Question

In this web page: CS302 — External Sorting

Merge the resulting runs together into successively bigger runs, until the file is sorted.

As I quoted, how can we merge the resulting runs together??? We don’t have that much memory.

Answer 1

Imagine you have the numbers 1 – 9

9  7  2  6  3  4  8  5  1

And let’s suppose that only 3 fit in memory at a time.

So you’d break them into chunks of 3 and sort each, storing each result in a separate file:

279
346
158

Now you’d open each of the three files as streams and read the first value from each:

2 3 1

Output the lowest value 1, and get the next value from that stream, now you have:

2 3 5

Output the next lowest value 2, and continue onwards until you’ve outputted the entire sorted list.