Share your favourite helper functions

[kikito]

I'm pretty sure it's doable if you have access to Lua's debug library. You can get a list of the current locals with their names, so printing the name of a particular one shouldn't be very different.

The debug lib is not guaranteed to be defined in all cases. A function which relies on debug should take this into account - maybe "failing gracefully" (supressing input from debug if it's not present, instead of just raising an error and dying).

[Robin]

I'm pretty sure it's doable if you have access to Lua's debug library.

Oh, yeah, that might yield some results. Something like:

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function getnameof(value)
  local idx = 1
  while true do
    local ln, lv = debug.getlocal(2, idx)
    if ln ~= nil then
      if value == lv then return ln end
    else
      error "no local name for value found"
    end
    idx = 1 + idx
  end
end

[spir]

I just realised that, apparently, the "%q" string format tag does not truely escaped newlines !!!

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print (string.format("%q", "XXX\tXXX\nXXX"))
--[[ output:
"XXX\9XXX\
XXX"
]]

Why so? It does escape tab properly, so why not newlines? And what's this backslash roaming there anyway? %q adds an '\' but only "graphical" (so to say), a real '\\' char without escaping the newline. (Hum, I'll check the bytes to see what's in there exactly). However, in the meanwhile, here is a func that does the job. Tell me if it's correct (I'm not sure of all what can be a special char in Lua), and if there is a simpler/better/more efficient way to do it. If you add (as I did) the following meta-incantations, then your strings will print out properly escaped (thus, ideal for programmer feedback) as default:

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string.quoted = function (s)
   local esc_s = s:gsub(".", function (c)
         if     c < ' '   then return '\\' .. string.byte(c)
         elseif c == '\"' then return '\\"'
         elseif c == '\\' then return '\\\\'
         else                  return c
         end
      end
   )
   return '"' .. esc_s .. '"'
end

-- quick test
print(string.quoted( " \t \n \" \\ \0 \31 "))

string_mt = getmetatable("")
string_mt.__tostring = string.quoted

-- re quick test
print " \t \n \" \\ \0 \31 "

Denis

[Robin]

I just realised that, apparently, the "%q" string format tag does not truely escaped newlines !!!

It does, though.

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somestring = "hello\
world"

is just a different way of saying

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somestring = "hello\nworld"

I agree you'll usually will want the latter way, though, so a replacement function makes sense.

Your string.quoted looks good to me, although I don't know how efficient gsub over "." is.

Two things:

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         elseif c == '"' then return '\\"'

You don't need the backslash there, and in other places you don't use it.

More importantly:

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string_mt = getmetatable("")
string_mt.__tostring = string.quoted

I wouldn't do this.

Normally, tostring is the identity function for strings. So tostring(foo) == foo if foo is a string. Those two lines break that.

[Nsmurf]

One of the most helpful libs(?) ever.

[Inny]

That implementation of math.round on https://love2d.org/wiki/Additional_math is incorrect.

Round(-1.5) == -2, as seen here: http://www.wolframalpha.com/input/?i=round%28-1.5%29
Floor(-1.5+0.5) == -1, as seen here: http://www.wolframalpha.com/input/?i=fl ... %2B+0.5%29

So, implementing round(x) as floor(x+0.5) is wrong.

[Robin]

So, implementing round(x) as floor(x+0.5) is wrong.

Not necessarily. There are a lot of different rules for tie-breaking, and one is not necessarily "right" or "wrong".

[Inny]

So, implementing round(x) as floor(x+0.5) is wrong.

Not necessarily. There are a lot of different rules for tie-breaking, and one is not necessarily "right" or "wrong".

Egad, I tested a bunch of values in wolfram alpha and it appears that they implement the Half To Even style. I'm not sure what to think about this now.

[spir]

Your string.quoted looks good to me, although I don't know how efficient gsub over "." is.

Yo, I'll change that (with a gsub mapping special chars, and another for the rest of those < 0x20).

Two things:

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         elseif c == '"' then return '\\"'

You don't need the backslash there, and in other places you don't use it.

I don't get what you mean here: I'm producing an escaped string (thus backslahes are all escaped in the replacement patterns).

More importantly:

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string_mt = getmetatable("")
string_mt.__tostring = string.quoted

I wouldn't do this.

Normally, tostring is the identity function for strings. So tostring(foo) == foo if foo is a string. Those two lines break that.

Yo, it's a nice property, but have we the choice. __tostring is launched by a call to tostring, isn't it? So, if I want string to be correct eg in table out put, I must hook tostring. So that eg --provided you have a proper tostring for table notation to your taste--, tostring(t) works fine:

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t = {[true]="true", ['nil']="nil", nltab="\n\t", [1]="1", [9]="9"}
print(t)
--> {true="true", 9="9", 1="1", nltab="\n\t", "nil"="nil"}
--  or
--> {true="true", 9="9", 1="1", nltab="\9\10", "nil"="nil"}
-- but not (!) that:
--[[
{true=true, 9=9, 1=1, nltab=
	, nil=nil}
]]

Grrr!!! What the f*$k? How can you debug? I want to string to show correctly. Far more important than tostring(s) == s (when is this useful, anyway, else theoretically?); and it gives another hugely useful (according ti me) property:

programmer --> notation -----> Lua --> tostring of s -------> notation --> programmer

With notations equal on both sides as far as possible (with funcs, threads, userdata, let's wait for next Xmas).
[/code]

[spir]

That implementation of math.round on https://love2d.org/wiki/Additional_math is incorrect.

Round(-1.5) == -2, as seen here: http://www.wolframalpha.com/input/?i=round%28-1.5%29
Floor(-1.5+0.5) == -1, as seen here: http://www.wolframalpha.com/input/?i=fl ... %2B+0.5%29

So, implementing round(x) as floor(x+0.5) is wrong.

I think as you (but my math knowledge is limited to faraway school). I use the following:

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math.round0 = function (n)       -- to next int (1.5 --> 2, -1.5 --> -2)
   local i = math.floor(n)
   if n >= 0 then
      if (n - i) < 0.5 then return i else return i + 1 end
   else
      if (n - i) <= 0.5 then return i else return i + 1 end
   end
end
math.round = function (n, p)     -- to given power-of-ten precision
   local factor = 10^p
   local mult = n * factor
   local mult_round0 = math.round0(mult)
   return mult_round0 / factor
end
end

with the following test func showing all case combinations I could think at:

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   note("=== rounding to next int ===")
   local rnd0 = math.round0
   for _,i in ipairs {0.0, 1.0, 1.1, 1.5, 1.9} do
      notef("math.round0(%.1f) : %s", i, rnd0(i)) 
   end
   for _,i in ipairs {-1.0, -1.1, -1.5, -1.9} do
      notef("math.round0(%.1f) : %s", i, rnd0(i)) 
   end
   line()
   
   note("=== rounding to given precision ===")
   local rnd = math.round
   local n = 123.4567890
   for _,p in ipairs {7,3,2,1,0,-1,-2} do
      notef("math.round(%f, %d) : %s", n, p, rnd(n,p)) 
   end
   local n = -123.4567890
   for _,p in ipairs {7,3,2,1,0,-1,-2} do
      notef("math.round(%f, %d) : %s", n, p, rnd(n,p)) 
   end

I this correct? And is there a shorter way to express that?
Also, why does do you think Lua provides us with floor and ceil? I think i never, ever needed one of them (and they are highly dangerous to use, due to those corner cases --and we humans seem to be unable to think right with negative numbers, as shown by this example). I constantly need instead int-part and round.

Denis