Danny Guo | 郭亚东

How to Concatenate Strings in Lua

  ·  458 words  ·  ~3 minutes to read

The most straightforward way to concatenate (or combine) strings in Lua is to use the dedicated string concatenation operator, which is two periods (..).

message = "Hello, " .. "world!"
-- message equals "Hello, World!"

Numbers are coerced to strings. For fine-grained control over number formatting, use string.format, which behaves mostly like C’s printf.

count = 42
message = "The count is: " .. count
-- message equals "The count is: 42"

Trying to concatenate other types, like nil or a table, will result in an error.

count = nil
message = "The count is: " .. count
-- results in an "attempt to concatenate a nil value" error

Note that Lua doesn’t have syntactic sugar for augmented assignment. The following is invalid syntax.

message = "Hello, "
message ..= "world!"
-- results in a "syntax error near '..'" error

Strings in Lua are immutable, so the concatenation result (message in this example) is a brand new string.

start = "Hello, "
message = start .. "world!"
start = "Bye, "
-- message still equals "Hello, World!"

table.concat

If you need to perform many concatenation operations, using the concatenation operator can be slow because Lua has to keep reallocating memory to create new strings.

message = ""
for i=1,100000 do
  message = message .. i
end

As a result, it can be much faster to use table.concat.

numbers = {}
for i=1,100000 do
  numbers[i] = i
end
message = table.concat(numbers)

Here’s a benchmark comparsion (using hyperfine) from running the .. example as slow.lua and running the table.concat example as fast.lua.

hyperfine 'lua slow.lua'
# Benchmark #1: lua slow.lua
#   Time (mean ± σ):      1.287 s ±  0.115 s    [User: 1.120 s, System: 0.078 s]
#   Range (min … max):    1.187 s …  1.528 s    10 runs
hyperfine 'lua fast.lua'
# Benchmark #1: lua fast.lua
#   Time (mean ± σ):      39.3 ms ±   3.8 ms    [User: 34.6 ms, System: 2.8 ms]
#   Range (min … max):    35.3 ms …  58.3 ms    48 runs

The difference probably doesn’t matter in most cases, but it’s a good optimization to be aware of.

table.concat can also be easier to use because it can take a separator argument to add between elements.

message = table.concat({1, 2, 3, 4, 5})
-- message equals "12345"
message = table.concat({1, 2, 3, 4, 5}, ", ")
-- message equals "1, 2, 3, 4, 5"

It can also take start and end indexes. Keep in mind that Lua arrays start with index 1.

message = table.concat({1, 2, 3, 4, 5}, ", ", 2, 4)
-- message equals "2, 3, 4"

Direct Approach

Depending on your use case, you might be able to save some memory usage over table.concat by generating the result directly.

for i=1,100000 do
  io.stdout:write(i)
end

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