Click here to download sortpoints.c.

/*************************************************
 ** This program reads in points from the user and
 ** prints them back out in sorted order from
 ** closest to the origin to farthest.
 *************************************************/
#include <stdio.h>
#include <stdlib.h>
#include <math.h>

/*************************************************
 ** PROTOTYPES
 *************************************************/
void selectionSort(double** data, int size);

/*************************************************
 ** MAIN
 *************************************************/
int main() {
  // Get number of points and create & read array
  int n;
  printf("How many points? ");
  fflush(stdout);
  scanf(" %d", &n);
  printf("Enter points (x,y,z): ");
  fflush(stdout);

  double** points = calloc(n, sizeof(double*));
  for(int i = 0; i < n; i++) {
    points[i] = calloc(3, sizeof(double));
    scanf(" ( %lg , %lg , %lg )", &points[i][0], &points[i][1], &points[i][2]);
  }

  // Sort points!
  selectionSort(points, n);

  // Print out points
  for(int i = 0; i < n; i++) {
    printf("(%g,%g,%g) ", points[i][0], points[i][1], points[i][2]);
  }
  printf("\n");

  // clean up
  for (int i=0; i < n; ++i) {
    free(points[i]);
  }
  free(points);

  return 0;
}

/*************************************************
 ** DEFINITIONS
 *************************************************/
int before(double* a, double* b) {
  return sqrt(a[0]*a[0] + a[1]*a[1] + a[2]*a[2])
         <
         sqrt(b[0]*b[0] + b[1]*b[1] + b[2]*b[2]);
}

void selectionSort(double** data, int size) {
  for(int length = size; length > 1; length--) {
    // Find imax, the index of the largest
    int imax = 0;
    for(int i = 1; i < length; i++) {
      if (before(data[imax], data[i])) {
        imax = i;
      }
    }
    // Swap data[imax] & the last element
    double* temp = data[imax];
    data[imax] = data[length - 1];
    data[length - 1] = temp;
  }
}