Integers, Floats and Doubles

Sprout counts streak days and measures minutes spent. "4 days" and "12.5 minutes" look alike on paper, but they are different kinds of number, and choosing the wrong one gives you either a compile error or a quietly wrong answer.

The rule

Counted things are Int; measured things are Double.

Int holds whole numbers, positive or negative: days in a streak, habits in a list, check-ins this week. On today's 64-bit devices it reaches past nine quintillion either way, so you rarely think about its size.

Swift also has sized and unsigned integers such as Int8 (-128 to 127) and UInt8 (0 to 255), used mostly when talking to files or hardware. For everyday code, use Int.

Double holds numbers with a fractional part: minutes, distances, percentages. It keeps about 15 significant digits. Float is a smaller cousin with about 6 digits; use it only when an API asks for it. A literal with a decimal point is inferred as Double.

Two behaviours to know:

• Integer division drops the fraction. 7 / 2 is 3, not 3.5. Divide Doubles to keep it: 7.0 / 2.0 is 3.5.
• Overflow stops the program. If an Int8 holding 127 has 1 added at runtime, Swift crashes on purpose rather than wrapping to -128. A wrong number is worse than a stop.

Floating-point values are close approximations: 0.1 + 0.2 == 0.3 is false, because neither side is stored exactly.

The shortcut

Ask "could this ever be half?" Half a streak day makes no sense, so Int. Half a minute does, so Double. Then glance at every /: if both sides are Int, expect the fraction to vanish.

Drills

Answer fast — trust the rule.

Remember

• Counted things are Int; measured things are Double.
• Ask "could this ever be half?", then check every / between two Ints.
• Overflow stops the program; decimals are approximate.