📏 Kotlin – Ranges: Work with Number Sequences Easily

🧲 Introduction – Why Learn Kotlin Ranges?

Ranges in Kotlin provide a simple way to represent sequences of values, typically numbers or characters. Whether you’re looping through numbers, checking if a value lies within a range, or creating slices—Kotlin’s range expressions make the task readable and concise.

🎯 In this guide, you’ll learn:

• How to define and use numeric and character ranges
• Use ranges in for loops and condition checks
• Customize ranges using step, downTo, and until
• Real-world use cases and best practices

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🔢 Kotlin Range Syntax

✅ Standard Range using ..

val range = 1..5

Represents numbers: 1, 2, 3, 4, 5
The .. operator is inclusive.

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🔁 Using Ranges in Loops

for (i in 1..3) {
    println(i)
}

🟢 Output:

1  
2  
3

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🔁 Reverse Ranges using downTo

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

🟢 Output:

5  
4  
3  
2  
1

• Useful for countdowns or reverse iterations.

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🔁 Excluding End with until

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

🟢 Output:

1  
2  
3  
4

• until excludes the end value (5 in this case).

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⏩ Skipping Steps with step

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

🟢 Output:

2  
4  
6  
8  
10

• step defines how much to increment/decrement.

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🔤 Character Ranges

Kotlin also supports character ranges.

for (ch in 'a'..'e') {
    println(ch)
}

🟢 Output:

a  
b  
c  
d  
e

✔️ Works similarly to numeric ranges.

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📌 Range Checks with in and !in

val x = 10

if (x in 1..20) {
    println("x is in range")
}

if (x !in 15..30) {
    println("x is NOT in the second range")
}

🟢 Output:

x is in range  
x is NOT in the second range

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📐 Range Types Summary

Expression	Description	Inclusive	Reversible	Step
a..b	Inclusive range from a to b	✅	❌	✅
a downTo b	Reverse from a to b	✅	✅	✅
a until b	Range from a to b-1	❌	❌	✅

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🧠 Best Practices for Ranges

Practice	Why It Helps
Use until to avoid off-by-one	Prevents logic errors in loops
Combine downTo with step	Enhances control in countdowns
Use in for readable conditionals	Replaces verbose >= and <= checks
Prefer ranges in for loops	Cleaner than indexed iterations

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🚫 Common Mistakes

❌ Mistake	✅ Fix
Using .. instead of until	Use until to exclude upper bound
Forgetting step with custom logic	Add step to control iteration
Assuming a..b works with Float	Use IntRange—floats aren’t supported

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📌 Summary – Recap & Next Steps

Kotlin ranges give you a powerful and readable way to handle sequences, loops, and conditional logic. Whether you’re looping through numbers or checking value bounds, ranges help keep your code clean, expressive, and bug-free.

🔍 Key Takeaways:

• Use .. for inclusive ranges and until to exclude the end.
• Combine with step for flexible iteration control.
• Use in/!in for intuitive range-based checks.
• Works with both numbers and characters.

⚙️ Practical Use:
Perfect for loop counters, character parsing, pagination, input validation, or mathematical operations in Android and server-side Kotlin.

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❓ FAQs – Kotlin Ranges

❓ What is the difference between .. and until in Kotlin?
✅ .. includes the end value, until excludes it.

1..5 → 1, 2, 3, 4, 5  
1 until 5 → 1, 2, 3, 4

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❓ Can Kotlin ranges be used with Double or Float?
✅ Not directly. Kotlin ranges support integral and character types only. Use step or manual loops for floating-point.

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❓ How do I loop backwards in Kotlin?
✅ Use downTo:

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

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❓ Can I check if a character is in a range?
✅ Yes:

if ('b' in 'a'..'z') { println("It's a letter") }

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❓ How do I skip items in a range?
✅ Use step:

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

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