Question

I have code that is writing a vector of size greater than 10million to a text file. I used clock() to time the writefile function and its the slowest part of my program. Is there a better way to write to file than my below method?

void writefile(vector<fields>& fieldsvec, ofstream& sigfile, ofstream& noisefile)
/* Writes clean and noise data to respective files
 *
 * fieldsvec: vector of clean data
 * noisevec: vector of noise data
 * sigfile: file to store clean data
 * noisefile: file to store noise data
 */
{
    for(unsigned int i=0; i<fieldsvec.size(); i++)
    {
        if(fieldsvec[i].nflag==false)
        {
            sigfile << fieldsvec[i].timestamp << ";" << fieldsvec[i].price << ";" << fieldsvec[i].units;
            sigfile << endl;
        }
        else
        {
            noisefile << fieldsvec[i].timestamp << ";" << fieldsvec[i].price << ";" << fieldsvec[i].units;
            noisefile << endl;
        }
    }
}

where my struct is:

struct fields
// Stores a parsed line of a file
{
public:
    string timestamp;
    float price;
    float units;
    bool nflag; //flag if noise (TRUE=NOISE)
};

Answer 1

I suggest getting rid of the endl. This effectively flushes the buffer every time and thus greatly increases the number of syscalls.

Writing '\n' instead of endl should be a very good improvement.

And by the way, the code can be simplified:

ofstream& files[2] = { sigfile, noisefile };
for(unsigned int i=0; i<fieldsvec.size(); i++)
  files[fieldsvec[i].nflag] << fieldsvec[i].timestamp << ';' << fieldsvec[i].price << ";\n";