Question

I have a string, which is long, and a sorted dictionary of indexes and values. I should go over the elements in the dictionary and insert the value to the specified index in the string. I wrote the following code, which works fine, but very slow:

private string restoreText(string text){
  StringBuilder sb = new StringBuilder(text);
  foreach(KeyValuePair<int, string> pair in _tags){
    sb.Insert(pair.Key, pair.Value);
  }
  return sb.ToString();
}

The dictionary might be very big and contain 500,000 elements.
I think that what makes this function slow is the Insert() method. For dictionary of 100,000 elements, it took almost 5 seconds.

Is there a more efficient way to write this method?

Thanks,

Maya

Answer 1

what I don’t get if you have your indices setup so that the insert won’t change the others but as your code says “yes” I’ll assume so too.

Can you test this one:

private string RestoreText(string text)
{
    var sb = new StringBuilder();
    var totalLen = 0;
    var orgIndex = 0;
    foreach (var pair in _tags.OrderBy(t => t.Key))
    {
        var toAdd = text.Substring(orgIndex, pair.Key - totalLen);
        sb.Append(toAdd);
        orgIndex += toAdd.Length;
        totalLen += toAdd.Length;

        sb.Append(pair.Value);
        totalLen += pair.Value.Length;
    }
    if (orgIndex < text.Length) sb.Append(text.Substring(orgIndex));
    return sb.ToString();
}

it only uses append while beeing the same as your original code