In early languages such as FORTRAN, the only built-in data structure was the array. The programmer thus implemented nodes and their noncontiguous memory for a singly linked structure by using two parallel arrays. One array contained the data items. The other array contained the index positions, for corresponding items in the data array, of their successor items in the data array. Thus, following a link meant using a data item’s index in the first array to access a value in the second array and then using that value as an index into another data item in the first array. The empty link was represented by the value -1. Figure 13.12 shows a linked structure and its array representation. As you can see, this setup effectively decouples the logical position of a data item in the linked structure from its physical position in the array