什么是分段数据最快的方法

Question

I am not sure what this is called so Ill explain by giving an example. I have an array of numbers

76425
234786234
56
4356
564356
456
94
900
725

Now I would like to join this data and make a new array that looks like this

76425,234786234,56,4356
564356,456,94,900
725

This is a string array that contains 3 items. Items per row is 4. As you can see the last row only has one item. That is ok. Here is the code I have written to do this:

 numberOfColumns = numberOfColumns > lineCount ? lineCount : numberOfColumns;
 int newLineCount = Convert.ToInt32(Math.Ceiling((Convert.ToDouble(lineCount))/numberOfColumns));
 StringBuilder sb = new StringBuilder();

 for (int i = 0; i < newLineCount; i++)
 {
      var page = lines.Skip(numberOfColumns * i).Take(numberOfColumns).Select(xx => xx.Trim());
      sb.AppendLine(string.Join(",",page));
 }

This code works just fine. But it is very slow. Do you have any ideas to make it faster.

Answer 1

Are you looking for something like this?

StringBuilder sb = new StringBuilder();
for (int i = 0; i < lines.Count; i++)
{
    sb.Append(lines[i]);
    if ((i % 4) == 3)
        sb.AppendLine();
    else
        sb.Append(',');
}

Answer 2

Reducing the number of iterations will increase performance.

List<string> result = new List<string>();

for (int i = 0; i < lines.Count; i+=4)
{
 result.Add(string.Format ("{0},{1},{2},{3}", lines[i], lines[i+1], lines[i+2], lines[i+3]));
}

Also, you could use parallel processing, if you have a multicore processor.

 var linesPerProcessor = (int)(((lines.Count/4) / Environment.ProcessorCount)*4);
 Parallel.For(0,Environment.ProcessorCount -1
            ) =>
                {
                   for (int i = (p * linesPerProcessor); i < (p+1)*linesPerProcessor; i+=4)
                   {
                          result.Add(string.Format ("{0},{1},{2},{3}", 
                                lines[i], lines[i+1], lines[i+2], lines[i+3]));
                   }
                }
            );

Answer 3

Use:

var arr = new[]
    {
        76425,
234786234,
56,
4356,
564356,
456,
94,
900,
725,
    };

var result =
    arr.Select((x, i) => new {x, i})
    .GroupBy(t => t.i / 4)
    .Select(t => string.Join(",", t.Select(v => v.x)));

Output:

76425,234786234,56,4356
564356,456,94,900
725