? That’s a lot of lines of code to write and potentially a lot of places we would need to update it every time we added or removed a creator.

Syntax (Arrays)

We can use for-in loops to do something with each individual element from an array. If we use our array of trending TikTok creators, we could write a for-in loop to do the same thing with each of the string elements.

var trendingCreators = ["@cosette", "@avani", "@lorengray", "@jamescharles"]

for creator in trendingCreators {
  print(creator)
}

Breaking it Down

Let’s try to understand what’s going on here.

var trendingCreators = ["@cosette", "@avani", "@lorengray", "@jamescharles"]

for creator in trendingCreators {
  print(creator)
}

// => @cosette
// => @avani
// => @lorengray
// => @jamescharles

The word creator right after the keyword for refers to each element of our array, and the trendingCreators after the in keyword is the name of the array we are telling the computer to iterate over. We’ve told our loop to run the same number of times as there are elements in the array. So, since we currently have 4 elements in the array, it will run 4 times. If we added another creator to our array, the loop would run 5 times.

By using the word creator after the for keyword, we’re telling our code that we can use creator like a variable (only inside of the code block, or the curly braces), and its value for each iteration of the loop will be whatever element of the array it’s currently iterating over.

for-in Loops and Dictionaries

For the most part, the syntax for iterating over a dictionary looks very similar. Check it out:

var capitals = ["France": "Paris", "Cuba": "Havana", "Japan": "Tokyo"]

for pair in capitals {
  print(pair)
}

Takeaways:

• The syntax for looping over a dictionary is the same as an array
• When looping over a dictionary, we can control how much information we want - both key and value, just the key, or just the value
• The loop will run a certain number of times - the number of key/value pairs that are in the dictionary it is looping over

Another Way to Iterate Over Dictionaries

If you are looping over a dictionary and want both key and value to use at some point, it may be beneficial to modify your syntax so that both have a label:

var capitals = ["France": "Paris", "Cuba": "Havana", "Japan": "Tokyo"]

for (country, capital) in capitals {
  print("The capital of \(country) is \(capital).")
}

In the code snippet above, we still get both the key and value, but this time they each have a label/variable name associated with it. So, it may be easier to work with if we want to interpolate with each of them.

Placeholders & Counters

Sometimes, when iterating over an array, our end goal won’t be to do the exact same thing to each element. Sometimes we’ll want to combine elements. Sometimes we’ll want to filter through them and only use some of them.

Here’s a real-life example: Tumblr has an array of posts related to each term. When we first search a term, we’ll see all the posts. If we select “Link” from the filter drop-down, we’ll only see the two posts that are links.

How did Tumblr do that? They iterated over the array of all posts, checked each one for a piece of criteria, and put those that did meet the criteria in a placeholder. After it finished iterating, it returned the placeholder and displayed them in the browser.

Let’s solve a slightly less involved challenge, but one that gets at the same idea. Our goal will be to take an array of numbers and return an array of only the numbers from the original array that are greater than 10.

// start with an array of numbers
var numbers = [1, 4, 23, 14, 5, 7, 11, 6, 92]

// start a placeholder array that will eventually store numbers over 10
var largeNumbers = [Int]()

// iterate over the array
for number in numbers {

}

In the code above, we set up our placeholder and our for in loop. Now, we need to do some work inside the curly braces of the loop.

var numbers = [1, 4, 23, 14, 5, 7, 11, 6, 92]
var largeNumbers = [Int]()

for number in numbers {
  // check if the current number being iterated over is greater than 10
  if number > 10 {

  }
}

In the code above, we wrote a conditional to check the value of the current number against 10. Now, we need to write code inside that conditional.

var numbers = [1, 4, 23, 14, 5, 7, 11, 6, 92]
var largeNumbers = [Int]()

for number in numbers {
  if number > 10 {
    largeNumbers.append(number)
  }
}

At this point, our largeNumbers array is holding all four numbers greater than 10, but we aren’t ever printing it out, so it’s hard to see that.

var numbers = [1, 4, 23, 14, 5, 7, 11, 6, 92]
var largeNumbers = [Int]()

for number in numbers {
  if number > 10 {
    largeNumbers.append(number)
  }
}

print(largeNumbers)

for-in Loops without Collections

We can also use for-in loops without a collection, like an array or dictionary. In this case, we would do this because we want to perform a specific action, a certain number of times. There are a couple of differences in the syntax you might see when we aren’t iterating over an array. See the code below:

for _ in 1...4 {
  print("Hello!")
}

Why didn’t we use a variable here? We didn’t need one. We weren’t iterating over an array and trying to do something with each element, so we use the underscore.

for number in 1...4 {
  print(number)
}

In the example above, we did declare a variable, number - it will behave as if we were iterating over [1, 2, 3, 4]. (This doesn’t mean 1...4 really is an array, though.) If you’re interested in learning more about this, read up on the Range documentation.

Loops

We’re really getting into some complicated code! Work through the practice below to make sure you have this down.