How to realloc( ) for M[m][n]

[PCHOME]

Hi!

I need to implement a matrix M which has the form:
double M[m][n];
, but the m and n are only known on RUN time.

I am not good in C,

I am thinking to use:

double **M;
int m, n;

int i = 0, j = 0, max = 100;
double entry[MAX];
M = ( double **) malloc(max * sizeof(double) );

if ( M == ( double **) NULL ) PRINT_ERR;

while (! EOF ) {
// read the first row and put all entries into entry[MAX],
// and decide the value n in RUN time
// n is the num of column in matrix
n = read_a_entry_in_a_row(infile, entry);
// now I have n

if ( n < max )
strcpy( &M[0][0] ,entry);

if ( n >= max )
{ max *=2 ;

M = ( double **) realloc( ( double **)M, max * sizeof(double) );

if ( M == ( double **) NULL ) PRINT_ERR;

strcpy( &M[0][0] ,entry);
}

max = 10 ;
// allocate max*n elements to M
M = ( double **) realloc( ( double **)M, max*n * sizeof(double) );

int cur_row = 1 ;
// Now read the following rows and decide the value m in RUN time
while (! EOF )
{
// read one more row and save all entries in the row
// to M[cur_row][0] - M[cur_row][n-1]
read_one_more_row(infile, &M[cur_row++])
if (cur_row > max )
{
max *= 2 ;
M = ( double **) realloc( ( double **)M, max*n * sizeof(double)
);
}
}

}

m = cur_row - 1 ;
M = ( double **) realloc( ( double **)M, m*n * sizeof(double) );

// print out all entries of Matrix
for (i = 0; i < m ; i++ )
for (j = 0; j < n ; j++ )
printf("M[%d][%d] = %f,", i, j, M[i][j]);
But I could NOT get what I want to,
something must wrong in my codes.
Would you please help?

Thanks!

[pete]

PCHOME wrote:

Hi!

I need to implement a matrix M which has the form:
double M[m][n];
, but the m and n are only known on RUN time.

I am not good in C,

I am thinking to use:

double **M;
int m, n;

int i = 0, j = 0, max = 100;
double entry[MAX];

M = ( double **) malloc(max * sizeof(double) );

if ( M == ( double **) NULL ) PRINT_ERR;

while (! EOF ) {
// read the first row and put all entries into entry[MAX],
// and decide the value n in RUN time
// n is the num of column in matrix
n = read_a_entry_in_a_row(infile, entry);
// now I have n

if ( n < max )
strcpy( &M[0][0] ,entry);

if ( n >= max )
{ max *=2 ;

M = ( double **) realloc( ( double **)M, max * sizeof(double) );

if ( M == ( double **) NULL ) PRINT_ERR;

strcpy( &M[0][0] ,entry);
}

max = 10 ;
// allocate max*n elements to M
M = ( double **) realloc( ( double **)M, max*n * sizeof(double) );

int cur_row = 1 ;
// Now read the following rows and decide the value m in RUN time
while (! EOF )
{
// read one more row and save all entries in the row
// to M[cur_row][0] - M[cur_row][n-1]
read_one_more_row(infile, &M[cur_row++])
if (cur_row > max )
{
max *= 2 ;
M = ( double **) realloc( ( double **)M, max*n * sizeof(double)
);
}
}

}

m = cur_row - 1 ;
M = ( double **) realloc( ( double **)M, m*n * sizeof(double) );

// print out all entries of Matrix
for (i = 0; i < m ; i++ )
for (j = 0; j < n ; j++ )
printf("M[%d][%d] = %f,", i, j, M[i][j]);

But I could NOT get what I want to,
something must wrong in my codes.
Would you please help?

/* BEGIN new.c */

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

int main (void)
{
double **M;
void *old;
size_t a, b, count_a, count_b;

b = 10;
a = 2;
M = malloc(a * sizeof *M);
if (M == NULL) {
fputs("malloc(a * sizeof *M) problem\n", stderr);
exit(EXIT_FAILURE);
}
for (count_a = 0; count_a != a; ++count_a) {
M[count_a] = malloc(b * sizeof *M[count_a]);
if (M[count_a] == NULL) {
fputs("malloc(b * sizeof *M[count_a]) problem\n", stderr);
exit(EXIT_FAILURE);
}
for (count_b = 0; count_b != b; ++count_b) {
M[count_a][count_b] = 10 * count_a + count_b;
}
}
for (count_a = 0; count_a != a; ++count_a) {
for (count_b = 0; count_b != b; ++count_b) {
printf("%f\n", M[count_a][count_b]);
}
}
putchar('\n');
++b;
for (count_a = 0; count_a != a; ++count_a) {
old = M[count_a];
M[count_a] = realloc(old, b * sizeof *M[count_a]);
if (M[count_a] == NULL) {
fputs("malloc(b * sizeof *M[count_a]) problem\n", stderr);
exit(EXIT_FAILURE);
}
M[count_a][b - 1] = 10 * count_a + count_b;
}
for (count_a = 0; count_a != a; ++count_a) {
for (count_b = 0; count_b != b; ++count_b) {
printf("%f\n", M[count_a][count_b]);
}
}
putchar('\n');
++a;
old = M;
M = realloc(old, a * sizeof *M);
if (M != NULL) {
M[a - 1] = malloc(b * sizeof *M[a - 1]);
if (M[a - 1] == NULL) {
fputs("malloc(b * sizeof *M[a - 1]) problem\n", stderr);
exit(EXIT_FAILURE);
}
for (count_b = 0; count_b != b; ++count_b) {
M[a - 1][count_b] = 10 * (a - 1) + count_b;
}
} else {
M = old;
--a;
}
for (count_a = 0; count_a != a; ++count_a) {
for (count_b = 0; count_b != b; ++count_b) {
printf("%f\n", M[count_a][count_b]);
}
}
while (a-- != 0) {
free(M[a]);
}
free(M);
return 0;
}

/* END new.c */

[PCHOME]

Thank you so much for your help!
It looks like your code allocates the number of row(int a) first, then
allocates the number of column(int b).

if the input file for the matrix M has the form
a_11, a_12, ... , a_1n
a_21, a_22, ... , a_2n
.... .... .... ....
.... .... .... ....
a_m1, a_m2, ... , a_mn
EOF

the n(number of column) can not be known until the first row is read,
and m(number of row) can not be known until EOF is reached.

Any suggestion?
Thanks!

[PCHOME]

BTW,

double **M;
......
M[count_a] = realloc(old, b * sizeof *M[count_a]); <---- *
....
does the line above( the one with <---- * ) equals

M[count_a] = realloc(old, b * sizeof (double));

?

Thanks again.

[pete]

PCHOME wrote:

BTW,

double **M;
.....
M[count_a] = realloc(old, b * sizeof *M[count_a]); <---- *
...
does the line above( the one with <---- * ) equals

M[count_a] = realloc(old, b * sizeof (double));

Yes.
ptr = malloc(n * sizeof *ptr);
is the generally preferred idiom on this newsgroup
for allocation of an array of n elements.

[pete]

PCHOME wrote:

Thank you so much for your help!
It looks like your code allocates the number of row(int a) first, then
allocates the number of column(int b).

if the input file for the matrix M has the form

a_11, a_12, ... , a_1n
a_21, a_22, ... , a_2n
... .... .... ....
... .... .... ....
a_m1, a_m2, ... , a_mn
EOF

the n(number of column) can not be known until the first row is read,
and m(number of row) can not be known until EOF is reached.

Any suggestion?

I think you can follow what I did in the example.
At the end I incremented ++a,
which is equivalent to ++m in your program.
At that time the value for b was known, or
the value for your n would have been known.

++a;
old = M;
M = realloc(old, a * sizeof *M);
if (M != NULL) {
M[a - 1] = malloc(b * sizeof *M[a - 1]);
if (M[a - 1] == NULL) {
fputs("malloc(b * sizeof *M[a - 1]) problem\n", stderr);
exit(EXIT_FAILURE);
}
for (count_b = 0; count_b != b; ++count_b) {
M[a - 1][count_b] = 10 * (a - 1) + count_b;
}
} else {
M = old;
--a;
}
while (a-- != 0) {
free(M[a]);
}
free(M);