Fastest way to read numerical values from text file in C++ (double in this case)

Question

Currently, my code is simply this:

void ReadFile(double Cst[][1000], char* FileName, int height)

FILE* ifp;
double value;
int nRead = 0;
int mRead = 0;

//o         


        

Answer 1

C/C++ parsing numbers from text is very slow. Streams are horribly slow but even C number parsing is slow because it's quite difficult to get it correct down to the last precision bit.

In a production application where reading speed was important and where data was known to have at most three decimal digits and no scientific notation I got a vast improvement by hand-coding a floating parsing function handling only sign, integer part and any number of decimals (by "vast" I mean 10x faster compared to strtod).

If you don't need exponent and the precision of this function is enough this is the code of a parser similar to the one I wrote back then. On my PC it's now 6.8 times faster than strtod and 22.6 times faster than sstream.

double parseFloat(const std::string& input)
{
    const char *p = input.c_str();
    if (!*p || *p == '?')
        return NAN_D;
    int s = 1;
    while (*p == ' ') p++;

    if (*p == '-') {
        s = -1; p++;
    }

    double acc = 0;
    while (*p >= '0' && *p <= '9')
        acc = acc * 10 + *p++ - '0';

    if (*p == '.') {
        double k = 0.1;
        p++;
        while (*p >= '0' && *p <= '9') {
            acc += (*p++ - '0') * k;
            k *= 0.1;
        }
    }
    if (*p) die("Invalid numeric format");
    return s * acc;
}