fastest way to convert array to multiline string

Question

what is the most efficient way to export array elements into a .csv excel file? Now I am doing like this, but it is very slow. Thank you for all your support.

int FrameWidth = 640;
int FrameHeight = 480;
Int16[] Values;  // 640 x 480, 307200 elements

/*
.. processing ...... 
*/

//Exporting Values to .csv file
string string2csv = null;
for (int y = 0; y < FrameHeight; y++)
{
    for (int x = 0; x < FrameWidth; x++)
    {
        string2csv = string2csv + Values[y * FrameWidth + x] + ";";
    }
    string2csv = string2csv + "\n";
}
File.WriteAllText("string2csv.csv", string2csv);

@DriesVB Note that String.Join uses a StringBuilder internally. — D Stanley
– D Stanley, Commented Feb 9, 2016 at 21:48

Joachim Isaksson · Accepted Answer · 2016-02-09 22:03:00Z

3

One alternative is to combine File.WriteAllLines and string.Join;

File.WriteAllLines ("test.txt", 
    Values.Select((x,i) => new {x, i})
          .GroupBy(x => x.i / FrameWidth)
          .Select(grp => string.Join(";", grp.Select(y => y.x)))
);

It will select all elements along with the index in the array.
It will then group the values by row (calculated by dividing the index by the number of columns).
The grouped values are then joined using ; into a string per row, and the rows are all fed into File.WriteAllLines.

answered Feb 9, 2016 at 22:03

Joachim Isaksson

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1 Comment

atteee Over a year ago

StringBuilder seams to be the fastest, with 27ms, then Joachim's with 38ms, my solution 21 496ms :) I am very very thankful for all your comments! You guys are all great!

D Stanley · Accepted Answer · 2016-02-09 22:04:11Z

2

"most efficient" wil take trial and error, but one easy alternative is to use StringBuilder:

StringBuilder string2csv = new StringBuilder();
for (int y = 0; y < FrameHeight; y++)
{
    for (int x = 0; x < FrameWidth; x++)
    {
        string2csv.Append(Values[y * FrameWidth + x] + ";");
    }
    string2csv.Append("\n");
}

Adding strings using + allocates memory for a new string, then copies the contents of the two strings into the new memory. StringBuilder pre-allocates a buffer in memory and adds characters to the buffer, extending it as necessary. There's much less memory allocation and copying when adding lots of strings.

That may be "fast enough". Other than that, you'd need to get a decent profiler to see where your code is spending the most time and attack that part. It's entirely possible that your "processing..." block is the bottleneck, not the CSV output.

edited Feb 9, 2016 at 22:04

answered Feb 9, 2016 at 21:46

D Stanley

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12 Comments

Jeff Mercado Over a year ago

You don't even need to do that. He's building up a string of the contents of the file when he could just write to the file directly.

paparazzo Over a year ago

@JeffMercado Not sure that would be more efficient

Steve Over a year ago

For the purpose of optimization is very important to set an initial capacity for the stringbuilder according to the size of the input array (ca 5 * 307K bytes).

Jeff Mercado Over a year ago

@Frisbee: How so? File streams are buffered by default. How would generating all the data then writing it be more efficient than writing the data as it's generated.

atteee Over a year ago

Yes, if I do it this way: File.WriteAllText("string2csv.csv", string.Join(";", Values)); it is super fast :) but only 1 long row in excel.

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