changing sprite frame depending on random value - list

Question

I have a list of buttons(currently images) and i want to change the image color depending on the value that is taken from the list
buttons = {"btn1","btn2","btn3","btn4"}
local buttonSheetData = {
width = 150,
height = 150,
numFrames = 2,
sheetContentWidth = 300,
sheetcontentheight = 150,
}
local buttonSheet = graphics.newImageSheet("image/buttonSS.png", buttonSheetData)
local sequenceData = {
{name = "black", start = 1, count = 1},
{name = "red", start = 2, count = 1}
}
local btn1 = display.newSprite(buttonSheet, sequenceData)
btn1.x = 100
btn1.y = 90
local btn2 = display.newSprite(buttonSheet, sequenceData)
btn2.x = 200
btn2.y = 230
local btn3 = display.newSprite(buttonSheet, sequenceData)
btn3.x = 300
btn3.y = 90
local btn4 = display.newSprite(buttonSheet, sequenceData)
btn4.x = 400
btn4.y = 230
x = buttons[math.random(#buttons)]
x:setFrame(2)
The circles are currently black. every time i run the code i want it to take a random value from the list and change the color to red. so there is a different red circle when i run the code
I keep getting the error:
"Attemp to call method 'setSequence' (a nill value)"

Answer 1

You're code is treating strings and variable names as if they are interchangeable. The first line:
buttons = {"btn1","btn2","btn3","btn4"}
creates a table of strings, so the line:
x = buttons[math.random(#buttons)]
will set x to be a random entry of buttons which are strings so the next line (x:setFrame(2)) is calling a method that doesn't exist on a string.
Instead, create a table of your buttons:
buttons = {btn1,btn2,btn3,btn4} -- creates a table of buttons
x = buttons[math.random(#buttons)] -- x is a random entry of buttons (a button)