Question

I’m trying to optimize my solution to a problem, which requires fast double scanning. I tried to implement a function which read a double from the standard input, but I failed. Could someone point me some simple code which implements this efficiently? Thanks in advance.

Note here is my attempt, which seems to have some problems:

#include <stdio.h>
#include <stdlib.h>

inline double getDouble(FILE *f = stdin) {
    char tmp[20], ch;
    bool seen = false;
    double sign = 1.0;
    short index = 0;

    while((ch = getc(stdin)) != EOF) {
        if(ch == '-') {
            sign = -1.0;
            continue;
        }
        if(ch == ' ' || ch == '\n') {
            if(seen) break;
        } else {
            seen = true;
            tmp[index++] = ch;
        }
    }
    return sign * (double)atof(tmp);
}

int main() {
    int n;
    scanf("%d", &n);

    double *d = new double[n];

    for(int i=0; i<n; ++i) {
        d[i] = getDouble();
    }

    for(int i=0; i<n; ++i) {
        printf("%.5lf\n", d[i]);
    }

    return 0;
}

Input:

16
-2 -1 -4 -5
1 1 1 1
1.233 -435 -2.44
3
2 3 42 4 

Answer 1

#include <stdio.h>
#include <stdlib.h>

inline double getDouble(FILE *f = stdin) {
    double d;
    scanf("%lf", &d);
    return d;
}

int main() {
    int n;
    scanf("%d", &n);

    double *d = new double[n];

    for(int i=0; i<n; ++i) {
        d[i] = getDouble();
    }

    for(int i=0; i<n; ++i) {
        printf("%.5lf\n", d[i]);
    }

    return 0;
}

Edit: It can indeed be some faster as this, I estimate the following to be 2 to 3 times as fast, it will pass your input, but takes quite some assumptions, no guarantees outside the test sample 🙂

inline double getDouble(FILE *f = stdin) {
    char ch;
    bool seen = false;
    bool sign = false;
    char values[10];
    double result =0.;
    bool beforeDot = true;
    int beforeLength = 0;
    double multiplier;

    while((ch = getc(stdin)) != EOF) {
        if(ch == '-') {
            sign = true;
            continue;
        }
        if(ch == ' ' || ch == '\n') {
            if(seen) break;
            continue;
        }
        if(ch == '.') {
            beforeDot = false;
            multiplier = 1.;
            while(beforeLength) {
                result += (double)(values[--beforeLength] - '0') * multiplier;
                multiplier *= 10.;
            }
            multiplier = 10.;
        }
        else {
            if(!beforeDot)
            {
                result += double(ch - '0') / multiplier;
                multiplier *= 10.;
            } else {
                values[beforeLength++] = ch;
            }
            seen = true;
        }
    }
    if(beforeDot) {
        multiplier = 1;
        while(beforeLength) {
            result += (double)(values[--beforeLength] - '0') * multiplier;
            multiplier *= 10.;
        }
    }

    if(sign) result *= -1.;

    return result;
}