Calculate vector size

Question

How to calculate the vector size?

My code:

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

int retornaMediana(int *vetor){

int tam=0;

    tam = sizeof(vetor);

    printf("TESTE: %d, %d, %d\n\n",vetor[0],vetor[1],vetor[2]);

    return tam;

}

void main(){

int vetor[] = {5,25,7,10,13,33,45,11,60};
int n = retornaMediana(&vetor);
int n2;

n2 = sizeof(vetor)/sizeof(int);

    printf("TAMANHO = %d\n\n",n);
    printf("TAMANHO = %d\n\n",n2);


}

How do I get the function to return the real size of the vector ? I’ve passed the parameter, reference and nothing, I only get this 4 as a response.

Answer 1

This line is calculating the size correctly:

int len = sizeof(vetor)/sizeof(int);

Better would be

int len = sizeof(vetor)/sizeof(vetor[0]);

because, in case you change the type of vetor, will not be surprised by a bug difficult to perceive depending on the context.

Important to understand that the sizeof is already solved by the compiler, because he simply already knows what was done in the variable definition, so if you need to know the size of a vector passed by parameter, the solution that remains is to always pass the vector and the size.

Back to your code, the problem is that you are calculating the size correctly, but you end up overwriting the value on this other line, this time with the pointer size:

tam = sizeof(vetor);

An output would be to calculate the size within the declaration scope, as you were already doing, and send the size to the median function.

See the difference:

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

int retornaMediana(int *vetor, int tam){
    // -- aqui você corre os ítens e calcula a mediana --
    // return mediana;
    printf("TESTE: %d, %d, %d\n\n",vetor[0],vetor[1],vetor[2]);
    return tam;
}

void main(){
    int vetor[] = {5,25,7,10,13,33,45,11,60};
    int len = sizeof(vetor)/sizeof(vetor[0]);

    int n  = retornaMediana( vetor, len );

    printf("TAMANHO = %d\n\n",n);
    printf("TAMANHO = %d\n\n",len);
}

Here’s a demonstration on IDEONE.

Answer 2

The vector inside the method is now a pointer, so the size will always be 8 (if your file is 64x), to calculate its size, you must create a method that traverses the pointer and count the positions.

int vec_len(int *vec){
    int c = -1;
    while(*(vec++))
        c++;
    return c;
}

I recommend you use some value like 0 to identify when to arrive at the end of the vector.