/* * * * * * * * * * * * * * * * * * * * * * * * * * * * * *

This program calculates the

1. Center of mass for a given system of particles.
   All particles need not have the same mass.

2. Distance to each particle from the center of mass.


INPUT :

Input file must have the name 'center-of-mass-input.xyz' and the data
must be in the following format (space or tab separated)

First Line   : Number of particles, N
Next N Lines : x, y, z and M of each particle


DESCRIPTION OF VARIABLES :

Nbuf     First line of the input file, stored as string
xbuf     x co-ordinate of the particle, stored as string
ybuf     y co-ordinate of the particle, stored as string
zbuf     z co-ordinate of the particle, stored as string
Mbuf     Mass of the particle, stored as string

N        Nbuf converted to integer, using atoi()
         Number of particles

x[i]     xbuf converted to double precision number, using atof()
         One dimensional array for storing x co-ordinate of N particles  

y[i]     ybuf converted to double precision number, using atof()
         One dimensional array for storing y co-ordinate of N particles   

z[i]     zbuf converted to double precision number, using atof()
         One dimensional array for storing z co-ordinate of N particles

M[i]     Mbuf converted to integer, using atoi()
         One dimensional array for storing mass of N particles

COM_X    Sum of all x co-ordinates
COM_Y    Sum of all y co-ordinates
COM_Z    Sum of all z co-ordinates
M_TOT    Total mass
DIST     Distance to each particle from the center of mass


COMPILATION/EXECUTION PROCEDURE :

gcc -o center-of-mass.x center-of-mass.c -lm
./center-of-mass.x


OUTPUT :

If the compilation and execution is successful, the program 
writes an output file called 'center-of-mass-output.xyz' 
containing the

1. Center of mass, and
2. Distance to each particle from the center of mass.


Writtten by Gowtham, with valuable tips from H. A. Sawant
(hasawant@mtu.edu) in using atoi().

Question and/or comments should be sent to mail@sgowtham.net

* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * */

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

main() 
{

/* Defines input and output files */
  FILE *input;
  FILE *output;

/* Declaration and initialization of variables */
  char   Nbuf[100]={'\0'};
  char   xbuf[100]={'\0'};
  char   ybuf[100]={'\0'};
  char   zbuf[100]={'\0'};
  char   Mbuf[100]={'\0'};

  double   x[100]={0};
  double   y[100]={0};
  double   z[100]={0};
  int      M[100]={0};

  int     i, N, M_TOT;
  double  COM_X, COM_Y, COM_Z, DIST;
  
  M_TOT=0;
  COM_X=0.0;
  COM_Y=0.0;
  COM_Z=0.0;
  DIST=0.0;

/* Opens the input file for reading */
  input = fopen("center-of-mass-input.xyz", "r");

  if (input == NULL) {
    printf("\nInput File empty or corrupted! Exiting...\n");
    exit (1);
  }

/* Creates the output file and opens it for writing */
  output = fopen("center-of-mass-output.xyz", "w");

/* Reads the first line of the input file and stores it as the
   number of particles */
  fscanf(input, "%s", Nbuf);  /* reads one line */
  N = atoi(Nbuf);

/* Reads and stores the x, y, z and M for each particle, first as
   a string (buffer) and later stores it as an array element using
   appropriate conversion mechanism */
  for(i=0; i<N; i++) {
    fscanf(input, "%s", xbuf);
    x[i]= atof(xbuf);
  
    fscanf(input, "%s", ybuf);
    y[i]= atof(ybuf);
  
    fscanf(input, "%s", zbuf);
    z[i]= atof(zbuf);
      
    fscanf(input, "%s", Mbuf);
    M[i]= atoi(Mbuf);
  }
  
/* Writes the co-ordinates of each particle 
   This is just to make the output file 'complete' */
  fprintf(output, "INPUT (center-of-mass-input.xyz)\n");
  fprintf(output, "\nParticle \tX \t\tY \t\tZ \t\tM");
  
  for(i=0; i<N; i++) {
    fprintf(output, "\n%d",    i+1);
    fprintf(output, "\t\t%lf", x[i]);
    fprintf(output, "\t%lf",   y[i]);
    fprintf(output, "\t%lf",   z[i]);
    fprintf(output, "\t%d",    M[i]);
  }

/* Calculates the center of mass and writes them in the 
   output file */
  for(i=0; i<N; i++) {
    COM_X = COM_X + x[i]*M[i];
    COM_Y = COM_Y + y[i]*M[i];
    COM_Z = COM_Z + z[i]*M[i];
    M_TOT = M_TOT + M[i];
  }

  COM_X = COM_X / M_TOT;  /* X co-ordinate */
  COM_Y = COM_Y / M_TOT;  /* Y co-ordinate */
  COM_Z = COM_Z / M_TOT;  /* Z co-ordinate */

  fprintf(output, "\n\nOUTPUT - Part 1 : Co-ordinates of center of mass\n\n");
  fprintf(output, "\tx : %lf", COM_X);
  fprintf(output, "\ty : %lf", COM_Y);
  fprintf(output, "\tz : %lf", COM_Z);


/* Calculates the distance to each particle from the center of 
   mass and writes them in the output file */
  fprintf(output, "\n\nOUTPUT - Part 2 : Distance to each particle from the center of mass\n\n");
  
  fprintf(output, "\tParticle \tDistance");
  
  for(i=0; i<N; i++) {
    DIST = sqrt((COM_X-x[i])*(COM_X-x[i]) + (COM_Y-y[i])*(COM_Y-y[i]) + (COM_Z-z[i])*(COM_Z-z[i]));
    
    fprintf(output, "\n\t %d \t\t %lf", i+1, DIST);
  }

/* Closes the input and output files */
  fclose(input);
  fclose(output);

/* Confirmation to the user */
  printf("Please open center-of-mass-output.xyz for results\n\n");

/* Program Ends */
}