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I am trying to create some functions to handle vector and matrix operations. The vector sum function works as expected, but the dot product function returns always zeros. What I am doing wrong?
Also, I am not sure if this is the best way to handle the problem. I am creating a script for nonlinear optimization. My idea is to allocate memory for the auxiliary scalars, vectors and matrices that are reused on each iteration. The functions are void type to so I can keep track of all variables created in the program.
#include <stdio.h>
void dot (const double *v_1, const double *v_2, double s_out)
{
s_out = v_1[0] * v_2[0] + v_1[1] * v_2[1];
}
void sum (double *v_1, double *v_2, double *v_out)
{
v_out[0] = v_1[0] + v_2[0];
v_out[1] = v_1[1] + v_2[1];
}
int main ()
{
double *r;
double *t;
r = malloc(sizeof(double)*2);
t = malloc(sizeof(double)*2);
r[0] = 1; r[1] = 2;
double rho_new = 0;
dot (r, r, rho_new);
printf("rho_new = %lf\n", rho_new);
sum (r, r, t);
printf("t = %lf %lf\n", t[0], t[1]);
}
Two options to return value from the function.
#include <stdio.h>
double dot (const double *v_1, const double *v_2)
{
return v_1[0] * v_2[0] + v_1[1] * v_2[1];
}
void sum (double *v_1, double *v_2, double *v_out)
{
v_out[0] = v_1[0] + v_2[0];
v_out[1] = v_1[1] + v_2[1];
}
int main ()
{
double *r;
double *t;
r = malloc(sizeof(double)*2);
t = malloc(sizeof(double)*2);
r[0] = 1; r[1] = 2;
double rho_new = 0;
rho_new = dot(r, r, rho_new);
printf("rho_new = %lf\n", rho_new);
sum (r, r, t);
printf("t = %lf %lf\n", t[0], t[1]);
}
#include <stdio.h>
void dot (const double *v_1, const double *v_2, double *s_out)
{
*s_out = v_1[0] * v_2[0] + v_1[1] * v_2[1];
}
void sum (double *v_1, double *v_2, double *v_out)
{
v_out[0] = v_1[0] + v_2[0];
v_out[1] = v_1[1] + v_2[1];
}
int main ()
{
double *r;
double *t;
r = malloc(sizeof(double)*2);
t = malloc(sizeof(double)*2);
r[0] = 1; r[1] = 2;
double rho_new = 0;
dot (r, r, &rho_new);
printf("rho_new = %lf\n", rho_new);
sum (r, r, t);
printf("t = %lf %lf\n", t[0], t[1]);
}
I'm new to C and I'm trying to calculate the kinetic energy from certain values (px and py) but my routine accuses always as main mistake subscripted value is neither array nor pointer vector nor. Can anyone help me because all I know I've tried.
The error occurs in the kinetic function
thanks
My routine
#include <stdio.h>
#include <stdlib.h>
#ifndef RAN2_H_
#define RAN2_H_
void UniformBox(long, long *, double, double, double *, double *, double *, double *);
void kinetic(long, double, double, double);
#endif /* RAN2_H_ */
#include "ran2.h"
float ran2(long*);
void UniformBox(long n, long *idum, double L, double p0, double *rx, double *ry, double *px, double *py){
long i;
for(i = 0 ; i < n ; i++){
rx[i] = ((double)ran2(idum))*L;
ry[i] = ((double)ran2(idum))*L;
px[i] = ((double)ran2(idum) -.5)*2*p0;
py[i] = ((double)ran2(idum) -.5)*2*p0;
}
return;
}
void kinetic(long n, double x, double y, double kint){
long i;
for(i = 0 ; i < n ; i++){
kint[i] = (x*x)+(y*y);
}
return;
}
void UniformBox(long, long *, double, double, double *, double *, double *, double *);
void kinetic(long n, double x, double y, double kint);
int main(){
long i, n, seed, idum;
double *rx, *ry, *px, *py;
double L, p0, kint;
n = 1000;
L = 2.0;
p0 = 1.22;
seed = 10;
idum = -seed;
FILE *init = fopen("initialPosition.dat", "w");
rx = (double *) malloc((double) n * sizeof(double));
ry = (double *) malloc((double) n * sizeof(double));
px = (double *) malloc((double) n * sizeof(double));
py = (double *) malloc((double) n * sizeof(double));
UniformBox(n, &idum, L, p0, rx, ry, px, py);
kinetic(n, px, py, kint);
for(i = 0 ; i < n ; i++){
printf("%lf\t%lf\t%lf\t%lf\t%lf\n", rx[i], ry[i], px[i], py[i], kint[i]);
fprintf(init,"%lf\t%lf\t%lf\t%lf\n", rx[i], ry[i], px[i], py[i]);
}
fclose(init);
free(rx);
free(ry);
free(px);
free(py);
return 0;
}
I've fixed the compile errors. A few things to note.
I removed multiple incompatible function prototypes.
Aside from kinetic's kint needing to be a pointer, x and y needed to be pointers. Note that this is conjecture on my part, based on the totality of the code. Also note the change inside the loop from x to x[i]. Otherwise, things didn't make sense [to me].
In main, I added a malloc for kint
Anyway, here's the code [please pardon the gratuitous style cleanup]:
#include <stdio.h>
#include <stdlib.h>
#ifndef RAN2_H_
#define RAN2_H_
float ran2(long *);
#endif /* RAN2_H_ */
//#include "ran2.h"
void
UniformBox(long n, long *idum, double L, double p0, double *rx, double *ry,
double *px, double *py)
{
long i;
for (i = 0; i < n; i++) {
rx[i] = ((double) ran2(idum)) * L;
ry[i] = ((double) ran2(idum)) * L;
px[i] = ((double) ran2(idum) - .5) * 2 * p0;
py[i] = ((double) ran2(idum) - .5) * 2 * p0;
}
return;
}
void
kinetic(long n, double *x, double *y, double *kint)
{
long i;
for (i = 0; i < n; i++) {
kint[i] = (x[i] * x[i]) + (y[i] * y[i]);
}
return;
}
int
main()
{
long i, n;
long seed, idum;
double *rx, *ry;
double *px, *py;
double L, p0;
double *kint;
n = 1000;
L = 2.0;
p0 = 1.22;
seed = 10;
idum = -seed;
FILE *init = fopen("initialPosition.dat", "w");
rx = (double *) malloc((double) n * sizeof(double));
ry = (double *) malloc((double) n * sizeof(double));
px = (double *) malloc((double) n * sizeof(double));
py = (double *) malloc((double) n * sizeof(double));
kint = (double *) malloc((double) n * sizeof(double));
UniformBox(n, &idum, L, p0, rx, ry, px, py);
kinetic(n, px, py, kint);
for (i = 0; i < n; i++) {
printf("%lf\t%lf\t%lf\t%lf\t%lf\n", rx[i], ry[i], px[i], py[i], kint[i]);
fprintf(init, "%lf\t%lf\t%lf\t%lf\n", rx[i], ry[i], px[i], py[i]);
}
fclose(init);
free(rx);
free(ry);
free(px);
free(py);
return 0;
}
In C , you must pass arrays into a function by reference. Thus, your function header should be
void kinetic(long n, double x, double y, double * kint);
You could also use the [] notation like is suggested in the comments.
I just got GSL set up on my windows box and I am trying to learn how to use the nonlinear fitting functions. First thing I did was pull an example directly off their website: https://www.gnu.org/software/gsl/manual/html_node/Example-programs-for-Nonlinear-Least_002dSquares-Fitting.html
which is here:
#include <stdlib.h>
#include <stdio.h>
#include <stdlib.h>
#include <stdio.h>
#include <gsl/gsl_rng.h>
#include <gsl/gsl_randist.h>
#include <gsl/gsl_vector.h>
#include <gsl/gsl_blas.h>
#include <gsl/gsl_multifit_nlin.h>
#define N 40
#define FIT(i) gsl_vector_get(s->x, i)
#define ERR(i) sqrt(gsl_matrix_get(covar,i,i))
struct data {
size_t n;
double * y;
double * sigma;
};
int expb_f (const gsl_vector * x, void *data, gsl_vector * f)
{
size_t n = ((struct data *)data)->n;
double *y = ((struct data *)data)->y;
double *sigma = ((struct data *) data)->sigma;
double A = gsl_vector_get (x, 0);
double lambda = gsl_vector_get (x, 1);
double b = gsl_vector_get (x, 2);
size_t i;
for (i = 0; i < n; i++)
{
/* Model Yi = A * exp(-lambda * i) + b */
double t = i;
double Yi = A * exp (-lambda * t) + b;
gsl_vector_set (f, i, (Yi - y[i])/sigma[i]);
}
return GSL_SUCCESS;
}
int expb_df (const gsl_vector * x, void *data, gsl_matrix * J)
{
size_t n = ((struct data *)data)->n;
double *sigma = ((struct data *) data)->sigma;
double A = gsl_vector_get (x, 0);
double lambda = gsl_vector_get (x, 1);
size_t i;
for (i = 0; i < n; i++)
{
/* Jacobian matrix J(i,j) = dfi / dxj, */
/* where fi = (Yi - yi)/sigma[i], */
/* Yi = A * exp(-lambda * i) + b */
/* and the xj are the parameters (A,lambda,b) */
double t = i;
double s = sigma[i];
double e = exp(-lambda * t);
gsl_matrix_set (J, i, 0, e/s);
gsl_matrix_set (J, i, 1, -t * A * e/s);
gsl_matrix_set (J, i, 2, 1/s);
}
return GSL_SUCCESS;
}
int expb_fdf (const gsl_vector * x, void *data, gsl_vector * f, gsl_matrix * J)
{
expb_f (x, data, f);
expb_df (x, data, J);
return GSL_SUCCESS;
}
void print_state (size_t iter, gsl_multifit_fdfsolver * s);
int main (void)
{
const gsl_multifit_fdfsolver_type *T;
gsl_multifit_fdfsolver *s;
int status;
unsigned int i, iter = 0;
const size_t n = N;
const size_t p = 3;
gsl_matrix *covar = gsl_matrix_alloc (p, p);
double y[N], sigma[N];
struct data d = { n, y, sigma};
gsl_multifit_function_fdf f;
double x_init[3] = { 1.0, 0.0, 0.0 };
gsl_vector_view x = gsl_vector_view_array (x_init, p);
const gsl_rng_type * type;
gsl_rng * r;
gsl_rng_env_setup();
type = gsl_rng_default;
r = gsl_rng_alloc (type);
f.f = &expb_f;
f.df = &expb_df;
f.fdf = &expb_fdf;
f.n = n;
f.p = p;
f.params = &d;
/* This is the data to be fitted */
for (i = 0; i < n; i++)
{
double t = i;
y[i] = 1.0 + 5 * exp (-0.1 * t) + gsl_ran_gaussian (r, 0.1);
sigma[i] = 0.1;
printf ("data: %u %g %g\n", i, y[i], sigma[i]);
};
T = gsl_multifit_fdfsolver_lmsder;
s = gsl_multifit_fdfsolver_alloc (T, n, p);
gsl_multifit_fdfsolver_set (s, &f, &x.vector);
print_state (iter, s);
do
{
iter++;
status = gsl_multifit_fdfsolver_iterate (s);
printf ("status = %s\n", gsl_strerror (status));
print_state (iter, s);
if (status)
break;
status = gsl_multifit_test_delta (s->dx, s->x,
1e-4, 1e-4);
}
while (status == GSL_CONTINUE && iter < 500);
gsl_multifit_covar (s->J, 0.0, covar);
{
double chi = gsl_blas_dnrm2(s->f);
double dof = n - p;
double c = GSL_MAX_DBL(1, chi / sqrt(dof));
printf("chisq/dof = %g\n", pow(chi, 2.0) / dof);
printf ("A = %.5f +/- %.5f\n", FIT(0), c*ERR(0));
printf ("lambda = %.5f +/- %.5f\n", FIT(1), c*ERR(1));
printf ("b = %.5f +/- %.5f\n", FIT(2), c*ERR(2));
}
printf ("status = %s\n", gsl_strerror (status));
gsl_multifit_fdfsolver_free (s);
gsl_matrix_free (covar);
gsl_rng_free (r);
return 0;
}
void print_state (size_t iter, gsl_multifit_fdfsolver * s)
{
printf ("iter: %3u x = % 15.8f % 15.8f % 15.8f "
"|f(x)| = %g\n",
iter,
gsl_vector_get (s->x, 0),
gsl_vector_get (s->x, 1),
gsl_vector_get (s->x, 2),
gsl_blas_dnrm2 (s->f));
}
Ideally it should simply generate a short data set that follows a decaying exponential with some white noise on top and then fit it.
To get it running in Code::Blocks in windows I followed the procedure outlined here: installing GSL on Windows XP 32bit for use with codeblocks
It compiles without warnings even with -Wall and -Wextra flags. However, it fails on the line: gsl_multifit_fdfsolver_set (s, &f, &x.vector); with the error: multifit\fdfsolver.c:132: ERROR: vector length does not match solver. Default GSL error handler invoked.
I was a little surprised to find this in what should be raw example code, but here we are. So I am hoping someone more knowledgeable than I can tell me what I am doing wrong with this simple example.
Figured it out: They were allocating their initial vector wrong. The fixed code is here:
#include <stdlib.h>
#include <stdio.h>
#include <stdlib.h>
#include <stdio.h>
#include <gsl/gsl_rng.h>
#include <gsl/gsl_randist.h>
#include <gsl/gsl_vector.h>
#include <gsl/gsl_blas.h>
#include <gsl/gsl_multifit_nlin.h>
#define N 40
#define FIT(i) gsl_vector_get(s->x, i)
#define ERR(i) sqrt(gsl_matrix_get(covar,i,i))
struct data {
size_t n;
double * y;
double * sigma;
};
int expb_f (const gsl_vector * x, void *data, gsl_vector * f)
{
size_t n = ((struct data *)data)->n;
double *y = ((struct data *)data)->y;
double *sigma = ((struct data *) data)->sigma;
double A = gsl_vector_get (x, 0);
double lambda = gsl_vector_get (x, 1);
double b = gsl_vector_get (x, 2);
size_t i;
for (i = 0; i < n; i++)
{
/* Model Yi = A * exp(-lambda * i) + b */
double t = i;
double Yi = A * exp (-lambda * t) + b;
gsl_vector_set (f, i, (Yi - y[i])/sigma[i]);
}
return GSL_SUCCESS;
}
int expb_df (const gsl_vector * x, void *data, gsl_matrix * J)
{
size_t n = ((struct data *)data)->n;
double *sigma = ((struct data *) data)->sigma;
double A = gsl_vector_get (x, 0);
double lambda = gsl_vector_get (x, 1);
size_t i;
for (i = 0; i < n; i++)
{
/* Jacobian matrix J(i,j) = dfi / dxj, */
/* where fi = (Yi - yi)/sigma[i], */
/* Yi = A * exp(-lambda * i) + b */
/* and the xj are the parameters (A,lambda,b) */
double t = i;
double s = sigma[i];
double e = exp(-lambda * t);
gsl_matrix_set (J, i, 0, e/s);
gsl_matrix_set (J, i, 1, -t * A * e/s);
gsl_matrix_set (J, i, 2, 1/s);
}
return GSL_SUCCESS;
}
int expb_fdf (const gsl_vector * x, void *data, gsl_vector * f, gsl_matrix * J)
{
expb_f (x, data, f);
expb_df (x, data, J);
return GSL_SUCCESS;
}
void print_state (size_t iter, gsl_multifit_fdfsolver * s);
int main (void)
{
const gsl_multifit_fdfsolver_type *T;
gsl_multifit_fdfsolver *s;
int status;
unsigned int i, iter = 0;
const size_t n = N;
const size_t p = 3;
gsl_matrix *covar = gsl_matrix_alloc (p, p);
double y[N], sigma[N];
struct data d = { n, y, sigma};
gsl_multifit_function_fdf f;
gsl_vector *x = gsl_vector_alloc(p);
for (i=0; i<p; i++)
{
gsl_vector_set(x,i,i==0 ? 1 : 0);
}
const gsl_rng_type * type;
gsl_rng * r;
gsl_rng_env_setup();
type = gsl_rng_default;
r = gsl_rng_alloc (type);
f.f = &expb_f;
f.df = &expb_df;
f.fdf = &expb_fdf;
f.n = n;
f.p = p;
f.params = &d;
/* This is the data to be fitted */
for (i = 0; i < n; i++)
{
double t = i;
y[i] = 1.0 + 5 * exp (-0.1 * t) + gsl_ran_gaussian (r, 0.1);
sigma[i] = 0.1;
printf ("data: %u %g %g\n", i, y[i], sigma[i]);
};
T = gsl_multifit_fdfsolver_lmsder;
s = gsl_multifit_fdfsolver_alloc (T, n, p);
gsl_multifit_fdfsolver_set (s, &f, x);
print_state (iter, s);
do
{
iter++;
status = gsl_multifit_fdfsolver_iterate (s);
printf ("status = %s\n", gsl_strerror (status));
print_state (iter, s);
if (status)
break;
status = gsl_multifit_test_delta (s->dx, s->x,
1e-4, 1e-4);
}
while (status == GSL_CONTINUE && iter < 500);
gsl_multifit_covar (s->J, 0.0, covar);
{
double chi = gsl_blas_dnrm2(s->f);
double dof = n - p;
double c = GSL_MAX_DBL(1, chi / sqrt(dof));
printf("chisq/dof = %g\n", pow(chi, 2.0) / dof);
printf ("A = %.5f +/- %.5f\n", FIT(0), c*ERR(0));
printf ("lambda = %.5f +/- %.5f\n", FIT(1), c*ERR(1));
printf ("b = %.5f +/- %.5f\n", FIT(2), c*ERR(2));
}
printf ("status = %s\n", gsl_strerror (status));
gsl_multifit_fdfsolver_free (s);
gsl_matrix_free (covar);
gsl_rng_free (r);
gsl_vector_free(x);
return 0;
}
void print_state (size_t iter, gsl_multifit_fdfsolver * s)
{
printf ("iter: %3u x = % 15.8f % 15.8f % 15.8f "
"|f(x)| = %g\n",
iter,
gsl_vector_get (s->x, 0),
gsl_vector_get (s->x, 1),
gsl_vector_get (s->x, 2),
gsl_blas_dnrm2 (s->f));
}
I'm not C expert and I've read through the forum, but I still need some advice regarding a sorting problem on C.
I have 4 dynamic arrays of doubles in C. All of them are the same size, and lets say n. What I want to do is to sort all of them using one of the arrays as first order and a second array as my second order. So if the arrays are *x, *y, *w and *z. I want to sort them according to the values of *x, then *y.
I must do this efficiently because the arrays are quite large.
Any help will be much appreciated.
The easy way to do this would be to map your four separate arrays onto a single array of a struct type like
struct rec {
double x;
double y;
double w;
double z;
};
struct rec *arr = malloc( sizeof *arr * N ); // where N is the number of
// elements in each array
if ( !arr )
// malloc failed, handle error somehow
for ( size_t i = 0; i < N; i++ )
{
arr[i].x = x[i];
arr[i].y = y[i];
arr[i].w = w[i];
arr[i].z = z[i];
}
and then create a comparison function to pass to qsort:
int cmpRec( const void *lhs, const void *rhs )
{
struct rec *l = lhs;
struct rec *r = rhs;
if ( l->x < r->x )
return -1;
else if ( l->x > r->x )
return 1;
else
{
if ( l->y < r->y )
return -1;
else if ( l->y > r->y )
return 1;
else
return 0;
}
return 0;
}
Now you can use the qsort library function to sort that array of struct:
qsort( arr, N, sizeof *arr, cmpRec );
Once that array is sorted, you can map the results back onto your four original arrays.
Clearly, sorting this using standard qsort() is not going to work; there isn't a mechanism for passing four arrays.
Equally clearly, if the data were structured as an array of structures, then using qsort() would be feasible.
Question 1: Is it feasible to create an array of structures, load it, sort it, and then unload back into the original arrays?
Question 2: Another option is to sort an array of integers:
int indexes[n];
for (int i = 0; i < n; i++)
indexes[i] = i;
qsort(indexes, n, sizeof(indexes[0]), comparator);
The comparator function would have to be able to access the x and y arrays as file scope variables:
int comparator(void const *v1, void const *v2)
{
int i1 = *(int *)v1;
int i2 = *(int *)v2;
extern double *x, *y;
if (x[i1] > x[i2])
return +1;
else if (x[i1] < x[i2])
return -1;
else if (y[i1] > y[i2])
return +1;
else if (y[i1] < y[i2])
return -1;
else
return 0;
}
You'd then be able to access the arrays using x[indexes[i]] etc to access the ith element in sorted order.
Is that acceptable?
If that is not convenient either, then you will end up writing your own sort; it isn't horribly painful, but will require some care.
I spent some time adapting an existing sort test framework to this scenario. The full code is quite large because it includes a lot of testing support code. The core function (compare, swap, partition and quicksort) are here (122 lines, including comment and blank lines):
/* SO 20271977 - sort arrays x, y, z, w (type double, size n) in parallel based on values in x and y */
/*
** To apply this to the real code, where there are 4 arrays to be sorted
** in parallel, you might write:
**
** Array4 a;
** a.x = x;
** a.y = y;
** a.z = z;
** a.w = w;
** a.n = n;
** quicksort_random(&a);
**
** Or even:
**
** quicksort_random((Array4){ .n = n, .x = x, .y = y, .z = z, .w = w });
**
** combining designated initializers and compound literals. Or you could write a
** trivial wrapper so that you can call:
**
** quicksort_random_wrapper(n, x, y, z, w);
*/
/* SOF so-20271977.h */
#include <stddef.h>
typedef struct Array4
{
size_t n;
double *x;
double *y;
double *z;
double *w;
} Array4;
extern void quicksort_random(Array4 *A);
/* EOF so-20271977.h */
#include <assert.h>
#include <stdlib.h> /* lrand48() */
/*
** Note that a more careful implementation would use nrand48() instead
** of lrand48() to prevent its random number generation from interfering
** with other uses of the x-rand48() functions.
*/
typedef size_t (*Part)(Array4 *A, size_t p, size_t r);
static void quicksort_partition(Array4 *A, size_t p, size_t r, Part partition);
static size_t partition_random(Array4 *A, size_t p, size_t r);
/* Quick Sort Wrapper function - specifying random partitioning */
void quicksort_random(Array4 *A)
{
quicksort_partition(A, 0, A->n - 1, partition_random);
}
/* Main Quick Sort function */
static void quicksort_partition(Array4 *A, size_t p, size_t r, Part partition)
{
if (p < r)
{
size_t q = (*partition)(A, p, r);
assert(p <= q && q <= r);
if (q > 0)
quicksort_partition(A, p, q-1, partition);
quicksort_partition(A, q+1, r, partition);
}
}
static inline int compare(Array4 const *A, size_t p, size_t r)
{
if (A->x[p] < A->x[r])
return -1;
else if (A->x[p] > A->x[r])
return +1;
if (A->y[p] < A->y[r])
return -1;
else if (A->y[p] > A->y[r])
return +1;
else
return 0;
}
static inline size_t random_int(size_t p, size_t r)
{
return(lrand48() % (r - p + 1) + p);
}
static inline void swap(Array4 *A, size_t i, size_t j)
{
double d;
d = A->x[i];
A->x[i] = A->x[j];
A->x[j] = d;
d = A->y[i];
A->y[i] = A->y[j];
A->y[j] = d;
d = A->z[i];
A->z[i] = A->z[j];
A->z[j] = d;
d = A->w[i];
A->w[i] = A->w[j];
A->w[j] = d;
}
static size_t partition_random(Array4 *A, size_t p, size_t r)
{
size_t pivot = random_int(p, r);
swap(A, pivot, r);
size_t i = p-1;
size_t j = p;
while (j <= r)
{
if (compare(A, j, r) <= 0)
swap(A, j, ++i);
j++;
}
return i;
}
The test framework (quite ridiculously elaborate if it weren't that I already had a variant of it on hand) is 369 lines including blank lines and comment lines — and all the code above:
#include <assert.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <time.h>
#define FLTFMT "%13.6f"
typedef struct Array4
{
size_t n;
double *x;
double *y;
double *z;
double *w;
} Array4;
static int trace = 0;
static void *xmalloc(size_t size)
{
void *space = malloc(size);
if (space == 0)
{
fprintf(stderr, "Out of memory (%zu)\n", size);
exit(1);
}
return space;
}
void quicksort_last(Array4 *A);
void quicksort_random(Array4 *A);
void selectionsort(Array4 *A);
static inline int compare(Array4 const *A, size_t p, size_t r)
{
if (A->x[p] < A->x[r])
return -1;
else if (A->x[p] > A->x[r])
return +1;
if (A->y[p] < A->y[r])
return -1;
else if (A->y[p] > A->y[r])
return +1;
else
return 0;
}
static void dump_array(char const *tag, Array4 const *A)
{
printf("%s [%zu..%zu]:\n", tag, (size_t)0, A->n-1);
for (size_t i = 0; i < A->n; i++)
printf("(" FLTFMT ", " FLTFMT ", " FLTFMT ", " FLTFMT ")\n",
A->x[i], A->y[i], A->z[i], A->w[i]);
}
static void chk_sort(Array4 const *A)
{
for (size_t i = 0; i < A->n - 1; i++)
{
//if (compare(A, i, i+1) > 0)
{
if (A->x[i] > A->x[i+1])
{
printf("Out of order: A.x[%zu] = " FLTFMT ", A.x[%zu] = " FLTFMT "\n",
i, A->x[i], i+1, A->x[i+1]);
}
else if ((A->x[i] == A->x[i+1] && A->y[i] > A->y[i+1]))
{
printf("Out of order: A.x[%zu] = " FLTFMT ", A.x[%zu] = " FLTFMT ", "
"A.y[%zu] = " FLTFMT ", A.y[%zu] = " FLTFMT "\n",
i, A->x[i], i+1, A->x[i+1], i, A->y[i], i+1, A->y[i+1]);
}
}
}
}
static inline void set(Array4 *A, size_t p, double d)
{
A->x[p] = d;
A->y[p] = d + drand48() - 0.5;
A->z[p] = d / 2.0;
A->w[p] = d * 2.0;
}
static void load_random(Array4 *A)
{
size_t size = A->n;
for (size_t i = 0; i < size; i++)
{
A->x[i] = drand48() * size;
A->y[i] = drand48() * size + drand48() - 0.5;
A->z[i] = drand48() * size / 2.0;
A->w[i] = drand48() * size * 2.0;
}
}
static void load_ascending(Array4 *A)
{
for (size_t i = 0; i < A->n; i++)
set(A, i, i);
}
static void load_descending(Array4 *A)
{
for (size_t i = 0; i < A->n; i++)
set(A, i, A->n - i);
}
static void load_uniform(Array4 *A)
{
for (size_t i = 0; i < A->n; i++)
set(A, i, A->n);
}
static void load_organpipe(Array4 *A)
{
for (size_t i = 0; i <= A->n / 2; i++)
set(A, i, i);
for (size_t i = A->n / 2 + 1; i < A->n; i++)
set(A, i, A->n - i);
}
static void load_invorganpipe(Array4 *A)
{
size_t range = A->n / 2;
for (size_t i = 0; i < A->n / 2; i++)
set(A, i, range - i);
for (size_t i = A->n / 2 + 1; i < A->n; i++)
set(A, i, i - range);
}
typedef void (*Load)(Array4 *A);
typedef void (*Sort)(Array4 *A);
typedef size_t (*Part)(Array4 *A, size_t p, size_t r);
static void test_one_sort(Array4 *A, Sort sort, char const *s_tag,
char const *l_tag, char const *z_tag)
{
if (trace)
{
printf("%s-%s-%s:", z_tag, l_tag, s_tag);
dump_array("Before", A);
}
clock_t start = clock();
(*sort)(A);
clock_t finish = clock();
double sec = (finish - start) / (double)CLOCKS_PER_SEC;
printf("%s-%s-%s: %13.6f\n", z_tag, l_tag, s_tag, sec);
chk_sort(A);
if (trace)
{
printf("%s-%s-%s:", z_tag, l_tag, s_tag);
dump_array("After", A);
}
fflush(stdout);
}
static Array4 *alloc_array(size_t size)
{
Array4 *A = xmalloc(sizeof(*A));
A->n = size;
A->x = xmalloc(size * sizeof(A->x[0]));
A->y = xmalloc(size * sizeof(A->y[0]));
A->z = xmalloc(size * sizeof(A->z[0]));
A->w = xmalloc(size * sizeof(A->w[0]));
return A;
}
static Array4 *dup_array(Array4 *A)
{
size_t size = A->n;
Array4 *B = alloc_array(size);
if (B != 0)
{
B->n = size;
memmove(B->x, A->x, size * sizeof(A->x[0]));
memmove(B->y, A->y, size * sizeof(A->y[0]));
memmove(B->z, A->z, size * sizeof(A->z[0]));
memmove(B->w, A->w, size * sizeof(A->w[0]));
}
return B;
}
static void free_array(Array4 *A)
{
free(A->x);
free(A->y);
free(A->z);
free(A->w);
free(A);
}
static void test_set_sorts(Array4 *A, char const *l_tag, char const *z_tag)
{
struct sorter
{
Sort function;
char const *tag;
} sort[] =
{
{ quicksort_last, "QS.L" },
{ quicksort_random, "QS.R" },
{ selectionsort, "SS.N" },
};
enum { NUM_SORTS = sizeof(sort) / sizeof(sort[0]) };
for (int i = 0; i < NUM_SORTS; i++)
{
Array4 *B = dup_array(A);
test_one_sort(B, sort[i].function, sort[i].tag, l_tag, z_tag);
free(B);
}
}
static void test_set_loads(size_t size, char const *z_tag)
{
struct loader
{
Load function;
char const *tag;
} load[] =
{
{ load_random, "R" },
{ load_ascending, "A" },
{ load_descending, "D" },
{ load_organpipe, "O" },
{ load_invorganpipe, "I" },
{ load_uniform, "U" },
};
enum { NUM_LOADS = sizeof(load) / sizeof(load[0]) };
Array4 *A = alloc_array(size);
for (int i = 0; i < NUM_LOADS; i++)
{
load[i].function(A);
test_set_sorts(A, load[i].tag, z_tag);
}
free_array(A);
}
/* Main Quick Sort function */
static void quicksort_partition(Array4 *A, size_t p, size_t r, Part partition)
{
if (p < r)
{
size_t q = (*partition)(A, p, r);
assert(p <= q && q <= r);
if (q > 0)
quicksort_partition(A, p, q-1, partition);
quicksort_partition(A, q+1, r, partition);
}
}
static size_t partition_random(Array4 *A, size_t p, size_t r);
static size_t partition_last(Array4 *A, size_t p, size_t r);
/* Quick Sort Wrapper function - specifying random partitioning */
void quicksort_random(Array4 *A)
{
quicksort_partition(A, 0, A->n - 1, partition_random);
}
/* Quick Sort Wrapper function - specifying partitioning about last element */
void quicksort_last(Array4 *A)
{
quicksort_partition(A, 0, A->n - 1, partition_last);
}
static inline size_t random_int(size_t p, size_t r)
{
return(lrand48() % (r - p + 1) + p);
}
static inline void swap(Array4 *A, size_t i, size_t j)
{
double d;
d = A->x[i];
A->x[i] = A->x[j];
A->x[j] = d;
d = A->y[i];
A->y[i] = A->y[j];
A->y[j] = d;
d = A->z[i];
A->z[i] = A->z[j];
A->z[j] = d;
d = A->w[i];
A->w[i] = A->w[j];
A->w[j] = d;
}
static size_t partition_random(Array4 *A, size_t p, size_t r)
{
size_t pivot = random_int(p, r);
swap(A, pivot, r);
size_t i = p-1;
size_t j = p;
while (j <= r)
{
if (compare(A, j, r) <= 0)
swap(A, j, ++i);
j++;
}
return i;
}
static size_t partition_last(Array4 *A, size_t p, size_t r)
{
size_t i = p-1;
size_t j = p;
while (j <= r)
{
if (compare(A, j, r) <= 0)
swap(A, j, ++i);
j++;
}
return i;
}
/* Selection Sort algorithm */
void selectionsort(Array4 *A)
{
size_t r = A->n;
for (size_t p = 0; p < r; p++)
{
for (size_t i = p; i < r; i++)
{
if (compare(A, p, i) > 0)
swap(A, p, i);
}
}
}
/*
** To apply this to the real code, where there are 4 arrays to be sorted
** in parallel, you might write:
**
** Array4 a;
** a.x = x;
** a.y = y;
** a.z = z;
** a.w = w;
** a.n = n;
** quicksort_random(&a);
**
** Or even:
**
** quicksort_random((Array4){ .n = n, .x = x, .y = y, .z = z, .w = w });
**
** combining designated initializers and compound literals. Or you could write a
** trivial wrapper so that you can call:
**
** quicksort_random_wrapper(n, x, y, z, w);
*/
int main(void)
{
srand48((long)time(0));
for (size_t i = 10; i <= 40; i += 10)
{
char buffer[10];
snprintf(buffer, sizeof(buffer), "%zuK", i);
test_set_loads(1000*i, buffer);
}
return 0;
}
If you can't use qsort with
typedef struct Point {
double x;
double y;
double w;
double z;
} Point;
Use qsort with
typedef struct UglyThing {
double x;
int i;
} UglyThing;
Create an array of size n, fill x with x values, i with index.
Call qsort. At the end, i will store the permutation order.
Swap the three other arrays according to the permutation order.
Then do the same with little arrays ("with same x") in the y direction.
If this ugly trick is not possible, then I don't see any other solution than reinventing the wheel.
(edit : I have just seen Andrew said something very close to this answer...sorry!)
Bye,
Francis
I'm not C expert and I've read through the forum, but I still need some advice regarding a sorting problem on C.
I have 4 dynamic arrays of doubles in C. All of them are the same size, and lets say n. What I want to do is to sort all of them using one of the arrays as first order and a second array as my second order. So if the arrays are *x, *y, *w and *z. I want to sort them according to the values of *x, then *y.
I must do this efficiently because the arrays are quite large.
Any help will be much appreciated.
The easy way to do this would be to map your four separate arrays onto a single array of a struct type like
struct rec {
double x;
double y;
double w;
double z;
};
struct rec *arr = malloc( sizeof *arr * N ); // where N is the number of
// elements in each array
if ( !arr )
// malloc failed, handle error somehow
for ( size_t i = 0; i < N; i++ )
{
arr[i].x = x[i];
arr[i].y = y[i];
arr[i].w = w[i];
arr[i].z = z[i];
}
and then create a comparison function to pass to qsort:
int cmpRec( const void *lhs, const void *rhs )
{
struct rec *l = lhs;
struct rec *r = rhs;
if ( l->x < r->x )
return -1;
else if ( l->x > r->x )
return 1;
else
{
if ( l->y < r->y )
return -1;
else if ( l->y > r->y )
return 1;
else
return 0;
}
return 0;
}
Now you can use the qsort library function to sort that array of struct:
qsort( arr, N, sizeof *arr, cmpRec );
Once that array is sorted, you can map the results back onto your four original arrays.
Clearly, sorting this using standard qsort() is not going to work; there isn't a mechanism for passing four arrays.
Equally clearly, if the data were structured as an array of structures, then using qsort() would be feasible.
Question 1: Is it feasible to create an array of structures, load it, sort it, and then unload back into the original arrays?
Question 2: Another option is to sort an array of integers:
int indexes[n];
for (int i = 0; i < n; i++)
indexes[i] = i;
qsort(indexes, n, sizeof(indexes[0]), comparator);
The comparator function would have to be able to access the x and y arrays as file scope variables:
int comparator(void const *v1, void const *v2)
{
int i1 = *(int *)v1;
int i2 = *(int *)v2;
extern double *x, *y;
if (x[i1] > x[i2])
return +1;
else if (x[i1] < x[i2])
return -1;
else if (y[i1] > y[i2])
return +1;
else if (y[i1] < y[i2])
return -1;
else
return 0;
}
You'd then be able to access the arrays using x[indexes[i]] etc to access the ith element in sorted order.
Is that acceptable?
If that is not convenient either, then you will end up writing your own sort; it isn't horribly painful, but will require some care.
I spent some time adapting an existing sort test framework to this scenario. The full code is quite large because it includes a lot of testing support code. The core function (compare, swap, partition and quicksort) are here (122 lines, including comment and blank lines):
/* SO 20271977 - sort arrays x, y, z, w (type double, size n) in parallel based on values in x and y */
/*
** To apply this to the real code, where there are 4 arrays to be sorted
** in parallel, you might write:
**
** Array4 a;
** a.x = x;
** a.y = y;
** a.z = z;
** a.w = w;
** a.n = n;
** quicksort_random(&a);
**
** Or even:
**
** quicksort_random((Array4){ .n = n, .x = x, .y = y, .z = z, .w = w });
**
** combining designated initializers and compound literals. Or you could write a
** trivial wrapper so that you can call:
**
** quicksort_random_wrapper(n, x, y, z, w);
*/
/* SOF so-20271977.h */
#include <stddef.h>
typedef struct Array4
{
size_t n;
double *x;
double *y;
double *z;
double *w;
} Array4;
extern void quicksort_random(Array4 *A);
/* EOF so-20271977.h */
#include <assert.h>
#include <stdlib.h> /* lrand48() */
/*
** Note that a more careful implementation would use nrand48() instead
** of lrand48() to prevent its random number generation from interfering
** with other uses of the x-rand48() functions.
*/
typedef size_t (*Part)(Array4 *A, size_t p, size_t r);
static void quicksort_partition(Array4 *A, size_t p, size_t r, Part partition);
static size_t partition_random(Array4 *A, size_t p, size_t r);
/* Quick Sort Wrapper function - specifying random partitioning */
void quicksort_random(Array4 *A)
{
quicksort_partition(A, 0, A->n - 1, partition_random);
}
/* Main Quick Sort function */
static void quicksort_partition(Array4 *A, size_t p, size_t r, Part partition)
{
if (p < r)
{
size_t q = (*partition)(A, p, r);
assert(p <= q && q <= r);
if (q > 0)
quicksort_partition(A, p, q-1, partition);
quicksort_partition(A, q+1, r, partition);
}
}
static inline int compare(Array4 const *A, size_t p, size_t r)
{
if (A->x[p] < A->x[r])
return -1;
else if (A->x[p] > A->x[r])
return +1;
if (A->y[p] < A->y[r])
return -1;
else if (A->y[p] > A->y[r])
return +1;
else
return 0;
}
static inline size_t random_int(size_t p, size_t r)
{
return(lrand48() % (r - p + 1) + p);
}
static inline void swap(Array4 *A, size_t i, size_t j)
{
double d;
d = A->x[i];
A->x[i] = A->x[j];
A->x[j] = d;
d = A->y[i];
A->y[i] = A->y[j];
A->y[j] = d;
d = A->z[i];
A->z[i] = A->z[j];
A->z[j] = d;
d = A->w[i];
A->w[i] = A->w[j];
A->w[j] = d;
}
static size_t partition_random(Array4 *A, size_t p, size_t r)
{
size_t pivot = random_int(p, r);
swap(A, pivot, r);
size_t i = p-1;
size_t j = p;
while (j <= r)
{
if (compare(A, j, r) <= 0)
swap(A, j, ++i);
j++;
}
return i;
}
The test framework (quite ridiculously elaborate if it weren't that I already had a variant of it on hand) is 369 lines including blank lines and comment lines — and all the code above:
#include <assert.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <time.h>
#define FLTFMT "%13.6f"
typedef struct Array4
{
size_t n;
double *x;
double *y;
double *z;
double *w;
} Array4;
static int trace = 0;
static void *xmalloc(size_t size)
{
void *space = malloc(size);
if (space == 0)
{
fprintf(stderr, "Out of memory (%zu)\n", size);
exit(1);
}
return space;
}
void quicksort_last(Array4 *A);
void quicksort_random(Array4 *A);
void selectionsort(Array4 *A);
static inline int compare(Array4 const *A, size_t p, size_t r)
{
if (A->x[p] < A->x[r])
return -1;
else if (A->x[p] > A->x[r])
return +1;
if (A->y[p] < A->y[r])
return -1;
else if (A->y[p] > A->y[r])
return +1;
else
return 0;
}
static void dump_array(char const *tag, Array4 const *A)
{
printf("%s [%zu..%zu]:\n", tag, (size_t)0, A->n-1);
for (size_t i = 0; i < A->n; i++)
printf("(" FLTFMT ", " FLTFMT ", " FLTFMT ", " FLTFMT ")\n",
A->x[i], A->y[i], A->z[i], A->w[i]);
}
static void chk_sort(Array4 const *A)
{
for (size_t i = 0; i < A->n - 1; i++)
{
//if (compare(A, i, i+1) > 0)
{
if (A->x[i] > A->x[i+1])
{
printf("Out of order: A.x[%zu] = " FLTFMT ", A.x[%zu] = " FLTFMT "\n",
i, A->x[i], i+1, A->x[i+1]);
}
else if ((A->x[i] == A->x[i+1] && A->y[i] > A->y[i+1]))
{
printf("Out of order: A.x[%zu] = " FLTFMT ", A.x[%zu] = " FLTFMT ", "
"A.y[%zu] = " FLTFMT ", A.y[%zu] = " FLTFMT "\n",
i, A->x[i], i+1, A->x[i+1], i, A->y[i], i+1, A->y[i+1]);
}
}
}
}
static inline void set(Array4 *A, size_t p, double d)
{
A->x[p] = d;
A->y[p] = d + drand48() - 0.5;
A->z[p] = d / 2.0;
A->w[p] = d * 2.0;
}
static void load_random(Array4 *A)
{
size_t size = A->n;
for (size_t i = 0; i < size; i++)
{
A->x[i] = drand48() * size;
A->y[i] = drand48() * size + drand48() - 0.5;
A->z[i] = drand48() * size / 2.0;
A->w[i] = drand48() * size * 2.0;
}
}
static void load_ascending(Array4 *A)
{
for (size_t i = 0; i < A->n; i++)
set(A, i, i);
}
static void load_descending(Array4 *A)
{
for (size_t i = 0; i < A->n; i++)
set(A, i, A->n - i);
}
static void load_uniform(Array4 *A)
{
for (size_t i = 0; i < A->n; i++)
set(A, i, A->n);
}
static void load_organpipe(Array4 *A)
{
for (size_t i = 0; i <= A->n / 2; i++)
set(A, i, i);
for (size_t i = A->n / 2 + 1; i < A->n; i++)
set(A, i, A->n - i);
}
static void load_invorganpipe(Array4 *A)
{
size_t range = A->n / 2;
for (size_t i = 0; i < A->n / 2; i++)
set(A, i, range - i);
for (size_t i = A->n / 2 + 1; i < A->n; i++)
set(A, i, i - range);
}
typedef void (*Load)(Array4 *A);
typedef void (*Sort)(Array4 *A);
typedef size_t (*Part)(Array4 *A, size_t p, size_t r);
static void test_one_sort(Array4 *A, Sort sort, char const *s_tag,
char const *l_tag, char const *z_tag)
{
if (trace)
{
printf("%s-%s-%s:", z_tag, l_tag, s_tag);
dump_array("Before", A);
}
clock_t start = clock();
(*sort)(A);
clock_t finish = clock();
double sec = (finish - start) / (double)CLOCKS_PER_SEC;
printf("%s-%s-%s: %13.6f\n", z_tag, l_tag, s_tag, sec);
chk_sort(A);
if (trace)
{
printf("%s-%s-%s:", z_tag, l_tag, s_tag);
dump_array("After", A);
}
fflush(stdout);
}
static Array4 *alloc_array(size_t size)
{
Array4 *A = xmalloc(sizeof(*A));
A->n = size;
A->x = xmalloc(size * sizeof(A->x[0]));
A->y = xmalloc(size * sizeof(A->y[0]));
A->z = xmalloc(size * sizeof(A->z[0]));
A->w = xmalloc(size * sizeof(A->w[0]));
return A;
}
static Array4 *dup_array(Array4 *A)
{
size_t size = A->n;
Array4 *B = alloc_array(size);
if (B != 0)
{
B->n = size;
memmove(B->x, A->x, size * sizeof(A->x[0]));
memmove(B->y, A->y, size * sizeof(A->y[0]));
memmove(B->z, A->z, size * sizeof(A->z[0]));
memmove(B->w, A->w, size * sizeof(A->w[0]));
}
return B;
}
static void free_array(Array4 *A)
{
free(A->x);
free(A->y);
free(A->z);
free(A->w);
free(A);
}
static void test_set_sorts(Array4 *A, char const *l_tag, char const *z_tag)
{
struct sorter
{
Sort function;
char const *tag;
} sort[] =
{
{ quicksort_last, "QS.L" },
{ quicksort_random, "QS.R" },
{ selectionsort, "SS.N" },
};
enum { NUM_SORTS = sizeof(sort) / sizeof(sort[0]) };
for (int i = 0; i < NUM_SORTS; i++)
{
Array4 *B = dup_array(A);
test_one_sort(B, sort[i].function, sort[i].tag, l_tag, z_tag);
free(B);
}
}
static void test_set_loads(size_t size, char const *z_tag)
{
struct loader
{
Load function;
char const *tag;
} load[] =
{
{ load_random, "R" },
{ load_ascending, "A" },
{ load_descending, "D" },
{ load_organpipe, "O" },
{ load_invorganpipe, "I" },
{ load_uniform, "U" },
};
enum { NUM_LOADS = sizeof(load) / sizeof(load[0]) };
Array4 *A = alloc_array(size);
for (int i = 0; i < NUM_LOADS; i++)
{
load[i].function(A);
test_set_sorts(A, load[i].tag, z_tag);
}
free_array(A);
}
/* Main Quick Sort function */
static void quicksort_partition(Array4 *A, size_t p, size_t r, Part partition)
{
if (p < r)
{
size_t q = (*partition)(A, p, r);
assert(p <= q && q <= r);
if (q > 0)
quicksort_partition(A, p, q-1, partition);
quicksort_partition(A, q+1, r, partition);
}
}
static size_t partition_random(Array4 *A, size_t p, size_t r);
static size_t partition_last(Array4 *A, size_t p, size_t r);
/* Quick Sort Wrapper function - specifying random partitioning */
void quicksort_random(Array4 *A)
{
quicksort_partition(A, 0, A->n - 1, partition_random);
}
/* Quick Sort Wrapper function - specifying partitioning about last element */
void quicksort_last(Array4 *A)
{
quicksort_partition(A, 0, A->n - 1, partition_last);
}
static inline size_t random_int(size_t p, size_t r)
{
return(lrand48() % (r - p + 1) + p);
}
static inline void swap(Array4 *A, size_t i, size_t j)
{
double d;
d = A->x[i];
A->x[i] = A->x[j];
A->x[j] = d;
d = A->y[i];
A->y[i] = A->y[j];
A->y[j] = d;
d = A->z[i];
A->z[i] = A->z[j];
A->z[j] = d;
d = A->w[i];
A->w[i] = A->w[j];
A->w[j] = d;
}
static size_t partition_random(Array4 *A, size_t p, size_t r)
{
size_t pivot = random_int(p, r);
swap(A, pivot, r);
size_t i = p-1;
size_t j = p;
while (j <= r)
{
if (compare(A, j, r) <= 0)
swap(A, j, ++i);
j++;
}
return i;
}
static size_t partition_last(Array4 *A, size_t p, size_t r)
{
size_t i = p-1;
size_t j = p;
while (j <= r)
{
if (compare(A, j, r) <= 0)
swap(A, j, ++i);
j++;
}
return i;
}
/* Selection Sort algorithm */
void selectionsort(Array4 *A)
{
size_t r = A->n;
for (size_t p = 0; p < r; p++)
{
for (size_t i = p; i < r; i++)
{
if (compare(A, p, i) > 0)
swap(A, p, i);
}
}
}
/*
** To apply this to the real code, where there are 4 arrays to be sorted
** in parallel, you might write:
**
** Array4 a;
** a.x = x;
** a.y = y;
** a.z = z;
** a.w = w;
** a.n = n;
** quicksort_random(&a);
**
** Or even:
**
** quicksort_random((Array4){ .n = n, .x = x, .y = y, .z = z, .w = w });
**
** combining designated initializers and compound literals. Or you could write a
** trivial wrapper so that you can call:
**
** quicksort_random_wrapper(n, x, y, z, w);
*/
int main(void)
{
srand48((long)time(0));
for (size_t i = 10; i <= 40; i += 10)
{
char buffer[10];
snprintf(buffer, sizeof(buffer), "%zuK", i);
test_set_loads(1000*i, buffer);
}
return 0;
}
If you can't use qsort with
typedef struct Point {
double x;
double y;
double w;
double z;
} Point;
Use qsort with
typedef struct UglyThing {
double x;
int i;
} UglyThing;
Create an array of size n, fill x with x values, i with index.
Call qsort. At the end, i will store the permutation order.
Swap the three other arrays according to the permutation order.
Then do the same with little arrays ("with same x") in the y direction.
If this ugly trick is not possible, then I don't see any other solution than reinventing the wheel.
(edit : I have just seen Andrew said something very close to this answer...sorry!)
Bye,
Francis