if...else statements
if...else and the if. The following code fragment is an example of a binary selection using the if...else statement.
import kotlin.math.sqrt
fun main() {
print("Enter a number: ");
val n = readln().toDouble()
if (n >= 0) {
println(sqrt(n))
} else {
println("Number can not be negative.")
}
}
n is checked to see if it is greater than or equal to zero. If it is, the program prints the square root of the number. If it is not, the statement performs the else clause and tells the user that the number cannot be negative.
if must be enclosed in parentheses. The body of the if and else clause is enclosed in braces. If there is exactly one statement in the clause, you may omit the braces, but doing so may lead to code that is ambiguous and difficult to read. Thus, we strongly recommend that you always use braces (with one exception that we will cover later).
fun main() {
print("Enter a score: ");
val score = readln().toInt()
var grade = ""
if (score >= 90) {
grade = "A"
} else {
if (score >= 80) {
grade = "B"
} else {
if (score >= 70) {
grade = "C"
} else {
if (score >= 60) {
grade = "D"
} else {
grade = "F"
}
}
}
}
println("The grade is $grade.")
}
score by printing the letter grade earned. If the score is greater than or equal to 90, the variable grade will be assigned "A". If it is not (else), the next question is asked. If the score is greater than or equal to 80, then it must be between 80 and 89 since the answer to the first question was false. In this case, the grade becomes "B" is printed. You can see that using correct indentation pattern helps to make sense of the association between if and else.
else immediately by an if with the resulting code:
fun main() {
print("Enter a score: ");
val score = readln().toInt()
var grade = ""
if (score >= 90) {
grade = "A"
} else if (score >= 80) {
grade = "B"
} else if (score >= 70) {
grade = "C"
} else if (score >= 60) {
grade = "D"
} else {
grade = "F"
}
println("The grade is $grade.")
}
else is still necessary to provide the default case if all other conditions fail.
else is optional for if statements
else clause is not required in Kotlin; an if without an else is called a single-way selection construct. In a single-way selection, if the condition is true, an action is performed. When the condition is false, processing simply continues on to the next statement after the if. For example, the following fragment will first check to see if the value of a variable n is negative. If so, then it is modified by the absolute value function. Regardless, the next action is to compute the square root.
import kotlin.math.abs
import kotlin.math.sqrt
fun main() {
print("Enter a value: ");
var n = readln().toDouble()
if (n < 0) {
n = abs(n)
}
println(sqrt(n))
}
if expressions
grade. Kotlin provides an if expression that we can use to make the code a bit briefer:
fun main() {
print("Enter a score: ");
val score = readln().toInt()
val grade = if (score >= 90) {
"A"
} else if (score >= 80) {
"B"
} else if (score >= 70) {
"C"
} else if (score >= 60) {
"D"
} else {
"F"
}
println("The grade is $grade.")
}
if is an expression that returns a value, which is then assigned to the variable grade. Again, this version if if is useful when every case assigns a value to the same variable. This form requires that there be an else case.
when statements and expressions
if...else statement or expression where at every step we check the same variable for equality to different values, we can use a when statement or expression instead. when results in slightly briefer code than if in this situation. Here are two examples with similar behavior, one using when as a statement, and one using it as an expression.
when statement.fun main() {
print("Enter a letter grade: ");
val grade = readln()
var score = 0
when (grade) {
"A" -> score = 90
"B" -> score = 80
"C" -> score = 70
"D" -> score = 60
else -> score = 0
}
println("The grade was at least $score")
}
when expression.fun main() {
print("Enter a letter grade: ");
val grade = readln()
val score = when (grade) {
"A" -> 90
"B" -> 80
"C" -> 70
"D" -> 60
else -> 0
}
println("The grade was at least $score")
}
while statement and a very powerful for statement. The while statement repeats a body of code as long as a condition evaluates to true. Generally, we use a while loop for indeterminate loops, where we do not know how many times the loop has to iterate. Consider this program, where we ask the user repeatedly enter numbers they want to sum, entering zero when they are finished. In this case, we have no idea how many times the user will enter numbers before finishing:
fun main() {
var sum = 0.0
print("Enter a number, or zero to quit: ")
var n = readln().toDouble()
while (n != 0.0) {
sum = sum + n
print("Enter another number, or zero to quit: ")
n = readln().toDouble()
}
println(String.format("The sum is %.3f%n", sum))
}
while statement is evaluated at the start of each repetition. If the condition evaluates to true, the body of the statement will execute. While Kotlin does not require you to indent the body of the loop, we strongly suggest you do so to make your code more readable.
while statement is a very general-purpose iterative structure that we will use in a number of different algorithms. In many cases, a compound condition will control the iteration. Consider the following variation of the above program, which will also stop when the sum goes over 10:
fun main() {
print("Enter a number, or zero to quit: ")
var n = readln().toDouble()
var sum = n
while (n != 0.0 && sum < 10) {
print("Enter another number, or zero to quit: ")
n = readln().toDouble()
sum = sum + n
}
println(String.format("The sum is %.3f%n", sum))
}
for loop in conjunction with many of the Kotlin collections to iterate over the collection’s members. So, for example,
for (item in listOf(1, 3, 6, 2, 5)) {
println(item)
}
item to be each successive value in the list [1, 3, 6, 2, 5]. The body of the iteration is then executed. This results in the following output:
1 3 6 2 5
for (item in 0 ..< 5) {
println(item * item)
}
println function five times. The expression 0 ..< 5 returns a range object representing the sequence 0, 1, 2, 3, 4 and each value will be assigned to the variable item. This value is then squared and printed.
0 .. 5 (without the <) is inclusive on both ends: it represents the sequence 0, 1, 2, 3, 4, 5. Here are some other examples:
0 ..< 5 step 2 : sequence starting at 0, ending with 4, each item increasing by 25 downTo 1: sequence starting at 5, ending at 1, each item decreasing by 1while loop to continually prompt the user to enter a number greater than zero until the user does so. (Hint: as long as the number is not greater than zero, you must keep asking.)
for loop.
if is your friend).
