This lesson continues our exploration of functions. We’ll present a bit more Kotlin syntax, and spend time reinforcing what we’ve learned about functions. Let’s get started!

`for in`

Loop`for in`

LoopTo get us warmed up and ready to go, let’s check out a new bit of Kotlin syntax!
Remember how we started with this common `while`

loop:

var values = intArrayOf(1, 9, 9)

var i = 0

while (i < values.size) {

// Do something with each value, like print it

println(i)

i++

}

and eventually arrived at this common `for`

loop:

var values = intArrayOf(1, 9, 9)

for (i in values.indices) {

// Do something with each value, like print it

println(i)

}

Well, that `for`

loop became so common that there’s an even simpler way to work through the values in array using Kotlin’s `for in`

loop:

var values = intArrayOf(1, 9, 9)

for (i in values.indices) {

// Do something with each value

}

`for`

LoopIndexed v. Non-Indexed

`for`

LoopWhen you should use the indexed `for`

(`for (i in 0 until value)`

) and when `for in`

?
(Technically they are both `for in`

loops, but the difference is whether the variable hold the index or a value from the array.)
Here are some things to consider:

- If you are iterating over an array—or other data structures that support iteration, which we’ll encounter soon—consider the
`for in`

loop - However, if you
*need*access to the array index, then you have no option but to use the indexed`for`

loop

For many common array-processing tasks that we’ve encountered, the `for in`

loop is a much better fit, since avoiding the extra index variable leads to a cleaner loop declaration and value access within the loop.
For example, counting:

var values = intArrayOf(1, 2, 4)

// Indexed for

var count = 0

for (i in values.indices) { // Extra variable i

if (values[i] > 1) { // must use bracket notation

count++

}

}

println(count)

// for in

count = 0

for (value in values) { // No i needed for this problem

if (value > 1) { // No bracket notation!

count++

}

}

println(count)

Write a method `arrayAllPairs`

that returns whether a passed `IntArray`

is composed entirely of adjacent pairs of
the same value.
For example, the array `{1, 1, 2, 2}`

and the array `{4, 4, -1, -1}`

are composed of adjacent pairs of the same
element, but `{2, 1, 1, 2}`

and `{4, 4, -1, 0}`

are not.
To be composed entirely of pairs of the same element, the array must contain an even number of elements.
If the passed array is empty, you should return `false`

.

You will need to construct your loop carefully to complete this problem! We suggest that you examine the array looking for a counterexample: meaning a pair of adjacent elements that do not have the same value.

Practice with Functions

Next let’s get some more practice with functions!
Together we’ll write a method that determines when an `IntArray`

is a palindrome array: meaning that the values it contains are the same forward and backward.
For example, `{1, 2, 4}`

is *not* an array palindrome, but `{1, 0, 2, 0, 1}`

is!

Let’s go step by step and see how to approach constructing this method. First, let’s determine our method signature, and practice calling it on some sample inputs.

Next, let’s begin work on the body of the method. Our array access pattern here is a bit different than what we’ve seen previously, so let’s proceed carefully.

As a next step, let’s complete the job by adding the decision-making logic we need to determine if the passed array is an array palindrome. This should remind us a bit of the search pattern that we covered yesterday.

Finally, let’s make one small improvement to our code.

Write a method named `arrayRangeSum`

.
It receives an `IntArray`

and a positive `Int`

`range`

as parameters.
You should return the sum of all the elements in the array that are between `range`

and `range`

* -1, non-inclusive.

For example, given the array `{1, -4, 2, 24, -124}`

and the range `8`

, you would return `-1`

: `1 + -4 + 2`

, since
these are the values in the array strictly greater than `-8`

and strictly less than `8`

.
Given the range `124`

, you should return `23`

: `1 + -4 + 2 + 24`

, since these are the values in the array strictly
greater than `-124`

and less than `124`

.

Note that this problem is a great fit for the `for in`

loop!

Write a method named `arrayMinMaxSum`

.
It receives an `IntArray`

and two `Int`

values, min and max, in that order, as parameters.
You should return the sum of all the elements in the array that are between the min the max, inclusive.

For example, given the array `{1, -4, 2, 24, -124}`

and the min and max `1`

and `24`

, you would return `27`

:
`1 + 2 + 24`

, since these are the values in the array greater than or equal to `1`

and less than or equal to `24`

.
Note that the minimum may be greater than the maximum, in which case you should return `0`

.
(But you don't need code to handle this specific case.)

Note that this problem is a great fit for the `for in`

loop!

CS People: Ruchi Sanghvi

Early Facebook has a well-deserved reputation for being a male-dominated testosterone-driven workplace. So it may surprise you to find out that one of the software developers who was a core contributor to the first version of the News Feed—way back in 2006—was Ruchi Sanghvi.

She was also involved in Facebook’s response to the initial feedback on the News Feed.
Users hated it!
But Facebook kept it for one simple reason: Users were spending more time on the site.
You should always keep this in mind when using Facebook, YouTube, TikTok, or any free site run by ad revenue.
Their primary and sometimes only goal is for you to spend *as much time as possible* on their site.
Regardless of whether that’s healthy or appropriate or useful to you.

Ruchi Sanghvi worked at several companies after leaving Facebook. In this video, she discusses some of what she learned along the way.

Need more practice? Head over to the practice page.