Lua string to int

Question

How can I convert a string to an integer in Lua?

I have a string like this:

a = "10"

I would like it to be converted to 10, the number.

The precise link is the section on coercion: [5.1](http://www.lua.org/manual/5.1/manual.html#2.2.1), [5.2](http://www.lua.org/manual/5.2/manual.html#3.4.2). — lhf, Jun 09 '12 at 18:53
Lua just does automatically conversion between strings and numbers. If you want ensure the type, use a = tonumber(a). — xpol, Feb 26 '16 at 03:52

Nicol Bolas · Accepted Answer · 2016-12-16T15:12:46.500

384

Use the tonumber function. As in a = tonumber("10").

edited Dec 16 '16 at 15:12

answered Jun 09 '12 at 15:14

Nicol Bolas

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In Lua 5.3, (64-bit default) integers are treated accordingly (http://www.lua.org/manual/5.3/manual.html): "A numeric constant with a fractional dot or an exponent denotes a float; otherwise it denotes an integer." – Kevin Lee Mar 19 '15 at 15:10

score 42 · Answer 2 · answered Jun 09 '12 at 18:52

42

You can force an implicit conversion by using a string in an arithmetic operations as in a= "10" + 0, but this is not quite as clear or as clean as using tonumber explicitly.

answered Jun 09 '12 at 18:52

lhf

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Nope, it'll convert "10" to integer and then add 0 to it. (The lack of clarity is all the more reason to use `tonumber` instead, though!) – Rena Jun 28 '15 at 06:32
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@Rena, there's no lack in clarity. `+` is always explicitly addition, `..` - concatenation. – Oleg V. Volkov Feb 20 '16 at 12:36
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@lhf: auto coercion will only work on numbers. And comparison operators (== ~= < > <= >=) do not convert their arguments. And for performance reasons you should avoid relying on automatic coercion too much – wsha Sep 20 '18 at 07:36

Stormswept · Answer 3 · 2015-10-23T15:09:53.690

All numbers in Lua are floats (edit: Lua 5.2 or less). If you truly want to convert to an "int" (or at least replicate this behavior), you can do this:

local function ToInteger(number)
    return math.floor(tonumber(number) or error("Could not cast '" .. tostring(number) .. "' to number.'"))
end

In which case you explicitly convert the string (or really, whatever it is) into a number, and then truncate the number like an (int) cast would do in Java.

Edit: This still works in Lua 5.3, even thought Lua 5.3 has real integers, as math.floor() returns an integer, whereas an operator such as number // 1 will still return a float if number is a float.

score 11 · Answer 4 · answered Mar 26 '15 at 16:34

local a = "10"
print(type(a))
local num = tonumber(a)
print(type(num))

Output

   string                                                                                                                                                                          
   number

Shubham · Answer 5 · 2019-07-03T10:20:18.083

tonumber (e [, base])

tonumber takes two arguments, first is string which is converted to number and second is base of e.

Return value tonumber is in base 10.

If no base is provided it converts number to base 10.

> a = '101'
> tonumber(a)
101

If base is provided, it converts it to the given base.

> a = '101'
> 
> tonumber(a, 2)
5
> tonumber(a, 8)
65
> tonumber(a, 10)
101
> tonumber(a, 16)
257
>

If e contains invalid character then it returns nil.

> --[[ Failed because base 2 numbers consist (0 and 1) --]]
> a = '112'
> tonumber(a, 2)
nil
> 
> --[[ similar to above one, this failed because --]]
> --[[ base 8 consist (0 - 7) --]]
> --[[ base 10 consist (0 - 9) --]]
> a = 'AB'
> tonumber(a, 8)
nil
> tonumber(a, 10)
nil
> tonumber(a, 16)
171

I answered considering Lua5.3

score 5 · Answer 6 · edited Mar 12 '15 at 16:56

5

say the string you want to turn into a number is in the variable S

a=tonumber(S)

provided that there are numbers and only numbers in S it will return a number, but if there are any characters that are not numbers (except periods for floats) it will return nil

edited Mar 12 '15 at 16:56

Aleksandr M

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answered Mar 12 '15 at 16:36

CORE craftX

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score 5 · Answer 7 · edited Jun 19 '18 at 13:46

The clearer option is to use tonumber.

As of 5.3.2, this function will automatically detect (signed) integers, float (if a point is present) and hexadecimal (both integers and floats, if the string starts by "0x" or "0X").

The following snippets are shorter but not equivalent :

a + 0 -- forces the conversion into float, due to how + works.

a | 0 -- (| is the bitwise or) forces the conversion into integer. 
-- However, unlike `math.tointeger`, it errors if it fails.

score 5 · Answer 8 · answered Jul 17 '18 at 18:05

It should be noted that math.floor() always rounds down, and therefore does not yield a sensible result for negative floating point values.

For example, -10.4 represented as an integer would usually be either truncated or rounded to -10. Yet the result of math.floor() is not the same:

math.floor(-10.4) => -11

For truncation with type conversion, the following helper function will work:

function tointeger( x )
    num = tonumber( x )
    return num < 0 and math.ceil( num ) or math.floor( num )
end

Reference: http://lua.2524044.n2.nabble.com/5-3-Converting-a-floating-point-number-to-integer-td7664081.html

score 3 · Answer 9 · edited Sep 10 '15 at 03:59

3

I would recomend to check Hyperpolyglot, has an awesome comparison: http://hyperpolyglot.org/

http://hyperpolyglot.org/more#str-to-num-note

ps. Actually Lua converts into doubles not into ints.

The number type represents real (double-precision floating-point) numbers.

http://www.lua.org/pil/2.3.html

edited Sep 10 '15 at 03:59

Julian

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answered Nov 28 '14 at 16:19

Marcs

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score 2 · Answer 10 · answered Jul 12 '15 at 09:19

2

You can make an accessor to keep the "10" as int 10 in it.

Example:

x = tonumber("10")

if you print the x variable, it will output an int 10 and not "10"

same like Python process

x = int("10")

Thanks.

answered Jul 12 '15 at 09:19

user5555332

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score 2 · Answer 11 · answered Feb 03 '21 at 06:00

Since lua 5.3 there is a new math.tointeger function for string to integer. Just for integer, no float.

For example:

print(math.tointeger("10.1")) -- nil
print(math.tointeger("10")) -- 10

If you want to convert integer and float, the tonumber function is more appropriate.

score 2 · Answer 12 · answered Sep 21 '21 at 05:35

Lua digital types are double precision types, which are implemented in luaconf. h in detail There are two general options:

[Recommended use] The Lua script in version 5.3 contains the global.lua file, which includes the tonumber() and tostring() methods, which can realize the conversion of numbers and strings.
math.tointeger() It can also be converted into numbers by using this method.

However, according to your actual scene, if it is only for calculation, Lua can be converted according to this operation symbol. eg

  s = "1" + 2; -- lua will convert "1" to 1
  print(s)

  s1 = "e" + 3; -- error
  print(s1)

more about lua,you can see lua official document

Hope to be useful to you and look forward to your reply,and looking forward to further communication with you!

score 2 · Answer 13 · answered May 28 '22 at 02:17

2

You can use tonumber() to convert the string to a number, can be a float or an int.

Ex: tonumber("11") -- return: 11

answered May 28 '22 at 02:17

Gabriel Modog

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score 1 · Answer 14 · answered Jul 01 '18 at 14:49

1

Lua 5.3.1  Copyright (C) 1994-2015 Lua.org, PUC-Rio
> math.floor("10");
10
> tonumber("10");
10
> "10" + 0;
10.0
> "10" | 0;
10

answered Jul 01 '18 at 14:49

vkatsuba

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score 0 · Answer 15 · answered Dec 16 '16 at 22:41

0

here is what you should put

local stringnumber = "10"
local a = tonumber(stringnumber)
print(a + 10)

output:

20

answered Dec 16 '16 at 22:41

theresaspyaroundhere

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a = tonumber(stringnumber) is sufficent – wsha Sep 20 '18 at 07:29

Lua string to int

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