How do I fix my string issue? - lua

Question

local background = display.newImage("black.png", 0, 0)
local submit = display.newImage("submit.png")
submit.x = display.contentWidth/2
submit.y = display.contentHeight-100
local nameInstructions = display.newText("Enter your name", 10, 50, native.systemFont, 24)
local usersName = native.newTextField(10, 100, 350, 50)
usersName.inputType = "default"
local function keyboardListener (event)
native.setKeyboardFocus(nil)
end
background:addEventListener("tap", keyboardListener)
local function reverseName(event)
reverseUsersName = string.reverse(usersName)
end
submit:addEventListener("tap", reverseName)
local reverse = display.newText(reverseUsersName)
reverse.x = display.contentWidth/2
reverse.y = display.contentHeight/2
Every time that I run this using my Corona SDK thing, I get this:
Bad argument #-1 to 'newText' (string expected, got nil)
stack traceback:
[C]: ?
[C]: in function 'newText'
...Corona Projects/Assignment 4.3/main.lua/src/main.lua:24: in main chunk

Answer 1

reverseName is in the local function reverseName(event), but this function is called once u press or tap on the submit . But here local reverse = display.newText(reverseUsersName), is called before you tap on the sumbit . That is why its giving you error.

Answer 2

The only place that reverseUsersName, accessed on line 24, is set, is inside the function reverseName(event). In this function, reverseUsersName is a global so after the function is run once, that variable will be accessible from other parts of your script, but until then, it does not exist.
Now in line 22 you've registered reverseName as event listener for "tap" events, but events are only generated after your script has been executed once (and in between calls to your script callbacks like reverseName and keyboardListener are callbacks), so when you create the display text just after, the variable does not yet exist.
So what you would have to do is update the text of the reverse display item in your reverseName listener so that every time you click on the button, the reversed name becomes visible. And because of that, you would have to declare your reverse variable above reverseName function so that it is available as an upvalue (read the Corona getting started docs, they are excellent and discuss this subtlety) in that function. And presumably you would want to initialize with showing the usersName rather than reverse.
So you would need something like
local reverse = display.newText(usersName)
local function reverseName(event)
reverseUsersName = string.reverse(usersName)
reverse.SetText(reverseUserName)
end
submit:addEventListener("tap", reverseName)
Note that if you want the string being displayed to be reversed every time you press tap, rather than only first time you press it, you will have to use
local function reverseName(event)
reverseUsersName = string.reverse(reverseUsersName)
reverse.SetText(reverseUserName)
end
reverseUsersName = usersName

Answer 3

Check your function reverseName and the text object(reverse) with the following code:
local reverse --[[ Initialize the object with a global scope,
so you can access it anywhere from the page. --]]
local function reverseName(event)
--[[ In the below line, usersName is a table value. It is the reason
of the error. For getting the string from the text field, you
have to provide 'usersName.text' --]]
reverseUsersName = string.reverse(usersName.text)
reverse.text = reverseUsersName -- Assign the text field value to your text object
end
submit:addEventListener("tap", reverseName)
reverse = display.newText("",20,20,nil,20) --see the parameters of display.newText()*
reverse.x = display.contentWidth/2
reverse.y = display.contentHeight/2
*Corona API: display.newText()