LESSON 380 OF 414 · PROJECTS: COMMAND-LINE TOOLS

CLI Tool: head v4 — Negative n drops the tail in Kotlin

Practice

Same signature as v2/v3, but now n may be negative. Real head -n -K prints all lines except the last K. So when n < 0 return all lines except the last |n|; when n ≥ 0 behave exactly like v3 (return the first n, capped at the input length). If |n| is at least the number of lines, nothing is left, so return the empty string.

Function signature:

fun head(lines: List<String>, n: Int): String

Example (negative n):

Input: lines = ["a", "b", "c", "d", "e"], n = -2

Output: "a\nb\nc"

When printed:
a
b
c

Kotlin's subList needs an explicit, non-negative end index. Branch on the sign of n: when n < 0, the end index is lines.size + n; otherwise it's minOf(n, lines.size). Don't forget maxOf(0, ...) for the negative branch in case |n| exceeds the list length.

val end: Int
if (n < 0) {
    end = maxOf(0, lines.size + n)
} else {
    end = minOf(n, lines.size)
}
return lines.subList(0, end).joinToString("\n")

The full solution:

class Solution {
    fun head(lines: List<String>, n: Int): String {
        val end: Int
        if (n < 0) {
            end = maxOf(0, lines.size + n)
        } else {
            end = minOf(n, lines.size)
        }
        return lines.subList(0, end).joinToString("\n")
    }
}

STEP 1 OF 5

Guided

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CLI Tool: head v4 — Negative n drops the tail in Kotlin

Head V4 in Kotlin