Program won't work when adding fopen for results - c

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 boption. 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.