LESSON 399 OF 415 · PROJECTS: COMMAND-LINE TOOLS

CLI Tool: tr v2 — Set Translation in Swift

Practice

Build a more powerful version of the Unix tr command. Given a list of lines and two equal-length strings set1 and set2, replace every character in each line that appears in set1 at index i with set2[i]. Characters not in set1 are unchanged. Return the lines joined by newline characters (\n).

Notes: set1 and set2 have the same length, contain only ASCII characters, and set1 has no repeated character. Characters are taken literally: there are no ranges like a-z. All replacements happen at once, so set1 = "ab", set2 = "ba" swaps the two letters. Lines may be empty.

Function signature:

func tr(_ lines: [String], _ set1: String, _ set2: String) -> String

Example:

Input: lines = ["banana", "cherry"], set1 = "abc", set2 = "ABC"

Output: "BAnAnA\nCherry"

When printed:
BAnAnA
Cherry

Build a [Character: Character] dictionary from each char in set1 to the matching char in set2. Then walk each line and use map[ch] ?? ch to translate.

var map = [Character: Character]()
for (a, b) in zip(set1, set2) {
    map[a] = b
}
// per char: map[ch] ?? ch

The full solution:

class Solution {
    func tr(_ lines: [String], _ set1: String, _ set2: String) -> String {
        var map = [Character: Character]()
        for (a, b) in zip(set1, set2) {
            map[a] = b
        }
        var result = [String]()
        for line in lines {
            var out = ""
            for ch in line {
                out += String(map[ch] ?? ch)
            }
            result.append(out)
        }
        return result.joined(separator: "\n")
    }
}

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Guided

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CLI Tool: tr v2 — Set Translation in Swift

Tr V2 in Swift