Whole Numbers and Decimals

Asha ran 12 laps of the 400-metre track. Is that 4 kilometres or 4.8? In Scala, both answers are one character apart, and the difference comes entirely from which number types are involved.

The rule

Int for counting, Double for measuring; whole divided by whole stays whole.

Scala has six number types. Four of them cover almost everything:

Byte (8 bits) and Short (16 bits) also exist, mostly for files and hardware. A whole-number literal is an Int unless you add L; a decimal literal is a Double unless you add f. Underscores can group digits, so 1_530 is the same as 1530.

When both sides of / are whole numbers, the result is whole and the remainder is dropped. If either side is a Double, the result is a Double.

Two more edges to know:

• Overflow wraps silently. Int.MaxValue + 1 is -2147483648, with no error. Use Long for counts that might pass two billion. Writing 3_000_000_000 without the L is rejected as "number too large".
• Decimals are approximate. 12 * 0.4 gives 4.800000000000001, because 0.4 has no exact binary form. That is fine for distances; for money, use BigDecimal.

The shortcut

Before reading any arithmetic, tag each number I or D. Whole numbers and Int names get I; decimals get D. Any operation with a D makes a D. I with I stays I, and I / I drops the remainder.

The trap is a total that looks plausible but has been quietly rounded down, like the 4 above. The tags catch it before the club sees the wrong number.

Drills

Answer fast — trust the rule.

Remember

• Int and Long count, Double and Float measure; plain literals are Int or Double.
• Tag each number I or D: any D makes a D, and I / I drops the remainder.
• Traps: silent overflow past about 2.1 billion, and decimals that are only approximate.