Performance issues with nested loops and string concatenations

Question

Can someone please explain why this code is taking so long to run (i.e. >24 hours): The number of rows is 5000, whilst the number of columns is 2000 (i.e. Approximately 10m loop

Answer 1

Because you are creating tons of strings.

You should use StringBuilder for this.

StringBuilder sb = new StringBuildeR();

for (int i = 0; i < m.rows; i++)
{
    bool first = true;

    for (int j = 0; j < m.cols; j++)
    {
        sb.Append(m[i, j]);

        if (first)
        {
            first = false;
        }
        else
        {
            sb.Append(",");
        }
    }

    sb.AppendLine();
}

string output = sb.ToString();