Question

I am trying to concatenate all the values and append them to the same variable, then write the content of the string. I keep getting the error: “attempt to concatenate global ‘arglist’ (a nil value)”

  eo_device = {}

  eo_device[1] = {node = "ME", t_object = "AV1", h_object = "AV2" }
  eo_device[2] = {node = "ME", t_object = "AV3", h_object = "AV4" }
  eo_device[3] = {node = "ME", t_object = "AV5", h_object = "AV6" }
  eo_device[4] = {node = "ME", t_object = "AV7", h_object = "AV8" }
  eo_device[5] = {node = "ME", t_object = "AV9", h_object = "AV10" }
  eo_device[6] = {node = "ME", t_object = "AV11", h_object = "AV12" }
  eo_device[7] = {node = "ME", t_object = "AV13", h_object = "AV14" }
  eo_device[8] = {node = "ME", t_object = "AV15", h_object = "AV16" }
  eo_device[9] = {node = "ME", t_object = "AV17", h_object = "AV18" }
  eo_device[10] = {node = "ME", t_object = "AV19", h_object = "AV20" }
  eo_device[11] = {node = "ME", t_object = "AV21", h_object = "AV22" }
  eo_device[12] = {node = "ME", t_object = "AV23", h_object = "AV24" }
  eo_device[13] = {node = "ME", t_object = "AV25", h_object = "AV26" }
  eo_device[14] = {node = "ME", t_object = "AV27", h_object = "AV28" }
  eo_device[15] = {node = "ME", t_object = "AV29", h_object = "AV30" }


-- Generate string with all arguments to be written to CSV
for deviceindex = 1,#eo_device do

 device_name = scl.nodes[eo_device[deviceindex].node][eo_device[deviceindex].t_object .. "_Object_Name"].value -- point to Object Names
 description = scl.nodes[eo_device[deviceindex].node][eo_device[deviceindex].t_object .. "_Description"].value -- point to Description of all objects     
 temperature = scl.nodes[eo_device[deviceindex].node][eo_device[deviceindex].t_object .. "_Present_Value"].value -- value of temperature object
 humidity = scl.nodes[eo_device[deviceindex].node][eo_device[deviceindex].h_object .. "_Present_Value"].value -- value of humidity object


 buflist = device_name .. "," .. description .. "," .. temperature .. "," .. humidity 

 arglist = arglist .. "," .. buflist  

 end -- end for

--then once outside of the loop:
arglist = arglist .. "," .. buflist 

buffer = os.date() .. arglist .. "\r\n"

file, info = io.open("trend.csv", "a+") -- write to CSV
if file then
file:write(buffer)
file:close()
end -- end if

Answer 1

I keep getting the error: “attempt to concatenate global ‘arglist’ (a nil value)”

Because you’re attempting to concatenate arglist (a nil value) on line 31 of your code:

arglist = arglist .. "," .. buflist  

arglist is never initialized, so when you attempt that concatenation on the right hand of the =, you get that error.

---

Side note about this loop:

for deviceindex = 1,#eo_device do

    description = scl.nodes[eo_device[deviceindex].node][eo_device[deviceindex].t_object .. "_Description"].value

Lua has an ‘generic’ for loop which will allow you to iterate over all the elements of eo_device, giving you the indices and values, so you don’t have to index inside your loop. This will clean your code up a bit:

for deviceindex, device in ipairs(eo_device) do

    description = scl.nodes[device.node][device.t_object .. "_Description"].value

---

Side note about this code:

buflist = device_name .. "," .. description .. "," .. temperature .. "," .. humidity 

Building strings like this is incredibly inefficient. Doing this in a loop will result in thousands of string concatenations and generate tons of garbage. A better approach is to add all the string parts to a table, then join them into a single, comma-delimited string via table.concat.

For example, to create a comma-delimited string containing all the strings in your eo_device table:

local buffer = {}

for i,device in ipairs(eo_device) do
    table.insert(buffer, device.node)
    table.insert(buffer, device.t_object)
    table.insert(buffer, device.h_object)
end

devices = table.concat(buffer, ',')