Copying data from one column to another

I have a table of data (with 20 columns) that I am displaying using dataTables. I also have an assignment button (id=assign). What I am trying to do is copy the data in column#14 to column#20 when i click the button. I might do this on all the rows or a selection of them. my code looks like this.

$(document).ready(function() {
var oTable = $('#myTable').dataTable();

$('#assign').click(function() {			
	var selectedRecords = oTable.fnSettings().aiDisplay; //gives an array of row indices
	for(i=0;i<selectedRecords.length; i++) {
		mynodes[i] = oTable.fnGetNodes(selectedRecords[i]);		//get row node from index				
		mydata[i] = oTable.fnGetData(mynodes[i],13);			//get column data from node and column number			
		oTable.fnUpdate(mydata[i],mynodes[i],19);				//copy column data to new column		
	}					
});
			
});

What is happening is that the copying occurs in the first row and then stops. I get no errors in firebug. when i do an alert(mydata); I see all the information that is supposed to be there. Any pointers to what i am missing would be greatly appreciated.