Ramón Casero Cañas on Thu, 16 Jan 2003 09:37:16 +0100 (MET)

 thank you for the inversion

```Thank you all for the help. At last my function takes a C++ matrix,
inverts it mudulo2, and overwrites the input matrix with the result. The
code is below, just in case you want to have a look at this PARI's
`Hello world'. Of course you're going to say `it's so easy, and
everything that's hard was already done'. Well, it's been very
interesting getting to know PARI/GP, and I hope using it further.

I have also tried to do it the u_FpM_inv() way, but I'm still struggling
trying to get that it compiles. I have some directions from some of you

Best,

Ramón.

--------------------------------------------------

bool inverse_mat_mod2(bool **_a, unsigned _size) {
return inverse_mat_mod2(_a, _size, true);
}

bool inverse_mat_mod2(bool **_a, const unsigned _size, const bool _CHECK) {
GEN a;           // temp matrix by columns
unsigned i, j;   // counter
unsigned sizepari = _size + 1;
GEN z;           // auxiliary element of the matrix
GEN v;           // auxiliary element of the matrix
long paristacksize = 500000;
long total_size = sizepari * sizepari + _size * _size * 9; // thanks,
Karim

paristacksize = sizeof(long) * total_size * 10; // thanks again
pari_init(paristacksize, 2); // init PARI stack

/* allocate memory for the temp matrix
* note: remember that PARI matrices of mxn need (m+1)x(n+1), and
* that elements go a[1], a[2], ..., while a[0] is for type
* information */
std::cout << "inverse_mat_mod2: size = " << _size << std::endl;
a = cgetg(sizepari, t_MAT); // `a' has _size columns
for (i = 1; i <= _size; ++i) { // allocate `a' columns of _size elements
a[i] = lgetg(sizepari, t_COL);
}
for (i = 1; i <= _size; ++i) { // allocate space for elements
v = (GEN)a[i];
for (j = 1; j <= _size; ++j) {
v[j] = lgetg(3, t_INTMOD);
z = (GEN)v[j];
z[1] = lstoi(2); // modulo 2
if (_a[j-1][i-1]) z[2] = un; else z[2] = zero;//lstoi(0);
}
}
std::cout << "inverse_mat_mod2: assigment done" << std::endl;

/* invert matrix */
GEN ainv;
ainv = ginv(a);

/* check whether the inverse is valid */
if (_CHECK) {
GEN a_ainv;
a_ainv = gmul(a, ainv);
for (i = 1; i <= _size; ++i) {
v = (GEN)a_ainv[i];
for (j = 1; j <= _size; ++j) {
z = (GEN)v[j];
if ((i == j) != itos((GEN)z[2])) return false;
}
}
}

/* for no _CHECK or valid inversion, overwrite input array with
inverse matrix */
for (i = 1; i <= _size; ++i) {
v = (GEN)ainv[i];
for (j = 1; j <= _size; ++j) {
z = (GEN)v[j];
_a[j-1][i-1] = (bool)itos((GEN)z[2]);
}
}

return true;
}

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```