Get name of argument of function in lua

Questions

When call a lua function like

PrintMe(MyVariableName)

I would like to be able to actually print "MyVariableName" and not it s value(well, for demo purposes).

Obviously I could just pass the string but that requires extra quotes and I also would like to print it s value.

e.g.,

MyVariable = 4
PrintVariable(MyVariable)

Would print "MyVariable is 4" or whatever

I do not want to have to duplicate the name and variable like

PrintVariable(MyVariable, "MyVariable")

as this is unnecessary duplication.

Can lua handle it?

What I m doing now is passing the variable name in quotes and using loadstring to get the value but I would like to just pass the variable directly without the extra unnecessary quotes(which I thought debug.getlocal did but it ends up returning the value instead of the name).

---

Here is mock example

function printme1(var, val)
    print(var.." = "..val)
end

function printme2(v)
    local r
    loadstring("r = "..v)() -- equivalent to r = a but must be used since v is a string representing a and not the object a
    print(v.." = "..tostring(r))
end

function printme3(v)
    -- unknown
end

a = 3

printme1("a", a)
printme2("a")
printme3(a) 

In this case all 3 should print the same thing. printme3 obviously is the most convenient.

Answers

You can t say PrintVariable(MyVariable), because Lua gives you no way of determining which variable (if any; a constant could have been used) was used to pass an argument to your function. However, you can say PrintVariable( MyVariable ) then used the debug API to look for a local variable in the caller s scope which has that name:

function PrintVariable(name)
  -- default to showing the global with that name, if any
  local value = _G[name]

  -- see if we can find a local in the caller s scope with that name
  for i=1,math.huge do
    local localname, localvalue = debug.getlocal(2,i,1)
    if not localname then
      break -- no more locals to check
    elseif localname == name then
      value = localvalue
    end
  end

  if value then
    print(string.format("%s = %s", name, tostring(value)))
  else
    print(string.format("No variable named  %s  found.", name))
  end
end

Now you can say:

PrintVariable( MyVariable )

While in this case will print "MyVariable = 4".

---

Not, if you really want to do this without the quotes, you could check the caller s locals for variables that have a supplied value, but that s occasionally going to give you the wrong variable name if there is more than one variable in the caller s scope with a given value. With that said, here s how you d do that:

function PrintVariable(value)
  local name

  -- see if we can find a local in the caller s scope with the given value
  for i=1,math.huge do
    local localname, localvalue = debug.getlocal(2,i,1)
    if not localname then
      break
    elseif localvalue == value then
      name = localname
    end
  end

  -- if we couldn t find a local, check globals
  if not name then
    for globalname, globalvalue in pairs(_G) do
      if globalvalue == value then
        name = globalname
      end
    end
  end

  if name then
    print(string.format("%s = %s", name, tostring(value)))
  else
    print(string.format("No variable found for the value  %s .", tostring(value)))
  end
end

Now you can say PrintVariable(MyVariable), but if there happened to be another variable in the caller s scope with the value 4, and it occurred before MyVariable, it s that variable name that will be printed.

Source

License : cc by-sa 3.0

http://stackoverflow.com/questions/10458306/get-name-of-argument-of-function-in-lua

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