Thread: what's wrong with this code? (lagange polynomial multiplier)

> pn = (double**)malloc((n)*sizeof(double*));
Don't cast the result of malloc in C programs.
FAQ > Casting malloc - Cprogramming.com
If you've done everything right, you don't need to cast.
If you've done something wrong, casting just hides the problem.

> pn[i] = (double*)malloc((2^n)*sizeof(double));
If you were hoping for 2 to the power of n here, then you're mistaken.
^ is the exclusive or operator.

Some better indentation would help as well. Additional comments in the code.

Code:

#include <stdio.h>
#include <conio.h>  //!! obsolete, don't use
#include <math.h>
#include <stdlib.h>

main()  //!! should explicitly return int, and have return 0; at the end
{
  FILE *exibir;
  int i, j, k, l, n = 3;
  //!! some of these could be localised to inner scopes
  double x[3] = { 3, 3.5, 4 }, 
        fx[3] = { 1.099, 1.253, 1.386}, 
        lag[n], c[n], d[n], e[n], vet[n], 
        **pn, vpx = 1, dif, vat;

  pn = (double **) malloc((n) * sizeof(double *));
  for (i = 0; i < n; i++) {
    pn[i] = (double *) malloc((2 ^ n) * sizeof(double));
  }

  for (i = 0; i < n; i++) {
    lag[i] = 1;
    for (j = 0; j < n; j++) {
      if (j != i) {
        lag[i] = lag[i] * (1 / (x[i] - x[j]));
      }
    }
  }

  for (i = 0; i < n; i++) {
    vpx = vpx * (-x[i]);
  }

  for (i = 0; i < n; i++) {
    vet[i] = 1;
  }

  for (k = 0; k < n; k++) {
    for (l = 0; l < n; l++) {
      for (i = n - 1; i > l; i--) {
        vet[i] = (vet[i] * x[i]);
        vat = vat + vet[i];
      }
      pn[k][l] = (vpx / vat);
      vet[l] = 1;
      vat = 0;
    }
  }

  for (i = 0; i < n; i++) {
    d[i] = lag[i] * fx[i];
  }

  for (i = 0; i < n; i++) {
    for (j = 0; j < n; j++) {
      c[i] = c[i] + pn[j][i];
    }
  }

  for (i = 0; i < n; i++) {
    e[i] = c[i] * d[i];
  }

  exibir = fopen("exemplo.txt", "w");
  fprintf(exibir, "lagrange: ");
  for (i = 0; i < n; i++) {
    fprintf(exibir, " %.5lf", e[i]);
  }
  fclose(exibir);

  //!! memory leak, you need to free each pn[i] first.
  free(pn);
}

> i can't multiply the polynomials in the part with three "for"
Probably because your pn array is broken.

Code:

$ gcc -Wall -Wextra -O2 -g foo.c
foo.c:6:1: warning: return type defaults to ‘int’ [-Wreturn-type]
foo.c: In function ‘main’:
foo.c:14:24: warning: unused variable ‘dif’ [-Wunused-variable]
foo.c:73:1: warning: control reaches end of non-void function [-Wreturn-type]
$ valgrind ./a.out
==5903== Memcheck, a memory error detector
==5903== Copyright (C) 2002-2010, and GNU GPL'd, by Julian Seward et al.
==5903== Using Valgrind-3.6.1-Debian and LibVEX; rerun with -h for copyright info
==5903== Command: ./a.out
==5903== 
==5903== Invalid write of size 8
==5903==    at 0x40082A: main (foo.c:44)
==5903==  Address 0x51d20a8 is 0 bytes after a block of size 8 alloc'd
==5903==    at 0x4C28F9F: malloc (vg_replace_malloc.c:236)
==5903==    by 0x4006D4: main (foo.c:18)
==5903== 
==5903== Invalid read of size 8
==5903==    at 0x400896: main (foo.c:56)
==5903==  Address 0x51d20a8 is 0 bytes after a block of size 8 alloc'd
==5903==    at 0x4C28F9F: malloc (vg_replace_malloc.c:236)
==5903==    by 0x4006D4: main (foo.c:18)
/// much more reportage snipped.