Lua Cheatsheet

53 essential methods for coding interviews

Showing 53 of 53 methods

Method	Syntax	Description	Time	Priority
`table.insert`	`table.insert(t, [pos,] value)`	Inserts value at position pos (default: end of table).	O(n) when inserting at position, O(1) at end	essential
`table.remove`	`table.remove(t [, pos])`	Removes and returns element at position pos (default: last element).	O(n) when removing from middle, O(1) from end	essential
`table.concat`	`table.concat(t [, sep [, i [, j]]])`	Concatenates table elements into a string with optional separator.	O(n)	essential
`table.sort`	`table.sort(t [, comp])`	Sorts table in place. Optional comparison function returns true if first arg should come first.	O(n log n)	essential
`#table (length)`	`#t`	Returns the length of a sequence (table with consecutive integer keys starting at 1).	O(1) for arrays, O(log n) for sparse tables	essential
`ipairs`	`for i, v in ipairs(t) do ... end`	Iterates over array part of table (consecutive integer keys from 1).	O(n)	essential
`pairs`	`for k, v in pairs(t) do ... end`	Iterates over all key-value pairs in a table.	O(n)	essential
`string.sub`	`string.sub(s, i [, j])`	Returns substring from index i to j (default: end). Negative indices count from end.	O(j - i)	essential
`string.len / #string`	`string.len(s) or #s`	Returns the length of a string in bytes.	O(1)	essential
`string.find`	`string.find(s, pattern [, init [, plain]])`	Finds first match of pattern. Returns start and end indices, plus captures.	O(n * m)	essential
`string.match`	`string.match(s, pattern [, init])`	Returns captures from pattern match, or whole match if no captures.	O(n * m)	essential
`string.gmatch`	`string.gmatch(s, pattern)`	Returns iterator over all pattern matches in string.	O(n * m) total	essential
`string.gsub`	`string.gsub(s, pattern, repl [, n])`	Returns string with pattern matches replaced. repl can be string, table, or function.	O(n * m)	essential
`math.max / math.min`	`math.max(...) / math.min(...)`	Returns maximum/minimum of arguments.	O(n)	essential
`math.floor / math.ceil`	`math.floor(x) / math.ceil(x)`	Rounds down/up to nearest integer.	O(1)	essential
`math.abs`	`math.abs(x)`	Returns absolute value of x.	O(1)	essential
`math.huge`	`math.huge`	Represents positive infinity.	O(1)	essential
`tonumber`	`tonumber(e [, base])`	Converts value to number. Returns nil if conversion fails.	O(n)	essential
`tostring`	`tostring(v)`	Converts value to string representation.	O(n)	essential
`type`	`type(v)`	Returns type of value as string.	O(1)	essential
`print`	`print(...)`	Prints values to stdout separated by tabs, with newline.	O(n)	essential
`Pattern: Deep Copy`	`function deepcopy(t) ... end`	Recursively copies table and all nested tables.	O(n) total elements	essential
`Pattern: Table Contains`	`function contains(t, value) ... end`	Checks if array-like table contains a value.	O(n)	essential
`Pattern: String Split`	`function split(s, sep) ... end`	Splits string by separator into array of substrings.	O(n)	essential
`table.unpack`	`table.unpack(t [, i [, j]])`	Returns elements from table as multiple values.	O(n)	common
`next`	`next(t [, index])`	Returns next key-value pair after index, or first pair if index is nil.	O(1)	common
`string.format`	`string.format(formatstring, ...)`	Returns formatted string (similar to C printf).	O(n)	common
`string.byte`	`string.byte(s [, i [, j]])`	Returns numeric byte values of characters s[i] through s[j].	O(j - i)	common
`string.char`	`string.char(...)`	Returns string from numeric byte values.	O(n)	common
`string.rep`	`string.rep(s, n [, sep])`	Returns string s repeated n times, with optional separator.	O(n * len(s))	common
`string.reverse`	`string.reverse(s)`	Returns reversed string.	O(n)	common
`string.upper / string.lower`	`string.upper(s) / string.lower(s)`	Returns uppercase/lowercase version of string.	O(n)	common
`math.sqrt`	`math.sqrt(x)`	Returns square root of x.	O(1)	common
`math.random`	`math.random([m [, n]])`	Returns random number. No args: [0,1). One arg: [1,m]. Two args: [m,n].	O(1)	common
`math.randomseed`	`math.randomseed(x)`	Sets seed for random number generator.	O(1)	common
`math.pow / ^ operator`	`math.pow(x, y) or x ^ y`	Returns x raised to power y.	O(1)	common
`setmetatable / getmetatable`	`setmetatable(t, mt) / getmetatable(t)`	Sets/gets metatable for table. Metatables control table behavior.	O(1)	common
`select`	`select(index, ...) or select("#", ...)`	Returns arguments after index, or count of arguments with "#".	O(n)	common
`pcall`	`pcall(f, arg1, ...)`	Calls function in protected mode. Returns success boolean and results/error.	O(1) + function time	common
`assert`	`assert(v [, message])`	Raises error if v is false or nil, otherwise returns v.	O(1)	common
`error`	`error(message [, level])`	Raises an error with given message. Level affects error position in traceback.	O(1)	common
`io.read`	`io.read([format1, ...])`	Reads from stdin. "l" for line, "n" for number, "*a" for all.	O(n)	common
`Pattern: Table Keys`	`function keys(t) ... end`	Returns array of all keys in a table.	O(n)	common
`Pattern: Table Values`	`function values(t) ... end`	Returns array of all values in a table.	O(n)	common
`table.move`	`table.move(a1, f, e, t [, a2])`	Moves elements from a1[f..e] to a2[t..], returns destination table.	O(n)	useful
`math.fmod`	`math.fmod(x, y)`	Returns remainder of x/y (same sign as x).	O(1)	useful
`math.modf`	`math.modf(x)`	Returns integral and fractional parts of x.	O(1)	useful
`math.log`	`math.log(x [, base])`	Returns logarithm of x. Default base is e (natural log).	O(1)	useful
`rawget / rawset`	`rawget(t, k) / rawset(t, k, v)`	Gets/sets table value bypassing metamethods.	O(1)	useful
`collectgarbage`	`collectgarbage([opt [, arg]])`	Controls garbage collector. "collect" runs full GC cycle.	O(n) heap size	useful
`load / loadstring`	`load(chunk [, chunkname [, mode [, env]]])`	Compiles string/function as Lua chunk, returns function or nil+error.	O(n) chunk size	useful
`os.time / os.date`	`os.time([table]) / os.date([format [, time]])`	os.time returns Unix timestamp. os.date formats time.	O(1)	useful
`os.clock`	`os.clock()`	Returns CPU time used by program in seconds.	O(1)	useful

Method

Syntax

Description

Time

Priority

table.insert

table.insert(t, [pos,] value)

Inserts value at position pos (default: end of table).

O(n) when inserting at position, O(1) at end

essential

table.remove

table.remove(t [, pos])

Removes and returns element at position pos (default: last element).

O(n) when removing from middle, O(1) from end

essential

table.concat

table.concat(t [, sep [, i [, j]]])

Concatenates table elements into a string with optional separator.

O(n)

essential

table.sort

table.sort(t [, comp])

Sorts table in place. Optional comparison function returns true if first arg should come first.

O(n log n)

essential

#table (length)

#t

Returns the length of a sequence (table with consecutive integer keys starting at 1).

O(1) for arrays, O(log n) for sparse tables

essential

ipairs

for i, v in ipairs(t) do ... end

Iterates over array part of table (consecutive integer keys from 1).

O(n)

essential

pairs

for k, v in pairs(t) do ... end

Iterates over all key-value pairs in a table.

O(n)

essential

string.sub

string.sub(s, i [, j])

Returns substring from index i to j (default: end). Negative indices count from end.

O(j - i)

essential

string.len / #string

string.len(s) or #s

Returns the length of a string in bytes.

O(1)

essential

string.find

string.find(s, pattern [, init [, plain]])

Finds first match of pattern. Returns start and end indices, plus captures.

O(n * m)

essential

string.match

string.match(s, pattern [, init])

Returns captures from pattern match, or whole match if no captures.

O(n * m)

essential

string.gmatch

string.gmatch(s, pattern)

Returns iterator over all pattern matches in string.

O(n * m) total

essential

string.gsub

string.gsub(s, pattern, repl [, n])

Returns string with pattern matches replaced. repl can be string, table, or function.

O(n * m)

essential

math.max / math.min

math.max(...) / math.min(...)

Returns maximum/minimum of arguments.

O(n)

essential

math.floor / math.ceil

math.floor(x) / math.ceil(x)

Rounds down/up to nearest integer.

O(1)

essential

math.abs

math.abs(x)

Returns absolute value of x.

O(1)

essential

math.huge

Represents positive infinity.

O(1)

essential

tonumber

tonumber(e [, base])

Converts value to number. Returns nil if conversion fails.

O(n)

essential

tostring

tostring(v)

Converts value to string representation.

O(n)

essential

type

type(v)

Returns type of value as string.

O(1)

essential

print

print(...)

Prints values to stdout separated by tabs, with newline.

O(n)

essential

Pattern: Deep Copy

function deepcopy(t) ... end

Recursively copies table and all nested tables.

O(n) total elements

essential

Pattern: Table Contains

function contains(t, value) ... end

Checks if array-like table contains a value.

O(n)

essential

Pattern: String Split

function split(s, sep) ... end

Splits string by separator into array of substrings.

O(n)

essential

table.unpack

table.unpack(t [, i [, j]])

Returns elements from table as multiple values.

O(n)

common

next

next(t [, index])

Returns next key-value pair after index, or first pair if index is nil.

O(1)

common

string.format

string.format(formatstring, ...)

Returns formatted string (similar to C printf).

O(n)

common

string.byte

string.byte(s [, i [, j]])

Returns numeric byte values of characters s[i] through s[j].

O(j - i)

common

string.char

string.char(...)

Returns string from numeric byte values.

O(n)

common

string.rep

string.rep(s, n [, sep])

Returns string s repeated n times, with optional separator.

O(n * len(s))

common

string.reverse

string.reverse(s)

Returns reversed string.

O(n)

common

string.upper / string.lower

string.upper(s) / string.lower(s)

Returns uppercase/lowercase version of string.

O(n)

common

math.sqrt

math.sqrt(x)

Returns square root of x.

O(1)

common

math.random

math.random([m [, n]])

Returns random number. No args: [0,1). One arg: [1,m]. Two args: [m,n].

O(1)

common

math.randomseed

math.randomseed(x)

Sets seed for random number generator.

O(1)

common

math.pow / ^ operator

math.pow(x, y) or x ^ y

Returns x raised to power y.

O(1)

common

setmetatable / getmetatable

setmetatable(t, mt) / getmetatable(t)

Sets/gets metatable for table. Metatables control table behavior.

O(1)

common

select

select(index, ...) or select("#", ...)

Returns arguments after index, or count of arguments with "#".

O(n)

common

pcall

pcall(f, arg1, ...)

Calls function in protected mode. Returns success boolean and results/error.

O(1) + function time

common

assert

assert(v [, message])

Raises error if v is false or nil, otherwise returns v.

O(1)

common

error

error(message [, level])

Raises an error with given message. Level affects error position in traceback.

O(1)

common

io.read

io.read([format1, ...])

Reads from stdin. "*l" for line, "*n" for number, "*a" for all.

O(n)

common

Pattern: Table Keys

function keys(t) ... end

Returns array of all keys in a table.

O(n)

common

Pattern: Table Values

function values(t) ... end

Returns array of all values in a table.

O(n)

common

table.move

table.move(a1, f, e, t [, a2])

Moves elements from a1[f..e] to a2[t..], returns destination table.

O(n)

useful

math.fmod

math.fmod(x, y)

Returns remainder of x/y (same sign as x).

O(1)

useful

math.modf

math.modf(x)

Returns integral and fractional parts of x.

O(1)

useful

math.log

math.log(x [, base])

Returns logarithm of x. Default base is e (natural log).

O(1)

useful

rawget / rawset

rawget(t, k) / rawset(t, k, v)

Gets/sets table value bypassing metamethods.

O(1)

useful

collectgarbage

collectgarbage([opt [, arg]])

Controls garbage collector. "collect" runs full GC cycle.

O(n) heap size

useful

load / loadstring

load(chunk [, chunkname [, mode [, env]]])

Compiles string/function as Lua chunk, returns function or nil+error.

O(n) chunk size

useful

os.time / os.date

os.time([table]) / os.date([format [, time]])

os.time returns Unix timestamp. os.date formats time.

O(1)

useful

os.clock

os.clock()

Returns CPU time used by program in seconds.

O(1)

useful