Question

How can I get the previous focus/be able to compare strings?

Answer 1

The FocusEvent.FOCUS_IN event contains also a reference to the interactive object losing the focus (relatedObject property).

In your code you could change:

textbox[i].addEventListener(KeyboardEvent.KEY_UP, k);
textbox[i].tabIndex= i; 

to:

textbox[i].addEventListener(KeyboardEvent.KEY_UP, k);
textbox[i].addEventListener(FocusEvent.FOCUS_IN, handleFocusIn);
textbox[i].tabIndex= i; 

function handleFocusIn(anEvent: FocusEvent): void
{
  if (anEvent.relatedObject is TextField)
  {
     var previousBox: TextField = anEvent.relatedObject as TextField;
     var currentBox: TextField = anEvent.target as TextField;
     // etc.
  }
}

Like the previous answer said, try to figure out how looping works. Also you can reference the TextField using []:

// to reference names_mc.box19_txt using an index
var index: int = 19;
var textbox: TextField = names_mc['box' + index + '_txt'];