Conflicting results passing an Array as parameter

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The idea is to create a function that, given a one-dimensional array/vector, returns the number of elements present in that array/Array:

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

#define MAX 6
#define TXT 20

int arraySize(char array[][TXT]);

int main(){
    char array[MAX][TXT]={"Test0", "Test1", "Test1", "Test2", "Test3", "Test3"};

    printf("\n Main: [%d]\n Func: [%d]\n\n", sizeof(array)/sizeof(array[0]), arraySize(array));

    return 0;
}

int arraySize(char array[][TXT]){
    return sizeof(array)/sizeof(array[0]);
}

The point here is that the main function works perfectly, so when done in an auxiliary function, the result, even using the same logic, is incorrect; I wonder if anyone can help me in this dilemma, I believe it has to do with the type of parameter passage, but I tried using pointers and the error persists.

OBS.: Using the MAX constant is not an option, since, not necessarily, I will know the value of MAX; the idea is, precisely, to discover the number of elements of this vector to, for example, use it as an argument of an iterator.

c array

  • Its array is of static size so it will always have the same size in memory. It makes no difference whether you are using memory or not, you have allocated a char[MAX][TXT] matrix. To do what you want you must mark which addresses of the array you are not using, e.g. by checking NULL. Then you change your counting function so that it counts the number of valid strings in the array.

  • I thought of that possibility, the question is, the way it is, run the line sizeof(array)/sizeof(array[0]) inside and outside the main with the same vector returns different results. In the above execution example it returns 6 inside the main (correct number of elements present in the vector) and in the function int arraySize(char array[][TXT]) it returns 0. This occurs because inside the main sizeof(array) returns 120 (20 * the number of elements of the vector), however the same function executed in the function int arraySize(char array[][TXT]) returns 4,

  • following the execution logic, the main fara 120/20=6 while the other function will 4/20=0, the question is why the other function returns 4 when I do sizeof(array)

  • So, sizeof is an operator that returns the value during COMPILATION and not during EXECUTION. Its code in the arraySize function defines another variable with a different type array name than the main array. Hence the compiler n knows which array size in the function (pq vc defined the array type in char[][MAX]) and returns a default value of 4 bytes.

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As far as I know, it is impossible to do what you are trying to do in C without some kind of access to the array data, such as counting up to the first NULL value. The way it is implemented, you expect for each function call with a different size array to return a different value, but it would be necessary to instantiate a new function implementation for that new type, recompiling the value of sizeof on the return.

The generic solution of having a variable of type char [][TXT] as parameter does not work pq when you call arraySize(array), the type of the variable array decal of char [MAX][TXT] for char*[TXT] so that the arraySize function has a single implementation. Soon part of the information about the vector size is lost and when you ask for the value of sizeof(array), it simply returns 4 bytes pq this is the size of a pointer on your platform.

#include <iostream>

#define MAX 6
#define TXT 20

template <typename T, std::size_t N>
constexpr std::size_t array_size(T (&a)[N])
{
    return sizeof(T) * N;
}

int main()
{
    char array[MAX][TXT];
    std::cout << sizeof(array)      << "/" << sizeof(*array)     << '\n'
              << array_size(array)  << "/" << array_size(*array) << '\n';

    return 0;
}

Since templates only exist in C++ and not in C, I posted an alternative to their C implementation++.

Note that ultimately it’s just a more elaborate way to use the operator sizeof. Without pointers and dynamic allocation, there is no way to get away from it much because you get stuck in the fact that values are computed during compilation. To calculate the size of vectors running vc need to use more complex data structures with additional variables for counting and dynamic allocation, such as a Std::vector.

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