passing arrays to function - error

[GMG Kurt]

Code: [Select]

#include <iostream>
#include <stdlib.h>

int * coins;
int * buffer;

int sumofarray(int* array) {
	int sum;
	int i  = (sizeof(array)/sizeof(array[0]));
	std::cout << i << "\n";

	return sum;
}

int main (int argc, char * const argv[]) {
    
	int coins[] = {7, 1, 2, 8};
	std::cout << (sizeof(coins)/sizeof(coins[0])) << "\n";
	std::cout << sumofarray(coins);
}

so theres a dynamic array with four used cells. In main() the equation to determine how many elements it has works correctly returning 4, but when it's executed in the function (in which the same array has been passed to) it incorrectly returns 2.
If anybody has any insight they could give to help me with this that'd be greatly appreciated.

[Connors]

I... Thought I would try to interpret the syntax but I'm thoroughly confused here, sorry Kurt. >_>

PS - it says this was Kurt's post #666
PPS - that's odd, I'm at 1777 o-o

[Gan]

c/c++ doesn't store the size of the array. sizeof(array) returns the size of the pointer, not the size of the array.

Basically sizeof calculates the size of the datatype. Not what's in the data type. So you gotta remember how large your array is when you make it. Of course if you want a dynamic array you can use a vector.

I'm sort of guessing what you're trying to do but this might work:

Code: [Select]

#include <iostream>
#include <stdlib.h>
using namespace std;

int * coins;

int sumofarray(int* array, int len) {
	int sum;
	for (int i = 0; i < len; i++) {
		sum += array[i];
	}

	return sum;
}

int main (int argc, char * const argv[]) {
    
	int coins[] = {7, 1, 2, 8};
	cout << "Number of elements: 4" << endl;
	cout << sumofarray(coins, 4);
}

[Connors]

Okay, well, that makes a lot more sense to me and I just have to ask...
How did you come up with

int i  = (sizeof(array)/sizeof(array[0]));

[GMG Kurt]

@Connors It's something I used all the time with calloc() and malloc() in my C days when I remembered how to use pointers. It's the size of the array in bytes divided by the size of the data type of the first (zeroth) spot of the array in bytes, yeilding the number of elements in the array.
@Gan I need a way to send a dynamic array of indeterminant length to the sumofarray() function and get it to test the length inside of there, and then use that in the for() loop; like what you did but a variable representing the length instead of the number 4
Note: in an unshown part of the main() the array is stored in a buffer reallocated and then reallocated with an arbitrary size.

[Gan]

If you don't know the length of an array, there isn't a way to get the length.

[Circuit]

Hey Kurt.  Gan mentioned earlier that you can use a vector instead of an array.  Vectors are easier to use because vectors can automatically resize themselves.  Another thing about vectors is that the vector class has a size() method which can be called from anywhere that the vector is passed to.  This means that when you pass a vector, you don't need to pass the size along with it, because the method can determine the size of the vector by calling size().  You just have to make sure to pass the vector by reference or as a pointer, because if you don't, it will automatically be passed by value.

I'm curious, why do you want the function to determine the size of the array?  Why not pass the size of the array as a parameter?

[GMG Kurt]

Gan was right I couldn't find the size of the array outside of the main() because sizeof() is a runtime function. Currently I'm going to keep everything inside of the main() instead of passing the arrays to the function, and will deffinetly look into vectors cause they sound interesting.
@ circuit I wanted to just pass the array because in this program I wanted an array I could keep deallocating and reallocating depending on the number of elements that I needed. While tracking the size and giving that as a parameter is definitely an option (and an easier alternative) I just felt like doing it this way to do it, wanted the challenge.

[Gan]

If you want a challenge, make your own array class that you can initialize with a certain amount of variables that keeps track of it's size.
You'll then be able to pass it by pointer or reference.