LESSON 380 OF 414 · PROJECTS: COMMAND-LINE TOOLS

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

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:

fn head(lines: Vec<String>, n: i32) -> String

Example (negative n):

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

Output: "a\nb\nc"

When printed:
a
b
c

Branch on the sign of n: when n < 0, the end index is lines.len() as i32 + n (with a max(0, ...) floor); otherwise it's min(lines.len() as i32, n). Then slice and call join("\n") — Rust string slices have a built-in join.

let end: usize;
if n < 0 {
    end = std::cmp::max(0, lines.len() as i32 + n) as usize;
} else {
    end = std::cmp::min(lines.len() as i32, n) as usize;
}
lines[0..end].join("\n")

The full solution:

impl Solution {
    fn head(lines: Vec<String>, n: i32) -> String {
        let end: usize;
        if n < 0 {
            end = std::cmp::max(0, lines.len() as i32 + n) as usize;
        } else {
            end = std::cmp::min(lines.len() as i32, n) as usize;
        }
        lines[0..end].join("\n")
    }
}

STEP 1 OF 5

Guided

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

Head V4 in Rust