Whole and Decimal Numbers

In the first lesson, you saw that 10 / 2 prints 5.0, not 5. Now you'll find out why. Lua has two kinds of numbers:

• Whole numbers, which programmers call integers: 120, 0, -3. Emma's coins, eggs and days are whole numbers.
• Decimal numbers, which programmers call floats (from floating point, the way computers store them): 2.5, 0.75, -1.5. They're written with a decimal point, never with a comma.

Lua always prints a float with a decimal point, even when it's whole: 5.0 is a float, 5 an integer. It's the same amount, but Lua keeps track of which kind each number is. The function math.type tells you:

print(math.type(5))
print(math.type(5.0))
print(math.type(2.5))

This prints:

integer
float
float

(math is Lua's collection of math functions. You'll use more of them in the next exercise.)

Whether a result is an integer or a float depends on the operator:

• +, - and * give an integer when both numbers are integers. When one of them is a float, the result is a float too: 3 * 2 is 6, but 3 * 2.0 is 6.0.
• / (division) always gives a float, even when the division works out: 10 / 2 is 5.0.
• // (floor division) divides and rounds the result down to a whole number: 7 // 2 is 3. It tells you how many times one number fits into another.
• % gives the remainder after division: 7 % 2 is 1, because 2 fits into 7 three times, and 1 is left over.
• ^ raises a number to a power, and always gives a float: 2 ^ 3 is 8.0.

print(7 / 2)
print(7 // 2)
print(7 % 2)
print(10 / 2)
print(2 ^ 3)

This prints:

3.5
3
1
5.0
8.0

// and % work well together. The strawberries will be ripe in 17 days: 17 // 7 is 2 whole weeks, and 17 % 7 is 3 days left over. The remainder also tells even numbers from odd ones: an even number divided by 2 leaves 0, an odd one leaves 1.

Typical mistakes:

• / where you want a whole number: 17 / 7 gives 2.4285714285714. Use //.
• Floor division by zero: 7 // 0 stops the program with the error attempt to divide by zero. (7 / 0 doesn't stop it, but gives inf, infinity, which isn't much use either.)
• A decimal comma: local price = 2,5 doesn't report an error, but price is just 2. In Lua, a comma separates two values. The variable gets the first one, and the 5 gets lost.

One more thing: computers store floats in binary, so many decimal numbers aren't exact. 0.1 + 0.2 prints 0.3, but the result is really a tiny bit more. That's one reason why money is best counted in whole numbers, as Emma does with her coins.