为结果添加fopen时程序无法正常工作

Question

I'm trying to generate a tabulation file for 1000 data points using this code below, the program works when I don't add the File Creation and Opening section, but when this is added the program doesn't end once I've entered the inputs.

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


void mapfun(int imap, double *c, double xi, double yi, double *xo, double *yo);
void mapspin(int imap, double *c, int nspin, double *xo, double *yo);

int main(){
   /* Defining values */
   int imap;
   int nspin=1000;
   double xo;
   double yo;
   double c[4];
   float mu;
   /* Ask for an input of 1 or 2 */
   printf("Input 1 or 2\n");
   scanf("%d", &imap);
   /* Assigning c when input is 1 and assigning mu */
   if(imap==1){
      c[1]=0.04;
      c[2]=0.04;
      printf("Input a value for mu between 2.8 and 3.8\n");
      scanf("%e:%d", &mu);
      if(mu>2.8 && mu<3.8){
         c[0]=mu;
         c[3]=mu;
      } else{
         printf("Invalid value for mu\n");
         exit(0);
      }             
      /* Assigning c when input is 2 and assigning mu */
   } else if(imap==2){
      c[1]=1;
      c[2]=1;
      printf("Input a value for mu between 1 and 3\n");
      scanf("%e:%d", &mu);
      if(mu>1 && mu<3){
         c[0]=mu;
         c[3]=mu;
      } else{
         printf("Invalid value for mu\n");
         exit(0);
      }
   } else{
      printf("Invalid value entered\n");
      exit(0);
   }

   mapspin(imap, c, nspin, &xo, &yo);

   /*File Creation and Opening*/
   FILE*orbit;
   orbit = fopen("orbit.dat", "rb+");
   if(orbit == NULL) {
      /* creates file if not there */
      orbit = fopen("orbit.dat", "wb");
   }

   double xi=xo;
   double yi=yo;
   int i;
   for(i=0; i<nspin; i+1){
      mapfun(imap, c, xi, yi, &xo, &yo);
      xi=xo;
      yi=yo;
      fprintf(orbit, "xo = %3f, yo = %3f\n", xo, yo);
   }
   orbit = fopen("orbit.dat", "r");
   printf("c[0]= %.2f, c[1]= %.2f, c[2]= %.2f, c[3]= %.2f\n", c[0], c[1], c[2], c[3]);
   exit(0);
}

Answer 1

Main problem:

The for loop is not well formed.

for(i=0; i<nspin; i+1){
                  ^^^ This does not change the value of i
                  ^^^ That explains why the loop never ends.

Use:

for(i=0; i<nspin; ++i){

Another issue:

I don't understand why you have the line:

   orbit = fopen("orbit.dat", "r");

That seems to be an unnecessary line.

Additional Info

The following program illustrates how you can accidentally modify a value in a function called from main .

#include <stdio.h>

void foo(int* x)
{
   x[1] = 20;
}

int main()
{
   int a = 10;
   int b;

   printf("Value of a before call to foo: %d\n", a);

   foo(&b); // foo has no direct access to "a" but it can 
            // indirectly access "b" by using an out of
            // bounds index.

   printf("Value of a after call to foo: %d\n", a);
}

I built the program using gcc 4.8.2. When I run the program, the output is:

Value of a before call to foo: 10
Value of a after call to foo: 20

Answer 2

You write ascii/text into a binary open file with the b option. I think if the file exists you just have to append by using a fopen option and ths will create the file if it doesn't exists:

orbit = fopen("orbit.dat", "a");
if(orbit == NULL) {
    //IO Error cannot open the file in append mode or create the file
    exit(1);
}

And don't reuse a file descriptor variable without doing fclose call:

    fprintf(orbit, "xo = %3f, yo = %3f\n", xo, yo);
    //Fprintf Return value must be checked too
}
if( 0 != fclose( orbit ) )
{
    //IO Error cannot close the opened file
    exit(1);
}

orbit = fopen("orbit.dat", "r");
if( orbit == NULL )
{
    //IO Error cannot open the file for reading
    exit(1);
}

Each IO call return value/errno must be checked!

And the main problem reported by @r-sahu is the for loop.