Numbers

Swift's main number types are Int (whole numbers) and Double (decimals). Int is 64 bits on modern devices, holding values up to about 9.2 quintillion.

Example

print(7 + 2)
print(7 - 2)
print(7 * 2)
print(7 / 2)
print(7 % 2)
print(7.0 / 2)
print(10.0 / 3)
print(1_000_000 + 1)

Expected output

9
5
14
3
1
3.5
3.3333333333333335
1000001

Line by line:

• 7 / 2 is 3. Dividing two Ints gives an Int, dropping the remainder (integer division).
• % gives the remainder: 7 % 2 is 1 — handy for checking whether a number is even.
• Dividing Doubles gives a Double.
• Doubles are approximate: 10.0 / 3 shows the tiny rounding error built into how computers store decimals.
• Underscores in number literals are just for readability.

Mixing Int and Double

Swift won't mix an Int variable with a Double automatically. Convert one side explicitly:

let items = 7
let people = 2
let share = Double(items) / Double(people)
print(share)

3.5

Useful number features

print(abs(-5))
print(max(3, 9))
print(min(3, 9))
print(10.isMultiple(of: 5))
print((3.7).rounded())
print(Int(3.99))
print(Int.random(in: 1...6) <= 6)

5
9
3
true
4.0
3
true

Int(3.99) truncates (cuts off the decimals) rather than rounding. Int.random(in: 1...6) picks a random number from 1 to 6 — perfect for dice.

Swift has no ** operator for powers; use pow(2.0, 10.0) after import Foundation.

Common mistakes

• Integer overflow. Going past Int.max doesn't wrap around silently — Swift stops the program with an overflow error. That's a safety feature.
• Using Double for money. 0.1 + 0.2 is 0.30000000000000004. Store whole cents as Int, or use Foundation's Decimal type.

Practice

A pizza has 8 slices shared by 3 people. Print how many whole slices each person gets, how many are left over, and the exact share as a decimal.

let slices = 8
let people = 3
print(slices / people)
print(slices % people)
print(Double(slices) / Double(people))

2
2
2.6666666666666665