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My idea is to declare a global vector of a structure x, but I will only have the size of the vector in main. How can I declare the vector and then tell how big it is? I have an equivalent solution in Java, but not in C.
1 answer
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Here are two possible solutions to your problem:
1) Solution with dynamic vector allocation with functions malloc() and free() of the standard library stdlib.h:
#include <stdlib.h>
typedef struct foobar_s
{
int a;
int b;
int c;
} foobar_t;
int main( int argc, char * argv[] )
{
foobar_t * v = NULL;
int n = 0;
int i = 0;
/* Recupera tamanho do vetor passado como parametro na linha de comando */
n = atoi( argv[1] );
/* Aloca memoria necessaria para armazenar o vetor */
v = (foobar_t*) malloc( n * sizeof(foobar_t) );
/* Inicializa todos os membros de cada elemento do vetor com o valor '0' */
for( i = 0; i < n; i++ )
{
v[i]->a = 0;
v[i]->b = 0;
v[i]->c = 0;
}
/* Faz alguma coisa com o vetor... */
/* Libera memoria alocada */
free( v );
return 0;
}
/* fim-de-arquivo */
2) Solution with static vector allocation using VLAs (Variable Length Array) permitted from the standard C99:
typedef struct foobar_s
{
int a;
int b;
int c;
} foobar_t;
int main( int argc, char * argv[] )
{
int n = 0;
int i = 0;
/* Recupera tamanho do vetor passado como parametro na linha de comando */
n = atoi( argv[1] );
/* Declara vetor estaticamente com tamanho 'n' */
foobar_t v[ n ];
/* Inicializa todos os membros de cada elemento do vetor com o valor '0' */
for( i = 0; i < n; i++ )
{
v[i].a = 0;
v[i].b = 0;
v[i].c = 0;
}
/* Faz alguma coisa com o vetor... */
return 0;
}
/* fim-de-arquivo */
I hope I’ve helped!
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But in the solution (2) the array
vwill not be local to main function?
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Take a look at the answers to this question that shows how you can use the
mallocto allocate memory as needed.– gato
Did you try to do anything? It’s not very secret. Do you have any specific questions?
– Maniero