Logic & Loops

In this lesson, you will learn about "Control Flow" in Kotlin. You will learn how to use conditional statements and loops to control the flow of your program.

Conditional Statements

Conditional statements allow you to execute different blocks of code based on certain conditions. In Kotlin, you can use the following conditional statements:

• if statement
• if-else statement
• when statement

if Statement

The if statement is used to execute a block of code if a condition is true. The syntax for the if statement is as follows:

if (condition) {
    // Code to be executed if the condition is true
}

If the condition is true, the code block inside the curly braces {} will be executed. If the condition is false, the code block will be skipped.

Example

val number = 10

if (number > 0) {
    println("Number is positive")
}

In the example above, we declare a variable number with a value of 10. We then use an if statement to check if number is greater than 0. Since number is 10, the condition is true, and the message "Number is positive" will be printed.

if-else Statement

The if-else statement is used to execute one block of code if a condition is true and another block of code if the condition is false. The syntax for the if-else statement is as follows:

if (condition) {
    // Code to be executed if the condition is true
} else {
    // Code to be executed if the condition is false
}

If the condition is true, the code block inside the first set of curly braces {} will be executed. If the condition is false, the code block inside the second set of curly braces {} will be executed.

Example

val number = -5

if (number > 0) {
    println("Number is positive")
} else {
    println("Number is negative")
}

In the example above, we declare a variable number with a value of -5. We then use an if-else statement to check if number is greater than 0. Since number is -5, the condition is false, and the message "Number is negative" will be printed.

You can also chain multiple if-else statements together to check for multiple conditions.

Example

val number = 0

if (number > 0) {
    println("Number is positive")
} else if (number < 0) {
    println("Number is negative")
} else {
    println("Number is zero")
}

In the example above, we declare a variable number with a value of 0. We then use multiple if-else statements to check if number is greater than 0, less than 0, or equal to 0. Since number is 0, the message "Number is zero" will be printed.

when Statement

The when statement allows you to execute different blocks of code based on a more complex state than true/false. this is useful when you have multiple conditions to check, so you don't have to chain multiple if-else statements.

The syntax for the when statement is as follows:

when (variable) {
    value1 -> {
        // Code to be executed if variable equals value1
    }
    value2 -> {
        // Code to be executed if variable equals value2
    }
    else -> {
        // Code to be executed if variable does not equal any of the values
    }
}

The when statement checks the value of the variable and executes the code block corresponding to the value. If the variable does not match any of the values, the code block inside the else block will be executed.

Example

val number = 10

when (number) {
    0 -> println("Number is zero")
    in 1..9 -> println("Number is between 1 and 9")
    else -> println("Number is 10 or greater")
}

In the example above, we declare a variable number with a value of 10. We then use a when statement to check the value of number. If number is 0, the message "Number is zero" will be printed. If number is between 1 and 9, the message "Number is between 1 and 9" will be printed. If number is 10 or greater, the message "Number is 10 or greater" will be printed.

Loops

Loops allow you to execute a block of code multiple times. In Kotlin, you can use the following loop structures:

• for loop
• while loop
• do-while loop

But first we need to quickly go over ranges and collections.

Ranges

You can create a range of numbers by specifying the start and end values separated by the .. operator. The range includes the start value and the end value.

val range = 1..5 // {1, 2, 3, 4, 5}

You can also create a range that counts down by using the downTo keyword.

val range = 5 downTo 1 // {5, 4, 3, 2, 1}

Collections

Collections are used to store multiple values. In Kotlin, there are many types of collections, but most of them are for you to learn about on your own. For this lesson we will just be talking about lists

A list is an ordered collection of elements. In Kotlin, you can create a list using the listOf function.

val numbers = listOf(1, 2, 3, 4, 5)

You can access elements in a list using the index of the element.

println(numbers[0]) // Output: 1

List indices start at 0, so the first element in the list has an index of 0.

for Loop

The for loop is used to iterate over a range, collection, or any other type of iterable. The syntax for the for loop is as follows:

for (element in range) {
    // Code to be executed for each element
}

The for loop will iterate over each element in the range and execute the code block for each element. the element variable will be assigned to the current element in the range.

Example

val numbers = listOf(1, 2, 3, 4, 5)

for (number in numbers) {
    println(number)
}

In the example above, we create a list of numbers 1, 2, 3, 4, 5 and use a for loop to iterate over each number and print it to the console.

You can use a shorthand for the for loop by using the .forEach function. When doing this you can use the it keyword to refer to the current element.

Example

val numbers = listOf(1, 2, 3, 4, 5)

numbers.forEach {
    println(it)
}

while Loop

The while loop is used to execute a block of code as long as a condition is true. The syntax for the while loop is as follows:

while (condition) {
    // Code to be executed while the condition is true
}

The while loop will continue to execute the code block as long as the condition is true.

Example

var number = 5

while (number > 0) {
    println(number)
    number--
}

In the example above, we declare a variable number with a value of 5. We then use a while loop to print the value of number and decrement it by 1 until number is less than or equal to 0.

You can also use the do-while loop, which is similar to the while loop but will always execute the code block at least once before checking the condition.

Example

var number = 5

do {
    println(number)
    number--
} while (number > 0)

In the example above, we declare a variable number with a value of 5. We then use a do-while loop to print the value of number and decrement it by 1 until number is less than or equal to 0.