Solution: Pretty time display
fun secondsToPrettyTime(seconds: Int): String {
if (seconds < 0) return "Invalid input"
if (seconds == 0) return "Now"
// or
// when {
// seconds < 0 -> return "Invalid input"
// seconds == 0 -> return "Now"
// }
val hours = seconds / SECONDS_IN_HOUR
val minutes = (seconds % SECONDS_IN_HOUR)/MINUTES_IN_HOUR
val secondsLeft = seconds % SECONDS_IN_MINUTE
var result = ""
if (hours > 0) {
result += "$hours h"
}
if (minutes > 0) {
result += " $minutes min"
}
if (secondsLeft > 0) {
result += " $secondsLeft sec"
}
return result.trim()
}
// Constants should be defined outside of the function
// we often name them using UPPER_SNAKE_CASE convention
// const modifier is a low-level optimization
private const val SECONDS_IN_MINUTE = 60
private const val MINUTES_IN_HOUR = 60
private const val SECONDS_IN_HOUR = 3600
This is not the only possible solution to this exercise. Especially transforming hours, minutes and secondsLeft into a string can be done in many different ways. I will show you a few popular solutions to this problem. Some of them use functions like listOfNotNull, joinToString, takeIf or buildString that are presented in the Functional Kotlin book (the next book I recommend you reading after Kotlin Essentials). Have patience, you will learn about them later, for now you can just treat them as curious examples of Kotlin code.
// Option 1 - This option is similar to the original,
// but uses if as an expression, and adds strings with +
val hoursString = if (hours > 0) "$hours h " else ""
val minutesString = if (minutes > 0) "$minutes min " else ""
val secondsString =
if (secondsLeft > 0) "$secondsLeft sec " else ""
return (hoursString + minutesString + secondsString).trim()
// Option 2 - This option uses listOfNotNull to construct
// a list of non-null elements, and then transforms those
// elements into a single string using joinToString
return listOfNotNull(
if (hours > 0) "$hours h" else null,
if (minutes > 0) "$minutes min" else null,
if (secondsLeft > 0) "$secondsLeft sec" else null
).joinToString(separator = " ")
// Option 3 - This option is similar to the previous one,
// but uses takeIf to filter out null elements
return listOfNotNull(
"$hours h".takeIf { hours > 0 },
"$minutes min".takeIf { minutes > 0 },
"$secondsLeft sec".takeIf { secondsLeft > 0 },
).joinToString(separator = " ")
// Option 4 - This option uses buildString function
// to construct a string by appending it with next parts
// inside lambda expression block
return buildString {
if (hours > 0) append("$hours h ")
if (minutes > 0) append("$minutes min ")
if (secondsLeft > 0) append("$secondsLeft sec ")
}.trim()
// Option 5 - This is the most complicated option,
// because it covers all possible cases in a when-expression.
// I do not recommend using this solution, but I wanted to
// show it as an example of how when-expression can be used.
return when {
hours == 0 && minutes == 0 && secondsLeft == 0 -> ""
hours == 0 && minutes == 0 -> "$secondsLeft sec"
hours == 0 && secondsLeft == 0 -> "$minutes min"
minutes == 0 && secondsLeft == 0 -> "$hours h"
minutes == 0 -> "$hours h $secondsLeft sec"
secondsLeft == 0 -> "$hours h $minutes min"
hours == 0 -> "$minutes min $secondsLeft sec"
else -> "$hours h $minutes min $secondsLeft sec"
}
Example solution in playground
import org.junit.Test
import kotlin.test.assertEquals
fun secondsToPrettyTime(seconds: Int): String {
if (seconds < 0) return "Invalid input"
if (seconds == 0) return "Now"
// or
// when {
// seconds < 0 -> return "Invalid input"
// seconds == 0 -> return "Now"
// }
val hours = seconds / SECONDS_IN_HOUR
val minutes = (seconds % SECONDS_IN_HOUR)/MINUTES_IN_HOUR
val secondsLeft = seconds % SECONDS_IN_MINUTE
var result = ""
if (hours > 0) {
result += "$hours h"
}
if (minutes > 0) {
result += " $minutes min"
}
if (secondsLeft > 0) {
result += " $secondsLeft sec"
}
return result.trim()
}
// Constants should be defined outside of the function
// we often name them using UPPER_SNAKE_CASE convention
// const modifier is a low-level optimization
private const val SECONDS_IN_MINUTE = 60
private const val MINUTES_IN_HOUR = 60
private const val SECONDS_IN_HOUR = 3600
fun main() {
println(secondsToPrettyTime(-1)) // Invalid input
println(secondsToPrettyTime(0)) // Now
println(secondsToPrettyTime(45)) // 45 sec
println(secondsToPrettyTime(60)) // 1 min
println(secondsToPrettyTime(150)) // 2 min 30 sec
println(secondsToPrettyTime(1410)) // 23 min 30 sec
println(secondsToPrettyTime(60 * 60)) // 1 h
println(secondsToPrettyTime(3665)) // 1 h 1 min 5 sec
}
class PrettyTimeTest {
@Test
fun testNegativeSeconds() {
val seconds = -1
val expected = "Invalid input"
assertEquals(expected, secondsToPrettyTime(seconds))
}
@Test
fun testOnlySeconds() {
val seconds = 45
val expected = "45 sec"
assertEquals(expected, secondsToPrettyTime(seconds))
}
@Test
fun testOnlyMinutes() {
val seconds = 60
val expected = "1 min"
assertEquals(expected, secondsToPrettyTime(seconds))
}
@Test
fun testMinutesAndSeconds() {
val seconds = 150
val expected = "2 min 30 sec"
assertEquals(expected, secondsToPrettyTime(seconds))
}
@Test
fun testMinutesAndSecondsWithRemainder() {
val seconds = 1410
val expected = "23 min 30 sec"
assertEquals(expected, secondsToPrettyTime(seconds))
}
@Test
fun testOnlyHours() {
val seconds = 3600
val expected = "1 h"
assertEquals(expected, secondsToPrettyTime(seconds))
}
@Test
fun testHoursMinutesAndSeconds() {
val seconds = 3665
val expected = "1 h 1 min 5 sec"
assertEquals(expected, secondsToPrettyTime(seconds))
}
@Test
fun testZeroSeconds() {
val seconds = 0
val expected = "Now"
assertEquals(expected, secondsToPrettyTime(seconds))
}
@Test
fun testHoursMinutesSecondsWithZeroMinutes() {
val seconds = 3605
val expected = "1 h 5 sec"
assertEquals(expected, secondsToPrettyTime(seconds))
}
@Test
fun testHoursMinutesWithZeroSeconds() {
val seconds = 7200
val expected = "2 h"
assertEquals(expected, secondsToPrettyTime(seconds))
}
@Test
fun testMinutesSecondsWithZeroHours() {
val seconds = 150
val expected = "2 min 30 sec"
assertEquals(expected, secondsToPrettyTime(seconds))
}
@Test
fun testLargeValue() {
val seconds = 123456789
val expected = "34293 h 33 min 9 sec"
assertEquals(expected, secondsToPrettyTime(seconds))
}
}
