Lua - Number to string behaviour

Question

I would like to know how Lua handles the number to string conversions using the tostring() function.

It is going to convert to an int (as string) if the number is round (i.e if number == (int) number) or is it always going to output a real (as string) like 10.0 ?

I need to mimic the exact behaviour of Lua's tostring in C, without using the Lua C API since, in this case, I'm not using a lua_State.

Solution 1

In Lua 5.2 or earlier, both tostring(10) and tostring(10.0) result as the string "10".

In Lua 5.3, this has changed:

print(tostring(10)) -- "10"
print(tostring(10.0)) -- "10.0"

That's because Lua 5.3 introduced the integer subtype. From Changes in the Language:

The conversion of a float to a string now adds a .0 suffix to the result if it looks like an integer. (For instance, the float 2.0 will be printed as 2.0, not as 2.) You should always use an explicit format when you need a specific format for numbers.

Solution 2

Lua converts the numbers as is:

print(tostring(10)) => "10"
print(tostring(10.0)) => "10.0"
print(tostring(10.1)) => "10.1"

If you want to play around with them, there's a small online parser for simple commands like this : http://www.lua.org/cgi-bin/demo This uses Lua 5.3.1

edit I must support Egor's comment, it's version dependent. I ran this locally on my system:

Lua 5.2.4  Copyright (C) 1994-2015 Lua.org, PUC-Rio
> print(tostring(10))
10
> print(tostring(10.0)) 
10

Solution 3

If you are using 5.3.4 and you need a quick hotfix, use math.floor - it casts it to an int-number. This beats @warspyking answer in efficiency, but lacks the coolness that is bunches of code.

>tostring(math.floor(54.0))
54
>tostring(54.0)
54.0
>type(math.floor(54.0))
integer
>type(54.0)
number

Comments

[Egor Skriptunoff]

It depends on Lua version.

[Virus721]

Thanks I will try this. Isn't there an official documentation about the lua's tostring function ? I've been searching but the information is really sparse and you have to read the whole book everytime you want a piece of info...

[codingbunny]

all I could find was lua.org/pil/2.4.html and lua.org/manual/5.1/manual.html#lua_tostring

[Virus721]

Well as I suspected, if number == (int) number, it prints an int. I can't beleive there is no official documentation of these basic functions. Thanks for the help anyway.

[Virus721]

Thanks for your help. So it's not as easy as I expected. I'm using 5.3.2 and not planning to use another version.