Lua Cheatsheet
Pitfalls to Avoid When Learning Lua
- Array indices start from 1 (unlike C, JavaScript, and other languages where they start from 0)
string.lendoes not return the number of characters in a string but the number of bytes- All uninitialized variables are
nil, and assigningnilto a value in a table removes it from the table - Only
nilandfalseare logically false; all other values, including0, are logically true - Strings and numbers are automatically converted when performing mathematical operations or comparisons, e.g.,
a = '1' + 2 - Two floating-point numbers (numbers with decimals) cannot be compared using the equality operator
==, e.g.,if 89.7 == (3 * 29.9) thenis incorrect - A floating-point number that can be fully represented as an integer is equal to its corresponding integer, e.g.,
1.0 == 1
Common Runtime Errors
| Error Message Fragment | Cause | Solution |
|---|---|---|
attempt to perform arithmetic on a | Attempted to perform arithmetic (+, -, *, /) on a non-numeric value | Check if all inputs for arithmetic operations are numbers |
attempt to compare | Attempted to perform comparison (>, <, >=, <=) on invalid values | Check if both inputs for comparison operations are valid for comparison |
attempt to concatenate a | Attempted to concatenate (..) a non-string value | Ensure both values being concatenated are strings |
attempt to call a | Attempted to call a variable that is not a function | Ensure the variable being called is a function |
attempt to index a | Attempted to index (subscript operation) a non-table variable | Ensure the variable being accessed as an array or associative array is a table |
attempt to yield across a C-call boundary | Attempted to yield in a non-yieldable call block | When using require, ensure the required module does not call yieldable functions (marked with an exclamation mark in the manual) before returning. Also, avoid using yieldable functions in functions with C callbacks (marked with an exclamation mark in the manual). |
invalid order function for sorting | Invalid sorting function, usually due to unclear sorting logic | Define clear sorting rules when calling the sorting function to avoid contradictory rules (e.g., a > b and a < b both being true) |
bad argument #1 to 'xxx' (number expected, got nil) | The first argument to a function is of the wrong type, expecting a number but got nil | Fix the argument and pass the correct value |
bad argument #2 to 'xxx' (number has no integer representation) | The second argument to a function cannot be converted to an integer | Fix the argument and pass the correct value |
bad argument #3 to 'xxx' | The third argument to a function is invalid | Fix the argument and pass the correct value |
Usage of os.date
| Format | Example | Description | Result |
|---|---|---|---|
%Y-%m-%d %H:%M:%S | os.date("%Y-%m-%d %H:%M:%S", 1487356783) | A common date-time format | 2017-02-18 02:39:43 |
%Y-%m-%d | os.date("%Y-%m-%d", 1487356783) | A common date format | 2017-02-18 |
%a | os.date("%a", 1487356783) | Abbreviated weekday name | Sat |
%A | os.date("%A", 1487356783) | Full weekday name | Saturday |
%b | os.date("%b", 1487356783) | Abbreviated month name | Feb |
%B | os.date("%B", 1487356783) | Full month name | February |
%c | os.date("%c", 1487356783) | Standard date-time string | Sat Feb 18 02:39:43 2017 |
%d | os.date("%d", 1487356783) | Day of the month, range 01 - 31 | 18 |
%H | os.date("%H", 1487356783) | Hour in 24-hour format, range 00 - 23 | 02 |
%I | os.date("%I", 1487356783) | Hour in 12-hour format, range 01 - 12 | 02 |
%j | os.date("%j", 1487356783) | Day of the year, range 001 - 366 | 049 |
%M | os.date("%M", 1487356783) | Minute, range 00 - 59 | 39 |
%m | os.date("%m", 1487356783) | Month, range 01 - 12 | 02 |
%p | os.date("%p", 1487356783) | AM or PM | AM |
%S | os.date("%S", 1487356783) | Second, may include leap second, range 00 - 61 | 43 |
%w | os.date("%w", 1487356783) | Day of the week (Sunday is 0), range 0 - 6 | 6 |
%x | os.date("%x", 1487356783) | Standard date string | 02/18/17 |
%X | os.date("%X", 1487356783) | Standard time string | 02:39:43 |
%y | os.date("%y", 1487356783) | Year without century | 17 |
%Y | os.date("%Y", 1487356783) | Year with century | 2017 |
%% | os.date("%%", 1487356783) | Percent sign | % |
Usage of the string Library
Basic Functions
| Function | Description | Example | Result |
|---|---|---|---|
string.len | Calculate the length of a string | string.len("abcd") | 4 |
string.rep | Return n copies of string s | string.rep("abcd", 2) | abcdabcd |
string.lower | Return the string in all lowercase | string.lower("AbcD") | abcd |
string.upper | Return the string in all uppercase | string.upper("AbcD") | ABCD |
string.format | Format a string | string.format("the value is: %d", 4) | the value is: 4 |
string.sub | Extract a substring from a string | string.sub("abcd", 2) | bcd |
string.sub("abcd", -2) | cd | ||
string.sub("abcd", 2, -2) | bc | ||
string.sub("abcd", 2, 3) | bc | ||
string.find | Find a substring in a string (returns position) | string.find("cdcdcdcd", "ab") | nil |
string.find("cdcdcdcd", "cd") | 1 2 | ||
string.find("cdcdcdcd", "cd", 7) | 7 8 | ||
string.match | Find a substring in a string (returns content) | string.match("cdcdcdcd", "ab") | nil |
string.match("cdcdcdcd", "cd") | cd | ||
string.gsub | Replace a substring in a string | string.gsub("abcdabcd", "a", "z") | zbcdzbcd 2 |
string.gsub("aaaa", "a", "z", 3) | zzza 3 | ||
string.byte | Return the integer representation of a character | string.byte("ABCD", 4) | 68 |
string.char | Convert integer numbers to characters and concatenate them | string.char(97, 98, 99, 100) | abcd |
Basic Patterns
| Character Class | Description | Example | Result |
|---|---|---|---|
. | Any character | string.find("", ".") | nil |
%s | Whitespace character | string.find("ab cd", "%s%s") | 3 4 |
%S | Non-whitespace character | string.find("ab cd", "%S%S") | 1 2 |
%p | Punctuation character | string.find("ab,.cd", "%p%p") | 3 4 |
%P | Non-punctuation character | string.find("ab,.cd", "%P%P") | 1 2 |
%c | Control character | string.find("abcd\t\n", "%c%c") | 5 6 |
%C | Non-control character | string.find("\t\nabcd", "%C%C") | 3 4 |
%d | Digit | string.find("abcd12", "%d%d") | 5 6 |
%D | Non-digit | string.find("12abcd", "%D%D") | 3 4 |
%x | Hexadecimal digit | string.find("efgh", "%x%x") | 1 2 |
%X | Non-hexadecimal digit | string.find("efgh", "%X%X") | 3 4 |
%a | Letter | string.find("AB12", "%a%a") | 1 2 |
%A | Non-letter | string.find("AB12", "%A%A") | 3 4 |
%l | Lowercase letter | string.find("ABab", "%l%l") | 3 4 |
%L | Uppercase letter | string.find("ABab", "%L%L") | 1 2 |
%u | Uppercase letter | string.find("ABab", "%u%u") | 1 2 |
%U | Non-uppercase letter | string.find("ABab", "%U%U") | 3 4 |
%w | Alphanumeric character | string.find("a1()", "%w%w") | 1 2 |
%W | Non-alphanumeric character | string.find("a1()", "%W%W") | 3 4 |
Escape Character %
| Character Class | Description | Example | Result |
|---|---|---|---|
% | Escape character | string.find("abc%..", "%%") | 4 4 |
string.find("abc..d", "%.%.") | 4 5 |
Using [] to Represent Character Sets, - for Ranges, and ^ for Complement of Character Sets
| Character Class | Description | Example | Result |
|---|---|---|---|
[01] | Match binary digits | string.find("32123", "[01]") | 3 3 |
[AB][CD] | Match AC, AD, BC, BD | string.find("ABCDEF", "[AB][CD]") | 2 3 |
[[]] | Match a pair of square brackets [] | string.find("ABC[]D", "[[]]") | 4 5 |
[1-3] | Match digits 1, 2, 3 | string.find("312", "[1-3][1-3][1-3]") | 1 3 |
[b-d] | Match letters b, c, d | string.find("dbc", "[b-d][b-d][b-d]") | 1 3 |
[^%s] | Match any non-whitespace character | string.find(" a ", "[^%s]") | 3 3 |
[^%d] | Match any non-digit character | string.find("123a", "[^%d]") | 4 4 |
[^%a] | Match any non-letter character | string.find("abc1", "[^%a]") | 4 4 |
Using () for Capturing
| Character Class | Description | Example | Result |
|---|---|---|---|
() | Capture string | string.find("12ab", "(%a%a)") | 3 4 ab |
string.find("ab12", "(%d%d)") | 3 4 12 |
Pattern Modifiers
| Modifier | Description | Example | Result |
|---|---|---|---|
+ | Matches 1 or more occurrences, matches the most | string.find("aaabbb", "(a+b)") | 1 4 aaab |
string.find("cccbbb", "(a+b)") | nil | ||
- | Matches 0 or more occurrences, matches the least | string.find("zzxyyy", "(xy-)") | 3 3 x |
string.find("zzzyyy", "(x-y)") | 4 4 y | ||
* | Matches 0 or more occurrences, matches the most | string.find("mmmnnn", "(m*n)") | 1 4 mmmb |
string.find("lllnnn", "(m*n)") | 4 4 n | ||
? | Matches 0 or 1 occurrence | string.find("aaabbb", "(a?b)") | 3 4 ab |
string.find("cccbbb", "(a?b)") | 4 4 b |
Common Usage of string.match
| Description | Example | Result |
|---|---|---|
| Match Chinese characters | string.match("男女abc123", "([^%w%p]+)") | 男女 |
| Match English letters | string.match("男女abc123", "(%a+)") | abc |
| Match digits | string.match("男女abc123", "(%d+)") | 123 |
| Match English letters and digits | string.match("男女abc123", "(%w+)") | abc123 |
Usage of the math Library
| Function Name | Description | Example | Result |
|---|---|---|---|
math.pi | Pi | math.pi | 3.1415926535898 |
math.abs | Absolute value | math.abs(-2012) | 2012 |
math.ceil | Ceiling | math.ceil(9.1) | 10 |
math.floor | Floor | math.floor(9.9) | 9 |
math.max | Maximum value | math.max(2, 4, 6, 8) | 8 |
math.min | Minimum value | math.min(2, 4, 6, 8) | 2 |
math.sqrt | Square root | math.sqrt(65536) | 256.0 |
math.modf | Integer and fractional parts | math.modf(20.12) | 20 0.12 |
math.randomseed | Set random seed | math.randomseed(os.time()) | |
math.random | Random number | math.random(5, 90) | |
math.rad | Degrees to radians | math.rad(180) | 3.1415926535898 |
math.deg | Radians to degrees | math.deg(math.pi) | 180.0 |
math.exp | Exponential function | math.exp(4) | 54.598150033144 |
math.log | Natural logarithm | math.log(54.598150033144) | 4.0 |
math.sin | Sine | math.sin(math.rad(30)) | 0.5 |
math.cos | Cosine | math.cos(math.rad(60)) | 0.5 |
math.tan | Tangent | math.tan(math.rad(45)) | 1.0 |
math.asin | Arcsine | math.deg(math.asin(0.5)) | 30.0 |
math.acos | Arccosine | math.deg(math.acos(0.5)) | 60.0 |
math.atan | Arctangent | math.deg(math.atan(1)) | 45.0 |