Kotlin for Loop

In Kotlin, the for loop is a versatile control structure that allows you to iterate through ranges, arrays, collections, and other sequences of data. This guide will take you through the fundamentals of the for loop in Kotlin, along with its usage, syntax, and examples to help you grasp this essential programming concept.

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Course Outline

1. Introduction to the for Loop
   • What is a for loop?
   • When to use a for loop in Kotlin
2. Syntax of the for Loop in Kotlin
   • Basic syntax for iterating over ranges and collections
3. Using for Loop with Ranges
   • for loop with a range of numbers
   • Ranges with step values
4. Using for Loop with Arrays and Collections
   • Iterating through arrays and lists
   • Accessing index and element pairs
5. Using for Loop with Maps
   • Iterating through key-value pairs in maps
   • Working with destructured entries
6. Nested for Loops
   • Creating loops within loops
   • Practical examples of nested for loops
7. Advanced Techniques with for Loops
   • Using for loop with conditions
   • Combining for loop with break and continue
8. Examples and Exercises

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1. Introduction to the for Loop

The for loop in Kotlin is primarily used to iterate over a sequence, which can be a range, array, list, map, or any iterable object. It’s especially useful when you know the number of iterations you need to perform.

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2. Syntax of the for Loop in Kotlin

The basic syntax of a for loop in Kotlin is:

for (item in sequence) {
    // Code to execute for each item
}

Here, item represents the element in each iteration, and sequence is the collection or range to loop over.

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3. Using for Loop with Ranges

Iterating Over a Range of Numbers

A range is defined by two values, separated by two dots (..). Here’s how you can use a for loop to iterate through a range:

for (i in 1..5) {
    println(i)
}

Output:

Copy code1
2
3
4
5

Using Step Values

You can specify a step value to control the increment in each iteration.

for (i in 1..10 step 2) {
    println(i)
}

Output:

Copy code1
3
5
7
9

Descending Order with downTo

To loop in reverse, use downTo:

for (i in 5 downTo 1) {
    println(i)
}

Output:

Copy code5
4
3
2
1

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4. Using for Loop with Arrays and Collections

Iterating Through Arrays

Use a for loop to iterate over each element in an array:

val fruits = arrayOf("Apple", "Banana", "Cherry")
for (fruit in fruits) {
    println(fruit)
}

Output:

Copy codeApple
Banana
Cherry

Using Indices

To get the index of each element, use .indices:

for (index in fruits.indices) {
    println("Index $index: ${fruits[index]}")
}

Output:

yamlCopy codeIndex 0: Apple
Index 1: Banana
Index 2: Cherry

Accessing Index and Element Pairs

You can also access both index and element with the withIndex() function:

for ((index, fruit) in fruits.withIndex()) {
    println("Index $index: $fruit")
}

Output:

yamlCopy codeIndex 0: Apple
Index 1: Banana
Index 2: Cherry

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5. Using for Loop with Maps

Maps store data as key-value pairs. To iterate through them, you can use the following syntax:

val ageMap = mapOf("Alice" to 23, "Bob" to 29, "Catherine" to 31)
for ((name, age) in ageMap) {
    println("$name is $age years old")
}

Output:

csharpCopy codeAlice is 23 years old
Bob is 29 years old
Catherine is 31 years old

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6. Nested for Loops

A nested for loop is when you have one for loop inside another. This is useful for iterating through multidimensional arrays or creating patterns.

for (i in 1..3) {
    for (j in 1..3) {
        println("i: $i, j: $j")
    }
}

Output:

yamlCopy codei: 1, j: 1
i: 1, j: 2
i: 1, j: 3
i: 2, j: 1
...

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7. Advanced Techniques with for Loops

Using Conditions in for Loops

You can add if conditions inside a for loop to filter elements.

for (i in 1..10) {
    if (i % 2 == 0) {
        println(i)
    }
}

Output:

Copy code2
4
6
8
10

Using break and continue

• break: Stops the loop entirely.
• continue: Skips to the next iteration of the loop.

for (i in 1..10) {
    if (i == 5) break
    println(i)
}

Output:

Copy code1
2
3
4

for (i in 1..10) {
    if (i % 2 == 0) continue
    println(i)
}

Output:

Copy code1
3
5
7
9

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8. Examples and Exercises

Example 1: Print Multiplication Table

Create a for loop that prints a multiplication table for a number:

val number = 5
for (i in 1..10) {
    println("$number * $i = ${number * i}")
}

Example 2: Count Elements in a List

Count the number of even numbers in a list.

val numbers = listOf(1, 2, 3, 4, 5, 6)
var evenCount = 0
for (number in numbers) {
    if (number % 2 == 0) evenCount++
}
println("There are $evenCount even numbers.")

Exercise 3: Display Names with a Prefix

Given a list of names, print each name with a “Hello” prefix.

val names = listOf("John", "Emma", "Chris")
for (name in names) {
    println("Hello, $name!")
}

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Conclusion

The for loop is a fundamental construct in Kotlin, enabling you to iterate over collections, arrays, and ranges with ease. It’s flexible, allowing for nested loops, conditions, and control flow statements like break and continue. By mastering for loops, you’ll be equipped to handle various data structures and algorithms, making your code more efficient and organized.