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

My version that handles exponents too:

template<class It>
double mystrtod(It &s, It const end)
{
    static double const pow10[] = { 1E-323, 1E-322, 1E-321, 1E-320, 1E-319, 1E-318, 1E-317, 1E-316, 1E-315, 1E-314, 1E-313, 1E-312, 1E-311, 1E-310, 1E-309, 1E-308, 1E-307, 1E-306, 1E-305, 1E-304, 1E-303, 1E-302, 1E-301, 1E-300, 1E-299, 1E-298, 1E-297, 1E-296, 1E-295, 1E-294, 1E-293, 1E-292, 1E-291, 1E-290, 1E-289, 1E-288, 1E-287, 1E-286, 1E-285, 1E-284, 1E-283, 1E-282, 1E-281, 1E-280, 1E-279, 1E-278, 1E-277, 1E-276, 1E-275, 1E-274, 1E-273, 1E-272, 1E-271, 1E-270, 1E-269, 1E-268, 1E-267, 1E-266, 1E-265, 1E-264, 1E-263, 1E-262, 1E-261, 1E-260, 1E-259, 1E-258, 1E-257, 1E-256, 1E-255, 1E-254, 1E-253, 1E-252, 1E-251, 1E-250, 1E-249, 1E-248, 1E-247, 1E-246, 1E-245, 1E-244, 1E-243, 1E-242, 1E-241, 1E-240, 1E-239, 1E-238, 1E-237, 1E-236, 1E-235, 1E-234, 1E-233, 1E-232, 1E-231, 1E-230, 1E-229, 1E-228, 1E-227, 1E-226, 1E-225, 1E-224, 1E-223, 1E-222, 1E-221, 1E-220, 1E-219, 1E-218, 1E-217, 1E-216, 1E-215, 1E-214, 1E-213, 1E-212, 1E-211, 1E-210, 1E-209, 1E-208, 1E-207, 1E-206, 1E-205, 1E-204, 1E-203, 1E-202, 1E-201, 1E-200, 1E-199, 1E-198, 1E-197, 1E-196, 1E-195, 1E-194, 1E-193, 1E-192, 1E-191, 1E-190, 1E-189, 1E-188, 1E-187, 1E-186, 1E-185, 1E-184, 1E-183, 1E-182, 1E-181, 1E-180, 1E-179, 1E-178, 1E-177, 1E-176, 1E-175, 1E-174, 1E-173, 1E-172, 1E-171, 1E-170, 1E-169, 1E-168, 1E-167, 1E-166, 1E-165, 1E-164, 1E-163, 1E-162, 1E-161, 1E-160, 1E-159, 1E-158, 1E-157, 1E-156, 1E-155, 1E-154, 1E-153, 1E-152, 1E-151, 1E-150, 1E-149, 1E-148, 1E-147, 1E-146, 1E-145, 1E-144, 1E-143, 1E-142, 1E-141, 1E-140, 1E-139, 1E-138, 1E-137, 1E-136, 1E-135, 1E-134, 1E-133, 1E-132, 1E-131, 1E-130, 1E-129, 1E-128, 1E-127, 1E-126, 1E-125, 1E-124, 1E-123, 1E-122, 1E-121, 1E-120, 1E-119, 1E-118, 1E-117, 1E-116, 1E-115, 1E-114, 1E-113, 1E-112, 1E-111, 1E-110, 1E-109, 1E-108, 1E-107, 1E-106, 1E-105, 1E-104, 1E-103, 1E-102, 1E-101, 1E-100, 1E-099, 1E-098, 1E-097, 1E-096, 1E-095, 1E-094, 1E-093, 1E-092, 1E-091, 1E-090, 1E-089, 1E-088, 1E-087, 1E-086, 1E-085, 1E-084, 1E-083, 1E-082, 1E-081, 1E-080, 1E-079, 1E-078, 1E-077, 1E-076, 1E-075, 1E-074, 1E-073, 1E-072, 1E-071, 1E-070, 1E-069, 1E-068, 1E-067, 1E-066, 1E-065, 1E-064, 1E-063, 1E-062, 1E-061, 1E-060, 1E-059, 1E-058, 1E-057, 1E-056, 1E-055, 1E-054, 1E-053, 1E-052, 1E-051, 1E-050, 1E-049, 1E-048, 1E-047, 1E-046, 1E-045, 1E-044, 1E-043, 1E-042, 1E-041, 1E-040, 1E-039, 1E-038, 1E-037, 1E-036, 1E-035, 1E-034, 1E-033, 1E-032, 1E-031, 1E-030, 1E-029, 1E-028, 1E-027, 1E-026, 1E-025, 1E-024, 1E-023, 1E-022, 1E-021, 1E-020, 1E-019, 1E-018, 1E-017, 1E-016, 1E-015, 1E-014, 1E-013, 1E-012, 1E-011, 1E-010, 1E-009, 1E-008, 1E-007, 1E-006, 1E-005, 1E-004, 1E-003, 1E-002, 1E-001, 1E+000, 1E+001, 1E+002, 1E+003, 1E+004, 1E+005, 1E+006, 1E+007, 1E+008, 1E+009, 1E+010, 1E+011, 1E+012, 1E+013, 1E+014, 1E+015, 1E+016, 1E+017, 1E+018, 1E+019, 1E+020, 1E+021, 1E+022, 1E+023, 1E+024, 1E+025, 1E+026, 1E+027, 1E+028, 1E+029, 1E+030, 1E+031, 1E+032, 1E+033, 1E+034, 1E+035, 1E+036, 1E+037, 1E+038, 1E+039, 1E+040, 1E+041, 1E+042, 1E+043, 1E+044, 1E+045, 1E+046, 1E+047, 1E+048, 1E+049, 1E+050, 1E+051, 1E+052, 1E+053, 1E+054, 1E+055, 1E+056, 1E+057, 1E+058, 1E+059, 1E+060, 1E+061, 1E+062, 1E+063, 1E+064, 1E+065, 1E+066, 1E+067, 1E+068, 1E+069, 1E+070, 1E+071, 1E+072, 1E+073, 1E+074, 1E+075, 1E+076, 1E+077, 1E+078, 1E+079, 1E+080, 1E+081, 1E+082, 1E+083, 1E+084, 1E+085, 1E+086, 1E+087, 1E+088, 1E+089, 1E+090, 1E+091, 1E+092, 1E+093, 1E+094, 1E+095, 1E+096, 1E+097, 1E+098, 1E+099, 1E+100, 1E+101, 1E+102, 1E+103, 1E+104, 1E+105, 1E+106, 1E+107, 1E+108, 1E+109, 1E+110, 1E+111, 1E+112, 1E+113, 1E+114, 1E+115, 1E+116, 1E+117, 1E+118, 1E+119, 1E+120, 1E+121, 1E+122, 1E+123, 1E+124, 1E+125, 1E+126, 1E+127, 1E+128, 1E+129, 1E+130, 1E+131, 1E+132, 1E+133, 1E+134, 1E+135, 1E+136, 1E+137, 1E+138, 1E+139, 1E+140, 1E+141, 1E+142, 1E+143, 1E+144, 1E+145, 1E+146, 1E+147, 1E+148, 1E+149, 1E+150, 1E+151, 1E+152, 1E+153, 1E+154, 1E+155, 1E+156, 1E+157, 1E+158, 1E+159, 1E+160, 1E+161, 1E+162, 1E+163, 1E+164, 1E+165, 1E+166, 1E+167, 1E+168, 1E+169, 1E+170, 1E+171, 1E+172, 1E+173, 1E+174, 1E+175, 1E+176, 1E+177, 1E+178, 1E+179, 1E+180, 1E+181, 1E+182, 1E+183, 1E+184, 1E+185, 1E+186, 1E+187, 1E+188, 1E+189, 1E+190, 1E+191, 1E+192, 1E+193, 1E+194, 1E+195, 1E+196, 1E+197, 1E+198, 1E+199, 1E+200, 1E+201, 1E+202, 1E+203, 1E+204, 1E+205, 1E+206, 1E+207, 1E+208, 1E+209, 1E+210, 1E+211, 1E+212, 1E+213, 1E+214, 1E+215, 1E+216, 1E+217, 1E+218, 1E+219, 1E+220, 1E+221, 1E+222, 1E+223, 1E+224, 1E+225, 1E+226, 1E+227, 1E+228, 1E+229, 1E+230, 1E+231, 1E+232, 1E+233, 1E+234, 1E+235, 1E+236, 1E+237, 1E+238, 1E+239, 1E+240, 1E+241, 1E+242, 1E+243, 1E+244, 1E+245, 1E+246, 1E+247, 1E+248, 1E+249, 1E+250, 1E+251, 1E+252, 1E+253, 1E+254, 1E+255, 1E+256, 1E+257, 1E+258, 1E+259, 1E+260, 1E+261, 1E+262, 1E+263, 1E+264, 1E+265, 1E+266, 1E+267, 1E+268, 1E+269, 1E+270, 1E+271, 1E+272, 1E+273, 1E+274, 1E+275, 1E+276, 1E+277, 1E+278, 1E+279, 1E+280, 1E+281, 1E+282, 1E+283, 1E+284, 1E+285, 1E+286, 1E+287, 1E+288, 1E+289, 1E+290, 1E+291, 1E+292, 1E+293, 1E+294, 1E+295, 1E+296, 1E+297, 1E+298, 1E+299, 1E+300, 1E+301, 1E+302, 1E+303, 1E+304, 1E+305, 1E+306, 1E+307, 1E+308 };
    long long b = 0, e1 = 0, e2 = 0;
    bool is_exp = false;
    do
    {
        bool negate = s != end && *s == '-';
        if (s != end && (*s == '-' || *s == '+')) { ++s; }
        bool decimal = false;
        long long &r = is_exp ? e2 : b;
        while (s != end && (*s == '.' || '0' <= *s && *s <= '9'))
        {
            if (*s != '.')
            {
                e1 -= decimal;
                char const digit = *s - '0';
                if (static_cast<unsigned long long>(r) < static_cast<unsigned long long>(r) * 10 + static_cast<unsigned char>(digit))
                {
                    r *= 10;
                    r += digit;
                }
            }
            else { decimal = true; }
            ++s;
        }
        r = negate ? -r : +r;
    } while ((is_exp = !is_exp, is_exp) && s != end && ((*s | ('e' ^ 'E')) == 'e') && (++s, is_exp));
    double const result = b * pow10[323 + (e1 + e2)];
    return result;
}

Answer 2

For C++, working with streams is both much easier and nearly always much slower than using the C interfaces. However, I suspect that the speed of various C interfaces will depend on their implementation. atof may be faster than strtod on one platform, and slower on another.

Personally, I would look at fast ways to read the file, not necessarily fast ways to parse the doubles. And your fastest ways to read files are almost always platform specific APIs (memory mapped files, scatter/gather I/O, etc.). So it's very hard to give you an answer that will be the fastest way possible, because that's very platform specific and will change in the future.