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.

Answer 1

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

Answer 2

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.

Answer 3

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

Output

   string                                                                                                                                                                          
   number

Answer 4

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.

Answer 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

Answer 6

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.
  

Answer 7

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

Answer 8

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

Answer 9

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.

Answer 10

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

Answer 11

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

Answer 12

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.

Answer 13

You can use the tonumber function, just like how you would use the tostring function, here's how do use it:

local x = tonumber("45")
print(x)

The result should be 45.

Answer 14

here is what you should put

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

output:

20