Formatting and Columns

You know string.format from the lesson on variables: it puts values into a text according to its format specifiers, %d for a whole number, %.2f for a decimal number with two decimal places, and %s for a text. format is a string function like the others, so you can also call it as a method on the text with the specifiers:

local name = "Bella"
local liters = 18.5
print(string.format("%s gave %.1f liters of milk", name, liters))
print(("%s gave %.1f liters of milk"):format(name, liters))

This prints:

Bella gave 18.5 liters of milk
Bella gave 18.5 liters of milk

Both lines do the same; the method needs the parentheses around the text, as you know. %s takes any value, not just a text: ("%s"):format(true) is "true". And because % starts a specifier, a percent sign is written as %%: ("%d%%"):format(75) is "75%".

A number between the % and the letter sets the width: how many characters the value takes at least. A shorter value is filled up with spaces on the left, so it's aligned to the right. With a minus, the spaces go on the right, and the value is aligned to the left. The brackets in this example show where each value starts and ends:

print(("[%5d]"):format(42))
print(("[%-5d]"):format(42))
print(("[%8s]"):format("hen"))
print(("[%-8s]"):format("hen"))
print(("[%7.2f]"):format(3.14159))

This prints:

[   42]
[42   ]
[     hen]
[hen     ]
[   3.14]

In %7.2f, 7 is the whole width, decimal point included, and 2 is the number of decimal places. Widths are what you need for a table that lines up. Without them, each line is only as long as its values, and the numbers end up all over the place:

print(("%s %d"):format("eggs", 14))
print(("%s %d"):format("milk", 9))
print(("%s %d"):format("strawberries", 120))

This prints:

eggs 14
milk 9
strawberries 120

With widths, the columns line up:

print(("%-12s %4d"):format("eggs", 14))
print(("%-12s %4d"):format("milk", 9))
print(("%-12s %4d"):format("strawberries", 120))

This prints:

eggs           14
milk            9
strawberries  120

Names go to the left, so %-12s; numbers go to the right, so that the ones and the tens are under each other, %4d. Choose the width by the longest value in the column: a longer value isn't cut off, it just breaks the alignment.

A typical mistake is a specifier without a value:

print(("%s and %s"):format("eggs"))

main.lua:1: bad argument #2 to 'format' (no value)

The message says that one of the specifiers got no value. Count the specifiers and the values: there must be as many values as specifiers (%% doesn't count, it needs no value). With the colon, Lua doesn't count the text before it as an argument, so the message says #2; string.format("%s and %s", "eggs") says #3, as in the lesson on variables.

One more thing about widths: they count bytes, and a letter with an accent takes two. A line with purée in it would be shifted by one place. That's why the names in this exercise have no accents; the next exercise shows how to deal with them.