Member Avatar for amadaeliseo

Pls help,

Can anyone help me to implement n-way merge sort (external merge sort) algorithm in c#.

Thank you.

  • 4 Contributors
  • 3 Replies
  • 101 Views
  • 1 Day Discussion Span
  • Latest Post Latest Post by Diamonddrake

Dani AI

Generated

— n-way external merge sort is the standard way to sort files larger than RAM. It’s disk-bound and more work than an in-memory sort (as hinted), but it’s straightforward to split into small, testable steps. : "n-way" just means merging N sorted runs at once. : if this is a duplicate thread, check any earlier replies for partial code.

Practical plan:

  1. Create initial runs: read as many records as safely fit in memory, sort them (List<T>.Sort or Array.Sort), and write each run to a temp file. Size runs using avg record size and available memory (keep headroom for buffers).
  2. Multi-way merge: open readers for each run (bounded by OS file-handle limits), keep a small read buffer per run, and use a min-heap (priority queue) keyed by the record value to pick the next output record.
  3. If the number of runs exceeds a safe fan-in, do multi-pass merging (merge groups of runs into intermediate runs, then merge those).

Example sketch (uses .NET 6+ PriorityQueue for clarity):

// after creating List<string> runs (paths) and readers:
var readers = runs.Select(p => new StreamReader(p)).ToArray();
var pq = new PriorityQueue<(string value, int runId), string>(Comparer<string>.Default);

for (int i = 0; i < readers.Length; i++)
{
    var line = readers[i].ReadLine();
    if (line != null) pq.Enqueue((line, i), line);
}

using var outWriter = new StreamWriter(outputPath);
while (pq.Count > 0)
{
    var item = pq.Dequeue();
    outWriter.WriteLine(item.value);
    var next = readers[item.runId].ReadLine();
    if (next != null) pq.Enqueue((next, item.runId), next);
}
foreach (var r in readers) r.Dispose();

Troubleshooting tips: always dispose streams (use using), delete temp files, tune buffer sizes for sequential reads, watch OS file-handle limits (or do multi-pass merges), and test on smaller data first. If you don’t need raw file-level control, consider letting a DB or the OS sort utility handle very large sorts.

Recommended Answers

All 3 Replies

Member Avatar for serkan sendur

what does that mean?

Member Avatar for IdanS

Why did you opened a new thread for that question?
You asked this question already in another thread.

Member Avatar for Diamonddrake

yeah wow, that's an incredibly complicated thing to do. Its only purpose is to sort large amounts of data but only use minimal system resources and is typically much slower. Its actually a lot of work.

The real question here, is do you really need it bad enough to put in that much work?

Be a part of the DaniWeb community

We're a friendly, industry-focused community of developers, IT pros, digital marketers, and technology enthusiasts meeting, networking, learning, and sharing knowledge.