String.split() / joined() / String Index

// Split a string (returns an array of Substring)
string.split(separator: "character")
string.split(separator: "character", maxSplits: limit)
string.split(separator: "character", omittingEmptySubsequences: false)

// Join an array
array.joined(separator: "delimiter")

// Index operations
string[string.startIndex]
string[string.index(string.startIndex, offsetBy: n)]
string.prefix(n)
string.suffix(n)
string.dropFirst(n)
string.dropLast(n)

// split: split a string
let sentence = "Swift は 型安全 な 言語 です"
let words = sentence.split(separator: " ")
print(words)  // ["Swift", "は", "型安全", "な", "言語", "です"]
print(type(of: words[0]))  // Substring

// Convert to String
let wordStrings = words.map { String($0) }

// Limit splits with maxSplits
let csv = "A,B,C,D,E"
let parts = csv.split(separator: ",", maxSplits: 2)
print(parts)  // ["A", "B", "C,D,E"]

// joined: join an array into a string
let fruits = ["りんご", "みかん", "ぶどう"]
let joined = fruits.joined(separator: "、")
print(joined)  // りんご、みかん、ぶどう

let path = ["Users", "taro", "Documents"]
print(path.joined(separator: "/"))  // Users/taro/Documents

// prefix / suffix / dropFirst / dropLast
let text = "Hello, World!"
print(text.prefix(5))    // Hello
print(text.suffix(6))    // World!
print(text.dropFirst(7)) // World!
print(text.dropLast(1))  // Hello, World

// Accessing characters with String.Index
let str = "あいうえお"
let startIdx = str.startIndex
let thirdIdx = str.index(startIdx, offsetBy: 2)  // 3rd character
print(str[thirdIdx])  // う

// Slice with a range
let range = str.index(startIdx, offsetBy: 1) ..< str.index(startIdx, offsetBy: 4)
print(str[range])  // いうえ

split(separator:) returns an array of Substring values. A Substring is a view that shares memory with the original string. If you need to store the result independently, copy it with String($0).

Swift string indices are of type String.Index, not integers. Because string length depends on Unicode, random access is not O(1). Be mindful of performance when doing many index-based lookups. prefix() and suffix() handle the index arithmetic internally and improve readability, so prefer them when possible.

For string basics such as count and isEmpty, see String.count / isEmpty / String Literals.