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Swift
String Conversion
Integer Parsing
Swift Programming
iOS Development

Get integer value from string in swift

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Introduction

Swift converts strings to integers using the failable Int() initializer, which returns an optional Int?. If the string contains a valid integer representation, you get the value; otherwise, you get nil. This design forces you to handle invalid input explicitly — there is no silent conversion to zero or crash on failure. For different number bases (hex, octal, binary), use Int("FF", radix: 16). For strings with whitespace, currency symbols, or locale-specific formatting, use NumberFormatter.

Basic Conversion with Int()

swift
1// Simple conversion — returns Int?
2let number = Int("42")        // Optional(42)
3let invalid = Int("hello")    // nil
4let decimal = Int("3.14")     // nil — Int does not parse decimals
5let empty = Int("")            // nil
6let negative = Int("-7")       // Optional(-7)
7let whitespace = Int(" 42 ")  // nil — no whitespace trimming
8
9// Unwrap with if-let
10if let value = Int("42") {
11    print("Parsed: \(value)")  // "Parsed: 42"
12} else {
13    print("Invalid number")
14}
15
16// Unwrap with guard
17func processInput(_ input: String) -> Int {
18    guard let value = Int(input) else {
19        fatalError("Expected a valid integer, got: \(input)")
20    }
21    return value
22}

Int() is strict — it requires the entire string to be a valid integer with no extra characters, whitespace, or decimal points.

Default Values with nil-Coalescing

swift
1// Provide a default when parsing fails
2let count = Int("abc") ?? 0           // 0
3let port = Int(portString) ?? 8080    // 8080 if invalid
4let page = Int(pageParam) ?? 1        // 1 if missing or invalid
5
6// Useful in SwiftUI / UIKit text field handling
7let textField = UITextField()
8textField.text = "25"
9let age = Int(textField.text ?? "") ?? 0  // 25

The nil-coalescing operator ?? provides a fallback value when parsing fails, avoiding force-unwrapping.

Parsing Different Number Bases

swift
1// Hexadecimal
2let hex = Int("FF", radix: 16)       // Optional(255)
3let hexPrefix = Int("0xFF", radix: 16)  // nil — "0x" prefix not supported
4let hexClean = Int("FF".replacingOccurrences(of: "0x", with: ""), radix: 16)  // 255
5
6// Binary
7let binary = Int("1010", radix: 2)   // Optional(10)
8
9// Octal
10let octal = Int("77", radix: 8)      // Optional(63)
11
12// Base 36 (0-9 + a-z)
13let base36 = Int("z", radix: 36)     // Optional(35)

The radix parameter accepts values from 2 to 36. Letters are case-insensitive ("ff" and "FF" both parse as 255 in base 16).

NumberFormatter for Locale-Aware Parsing

swift
1import Foundation
2
3// Parse numbers with thousands separators, currency, etc.
4let formatter = NumberFormatter()
5formatter.numberStyle = .decimal
6formatter.locale = Locale(identifier: "en_US")
7
8let number = formatter.number(from: "1,234,567")?.intValue  // Optional(1234567)
9
10// European format (period as thousands separator)
11formatter.locale = Locale(identifier: "de_DE")
12let german = formatter.number(from: "1.234.567")?.intValue   // Optional(1234567)
13
14// Currency
15formatter.numberStyle = .currency
16formatter.locale = Locale(identifier: "en_US")
17let price = formatter.number(from: "$99")?.intValue           // Optional(99)

NumberFormatter handles locale-specific formatting that Int() cannot parse — thousands separators, decimal commas, currency symbols, and percentage signs.

Trimming Whitespace Before Parsing

swift
1let input = "  42  \n"
2
3// Int() fails with whitespace
4let direct = Int(input)  // nil
5
6// Trim first
7let trimmed = input.trimmingCharacters(in: .whitespacesAndNewlines)
8let value = Int(trimmed)  // Optional(42)
9
10// One-liner
11let result = Int(input.trimmingCharacters(in: .whitespacesAndNewlines)) ?? 0

Extracting Numbers from Mixed Strings

swift
1import Foundation
2
3let text = "Order #12345 confirmed"
4
5// Using Scanner
6let scanner = Scanner(string: text)
7scanner.charactersToBeSkipped = CharacterSet.decimalDigits.inverted
8var number: Int = 0
9scanner.scanInt(&number)
10print(number)  // 12345
11
12// Using regular expressions (Swift 5.7+)
13if let match = text.firstMatch(of: /\d+/) {
14    let extracted = Int(match.output)  // Optional(12345)
15}
16
17// Extract ALL numbers
18let allNumbers = text.matches(of: /\d+/).compactMap { Int($0.output) }
19print(allNumbers)  // [12345]

Converting Between Integer Types

swift
1let intValue = Int("42")!           // Int (platform-dependent: 32 or 64 bit)
2let int8 = Int8("127")              // Optional(127) — max is 127
3let int8Overflow = Int8("128")      // nil — exceeds Int8.max
4let uint = UInt("42")               // Optional(42)
5let uintNeg = UInt("-1")            // nil — unsigned cannot be negative
6let int64 = Int64("9223372036854775807")  // Optional(Int64.max)
7
8// Convert between types
9if let intVal = Int("1000"), let uint8Val = UInt8(exactly: intVal) {
10    print(uint8Val)  // Only succeeds if value fits in UInt8
11} else {
12    print("Value out of range for UInt8")
13}

Each integer type (Int8, Int16, Int32, Int64, UInt, etc.) has its own failable init(_ description: String). Parsing fails if the value exceeds the type's range.

Common Pitfalls

  • Force-unwrapping Int() without validation: Int("abc")! crashes at runtime. Always use if let, guard let, or ?? to handle the nil case. User input and API responses are never guaranteed to be valid integers.
  • Expecting Int() to trim whitespace: Int(" 42") returns nil, not 42. Call .trimmingCharacters(in: .whitespacesAndNewlines) on the string before parsing.
  • Confusing Int() with Double() for decimal strings: Int("3.14") returns nil. If you need to truncate a decimal string to an integer, parse as Double first and then convert: Int(Double("3.14")!).
  • Using atoi or NSString.intValue from Objective-C: NSString.intValue returns 0 for invalid input instead of nil, silently hiding errors. atoi has the same problem. Always use Swift's failable Int() initializer for safe parsing.
  • Ignoring overflow for specific integer types: Int8("200") returns nil because 200 exceeds Int8.max (127). When parsing into narrow types, check the range or use Int first and then validate with Int8(exactly:).

Summary

  • Use Int("string") for basic string-to-integer conversion — returns Int? (optional)
  • Use ?? for default values: Int(input) ?? 0
  • Use Int("FF", radix: 16) for hex, binary, and other bases
  • Use NumberFormatter for locale-aware parsing (thousands separators, currency)
  • Trim whitespace before parsing: Int(str.trimmingCharacters(in: .whitespacesAndNewlines))
  • Each integer type (Int8, UInt, Int64) has its own failable initializer with range checking
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