Lua Script Error - lua

Question

Situation:
I want to save the record of a data which is a value of the sensor in the certain file.
Code is..
--Header file
require("TIMER")
require("IPBOX")
require("ANALOG_IN")
require("LOG")
function OnExit()
print("Exit code...do something")
end
function main()
timer = "Timer"
local analogsensor_1 = "AIR_1"
local analogsensor_2 = "AIR_2"
local timestr = os.data("%Y-%m-%d %H:%M:%S")
-- open the file for writing binary data
local filehandle = io.open("collection_of_data.txt", "a")
while true do
valueOfSensor_1 = ANALOG_IN.readAnalogIn(analogsensor_1);
valueOfSensor_2 = ANALOG_IN.readAnalogIn(analogsensor_2);
if (valueOfSensor_1 > 0 and valueOfSensor_2 > 0) then
-- save values of sensors
filehandle:write(timestr, " -The Value of the Sensors: ", tostring(valueOfSensor_1), tostring(valueOfSensor_2)"\n");
-- save values using rolling log appender:
LOG.log_event( ScenarioLoggerDevicenameInDeviceList, " -The Value of the Sensors: ", tostring(valueOfSensor_1), tostring(valueOfSensor_2), "any other string you wish to add", "etc", "etc")
LOG.log_event( ScenarioLoggerDevicenameInDeviceList, " -The Value of the Sensors: ", tostring(valueOfSensor_1))
print("Hello3"..valueOfSensor_1)
end
TIMER.sleep(timer,500)
end
-- close the file
filehandle:close()
end
print("start main")
main()
In this line:
filehandle:write(timestr, " -The Value of the Sensors: ", tostring(valueOfSensor_1), tostring(valueOfSensor_2)"\n");
I get an error:
"attemp to index global 'filehandle' (a nil value)"
How can I fix it?

Answer 1

io.open returns nil if it cannot open the file. This can be due to "file not found", "permissions denied" and maybe other reasons. To figure out the problem, io.open has a second return value, which lets you inspect the error (in fact, it even returns a third value, which is an error-code integer - but its meaning is system dependent).
Change:
local filehandle = io.open("collection_of_data.txt", "a")
to
local filehandle, message = io.open("collection_of_data.txt", "a")
if not filehandle then
print(message)
end
You can also use the following Lua idiom:
local filehandle = assert(io.open("collection_of_data.txt", "a"))
This will do the same. If the first argument to assert is nil, then the second argument (the second return value of io.open will be printed. If the first argument is not nil, it will simply be returned.