编写double转字符串函数：

不用库函数，自己编写一个函数，编写double转字符串函数：void DouleToStr(double d,int n,char s[])，n是小数位数（精度），支持负数；

#define _CRT_SECURE_NO_WARNINGS
#include <iostream>
#include <cstring>
#include <cstdlib>
using namespace std;

void DouleToStr(double d, int n, char s[])
{
    n = n < 0 ? 0 : n;
    char dd[20];
    switch (n) {
    case 0:
        sprintf(dd, "%.0lf", d);
        break;
    case 1:
        sprintf(dd, "%.1lf", d);
        break;
    case 2:
        sprintf(dd, "%.2lf", d);
        break;
    case 3:
        sprintf(dd, "%.3lf", d);
        break;
    case 4:
        sprintf(dd, "%.4lf", d);
        break;
    case 5:
        sprintf(dd, "%.5lf", d);
        break;
    case 6:
        sprintf(dd, "%.6lf", d);
        break;
    case 7:
        sprintf(dd, "%.7lf", d);
        break;
    default:
        sprintf(dd, "%.8lf", d);
        break;
    }
    char *p = (char*)malloc(strlen(dd));
    strcpy(p, dd);
    strcpy(s, p);
}

int main()
{
    double d = -3.1415926;
    char *dstr = (char*)malloc(20);;
    DouleToStr(d,8,dstr);
    cout << dstr;
    return 0;
}