c++ - Reading and writing double precision from/to files -

i trying write double arrays files , read them again. below code, there missing. sounds silly cannot right.

#include <stdio.h> #include <stdlib.h>  int main(){    int i,j,k;   int n = 10;    double* readin = new double[n];   double* ref = new double[n];   file* ptr1, *ptr2;    ptr1 = fopen("output.txt","w");   //write out   (i = 0; < n;i++){     ref[i] = (double)i;     fprintf(ptr1,"%g\n",(double)i);   }   fclose(ptr1);   //read file   ptr2 = fopen("output.txt","r+");   //read in   for(i = 0;i < n;i++)     fscanf(ptr2, "%g", &readin[i]);      fclose(ptr2);    for(i = 0;i<n;i++)     if(ref[i] != readin[i]){       printf("error:  %g   %g\n",ref[i], readin[i]);     }    return 0;  } 

your fscanf using wrong format string (which gcc tell if enable sufficient warnings).

so double filled float value, of course leads rather "random" errors.

if change "%g" "%lg", should work fine (at least on linux box).

of course, if use c++ streams, e.g.

 #include <fstream>   std::ofstream file1;  std::ifstream file2;   file1.open("output.txt");   (i = 0; < n;i++){     ref[i] = (double)i;     file1 << (double)i << std::endl;  } 

and

   file2.open("output.txt");    for(i = 0;i < n;i++)       file2 >> readin[i];  

the whole problem have been avoided - , if edit readin variable float, long values valid that, possible read values without chaning else [assuming output using cout instead of printf, of course].