Strings & Interpolation

Almost every app shows text: names, addresses, prices, messages. In Swift, text is stored in a String, a sequence of Character values. Swift strings are fully Unicode, so Hindi, Tamil, the ₹ sign and accented letters all work without special effort.

In this lesson you will build strings with interpolation, write multi-line and raw strings, use the most common string methods, and learn why Swift does not let you write name[0].

String literals and interpolation

A string literal is text in double quotes. String interpolation inserts values into a string: write \( and ) around any expression, and Swift converts the result to text.

let name = "Farhan"
let city = "Lucknow"
let age = 21
print("Hi, I am \(name) from \(city).")
print("Next year I will be \(age + 1).")

let price = 45
let quantity = 4
print("Total: ₹\(price * quantity)")

Hi, I am Farhan from Lucknow.
Next year I will be 22.
Total: ₹180

Interpolation works with any type: numbers, Bool values, and your own types. It is the normal way to build text in Swift, and it avoids the type errors you get when joining numbers to text with +.

Joining, counting and multi-line strings

var greeting = "Good"
greeting += " morning"
greeting = greeting + ", Kochi"
print(greeting)
print(greeting.count)
print("".isEmpty)

let address = """
    Sharma Sweets
    MI Road, Jaipur
    Rajasthan 302001
    """
print(address)

Good morning, Kochi
19
true
Sharma Sweets
MI Road, Jaipur
Rajasthan 302001

+ joins two strings and += appends to a var. count gives the number of characters, and isEmpty checks for an empty string (it is clearer and faster than count == 0).

A multi-line string starts and ends with three double quotes on their own lines. The indentation of the closing """ decides how much leading space is removed from every line, so you can indent the text neatly with your code and still print it without the extra spaces.

Escape sequences and raw strings

Some characters cannot be typed directly inside quotes. You write them with a backslash escape sequence: \" for a double quote, \\ for a backslash, \n for a new line, \t for a tab and \u{...} for any Unicode character by its code.

let student = "Farhan"
print("She said \"Namaste\"")
print("Folder: C:\\Users\\\(student)")
print("Line one\nLine two")
print("Rupee sign: \u{20B9}")
print(#"A raw string keeps \n and "quotes" as they are"#)
print(#"Interpolation in a raw string: \#(student)"#)

She said "Namaste"
Folder: C:\Users\Farhan
Line one
Line two
Rupee sign: ₹
A raw string keeps \n and "quotes" as they are
Interpolation in a raw string: Farhan

A raw string is wrapped in #" and "#. Backslashes and quotes inside it are ordinary characters, which is very useful for regular expressions and Windows paths. To interpolate inside a raw string, add the same number of # signs after the backslash: \#(student).

Common string methods

Some useful methods come from the standard library and some from Foundation, Apple's basic library that is also available on Linux and Windows. Add import Foundation at the top of the file when you use them; SwiftUI apps import it automatically through import SwiftUI.

import Foundation

let station = "  New Delhi Junction  "
let clean = station.trimmingCharacters(in: .whitespaces)
print(clean)
print(clean.uppercased())
print(clean.lowercased())
print(clean.hasPrefix("New"), clean.hasSuffix("Station"))
print(clean.contains("Delhi"))
print(clean.replacing("Junction", with: "Jn"))

let average = 2.0 / 3
print(average)
print(String(format: "%.2f", average))

New Delhi Junction
NEW DELHI JUNCTION
new delhi junction
true false
true
New Delhi Jn
0.6666666666666666
0.67

Strings are values: methods like uppercased() return a new string and leave the original unchanged. String(format: "%.2f", x) (from Foundation) shows a Double with exactly two decimal places, rounded, which is what you want for prices and percentages.

Characters, indexes and substrings

You cannot write pnr[0] in Swift. The reason is Unicode: one visible character can take one to several bytes in memory, so finding "character number 3" means walking through the string. Swift makes that cost visible with String.Index instead of hiding it behind an Int.

let pnr = "PNR4521"
print(pnr.prefix(3))
print(pnr.suffix(4))
print(pnr.dropFirst(3))

let fourth = pnr[pnr.index(pnr.startIndex, offsetBy: 3)]
print(fourth)

for ch in pnr.prefix(3) {
    print(ch, terminator: "-")
}
print()

let price = "₹500"
print(price.count, price.utf8.count)

PNR
4521
4521
4
P-N-R-
4 6

prefix, suffix and dropFirst cover most everyday needs without indexes. When you do need a position, index(_:offsetBy:) moves from startIndex. A for loop visits each Character; here terminator: keeps the three letters on one line, and the plain print() after the loop ends that line. The last line shows the Unicode point: "₹500" has 4 characters but takes 6 bytes in UTF-8, because ₹ alone needs 3 bytes.

The results of prefix, suffix and split are of type Substring: a view into the original string that avoids copying. Convert one with String(...) when you want to keep it for long or pass it to a function that needs a String.

let csvLine = "Aarav,Priya,Kabir,Meera"
let names = csvLine.split(separator: ",")
print(names.count)
print(names[1])

let firstName = String(names[0])
print(firstName.count)
print(names.joined(separator: " | "))

4
Priya
5
Aarav | Priya | Kabir | Meera

split(separator:) breaks a string into an array of substrings, and joined(separator:) does the opposite.

Common mistakes

• Trying name[0]. Use name.first, prefix(1) or index(_:offsetBy:).
• Forgetting import Foundation for methods like trimmingCharacters(in:) or String(format:) in a plain Swift file.
• Keeping substrings for a long time. A Substring keeps the whole original string in memory; convert it with String(...).
• Using count to check for empty text. Write text.isEmpty.

Key points

• Use interpolation, "\(value)", to build text from values and expressions.
• Multi-line strings use """; raw strings use #"..."# and interpolate with \#(...).
• Escape sequences include \", \\, \n, \t and \u{20B9}.
• count, isEmpty, uppercased(), hasPrefix, contains, replacing and split cover most daily work; some helpers need Foundation.
• Strings are indexed with String.Index, not Int, because characters vary in size.
• prefix, suffix and split return Substring values; convert with String(...) to keep them.

import Foundation

let raw = "  kabir singh rathore "
let words = raw.trimmingCharacters(in: .whitespaces).split(separator: " ")
print(words.count)
print(words[0].uppercased())

var initials = ""
for word in words {
    initials += word.prefix(1).uppercased() + "."
}
print(initials)

3
KABIR
K.S.R.