Problem with modifying array argument in function

[hiephoi]

Hello everyone!
I've got some experience with Java and C, but I'm very new to C++. Therefore, I guess the answer to my question will be obvious, but I just can't figure it out on my own.

I've got the following code:

#include <gmp.h>
#include <flint.h>
#include <fmpz.h>
#include <fmpz_poly.h>
#include <iostream>

using namespace std;

template<int length>

void random_code(fmpz_t (&result)[length]) {
	for(int i=0; i<length; i++) {
		fmpz_set_ui(result[i], 1); //not quite random, but for now it will do
	}
}

int main() {
	int len = 4;
	fmpz_t test[len];
	for(int i=0; i<len; i++) {
		fmpz_init(test[i]);
	}
	random_code(test);
	cout << "value of test: ";
	for(int i=0; i<len; i++) {
		fmpz_print(test[i]);
	}
	cout << endl;

	return 0;
}

I want to initialize an array of fmpz_t's and then set each component in the array to a certain value. The problem is that I want to modify the array argument in random_code, so I made the argument a reference to that array, but it doesn't seem to work.

When I try to compile it, I get the following error and notes:

gen_code.cpp: In function 'int main()':
gen_code.cpp:23:18: error: no matching function for call to 'random_code(fmpz [len][1])'
random_code(test);

gen_code.cpp:11:6 note: candidate: template<int length> void random_code(fmpz (&)[length][1])
void random_code(fmpz_t (&result)[length]) {

gen_code.cpp:11:6: note: template argument deduction/substitution failed:
gen_code.cpp:23:18: note: variable-sized array type 'int' is not a valid template argument
random_code(test);

I know that there is something wrong with the line 'random_code(test)' in main() and that I don't call the argument correctly, but I just don't see the solution, so I hope there's someone who can help me. Thanks in advance!

[Chervil]

I just threw this together to get it to compile and run.
The typedef was just a temporary workaround, you don't need that.
Note len in main() must be const.

The function bodies at lines 7, 8 and 9 are just random nonsense I made up, again to get it to compile and link.

#include <iostream>

using namespace std;

typedef int fmpz_t;

void fmpz_set_ui(fmpz_t & a, int b ) { a = b; }
void fmpz_init(fmpz_t & a) { a = 0; }
void fmpz_print(fmpz_t a) { std::cout << a << ' '; }

void random_code(fmpz_t result[], int length) {
    for(int i=0; i<length; i++) {
        fmpz_set_ui(result[i], 1); //not quite random, but for now it will do
    }
}

int main() 
{
    const int len = 4;
    fmpz_t test[len];
	
    for (int i=0; i<len; i++) {
        fmpz_init(test[i]);
    }
	
    random_code(test, len);
	
    cout << "value of test: ";
    for (int i=0; i<len; i++) {
        fmpz_print(test[i]);
    }
    cout << endl;

    return 0;
}

[hiephoi]

Thanks a lot, it works!! But I'm a bit confused now, because I searched on Google and I learned that when you want to modify an argument of a function inside the body of that function, you should put the reference to that argument in the function (i.e. & before the argument). But now you didn't and it still works. Could you perhaps explain me that?

[Chervil]

My code does use references in

void fmpz_set_ui(fmpz_t & a, int b ) { a = b; }
void fmpz_init(fmpz_t & a) { a = 0; }

At line 11,

void random_code(fmpz_t result[], int length) {

When an array is passed as a parameter to a function, it actually passes the address of the first element of the array. See "Arrays as parameters" in the tutorial.
http://www.cplusplus.com/doc/tutorial/arrays/

[hiephoi]

Thanks for your answer! But the library fmpz doesn't use references in those functions, unlike your implementation of the functions.

[Chervil]

Well, I had no idea what the functions fmpz_set_ui() and fmpz_init() were supposed to do, I've no idea what type of animal a fmpz represents. I just wrote some code which would compile and run, that's all.

edit:
A bit of googling, it looks like type fmpz_t is actually a typdef of a pointer type, thus it uses a pointer rather than a reference.

[hiephoi]

Thanks for your help!! I've got one last question. I'm not completely comfortable with pointers and references yet, especially not in the case of arrays. Does the fact that fmpz_t uses a pointer rather than a reference mean that I have to change anything to the code? The function random_code now looks like this:

void random_code(fmpz_t result[], size_t length) {

I mean, it works, but as I learned to use references when modifying an argument in a function, I'm still a bit confused.

[Chervil]

I would think your code is fine as it stands now,

Pointers and references are loosely related. They are not the same thing. But both of them allow a function to change the contents of a parameter.

Consider the following example. I tried to keep it as simple as possible.

#include <iostream>

using namespace std;

void changex(int x)
{
    x = x + 1;
}

void changey(int* y)
{
    *y = *y + 1;
}

void changez(int &z)
{
    z = z + 1;
}

int main()
{
    int x = 5;
    int y = 5;
    int z = 5;

    changex(x);    // x is passed by value
    changey(&y);   // address of y is passed
    changez(z);    // z is passed by reference

    cout << "x: " << x << "    y: " << y << "    z: " << z << '\n';
  
    return 0;
}

x: 5    y: 6    z: 6

1. changex(int x) This example fails to work. Depending on the compiler, there may be a warning message, mine gives 'x' is assigned a value that is never used at line 7. The variable x is a local variable which exists only for the lifetime of the function. Its value is a copy of the parameter given when the function is called

2. changey(int* y) y is a pointer. When the function is called, the parameter must be a pointer. Notice also within the function the pointer must be dereferenced to access the thing which is pointed to.

3. changez(int &z) Often this is the preferred approach. z is passed by reference. This means that in effect the variable z inside the function is the same object as that in the calling code (though it need not have the same name).

[Chervil]

As for arrays and pointers, when passing an array to a function, it is the address of the array start which is passed, and though an array is not the same thing as a pointer, they have similarities:

#include <iostream>

using namespace std;

void func1(char * str)
{
    *str = 'L';
}

void func2(char str[])
{
    str[0] = 'B';
}

int main()
{
    char word1[] = "dog";
    char word2[] = "cat";

    func1(word1);
    func2(word2);

    cout << "word1: " << word1 << '\n';
    cout << "word2: " << word2 << '\n';
}

word1: Log
word2: Bat

[hiephoi]

Thanks a lot for your explanation Chervil!!! I think I understand it better now.

[Cubbi]

Your first program was perfectly fine, it has only one error, at line 18:

	int len = 4; // len is not const, cannot be used as array size
	fmpz_t test[len]; // error here 

which should have said

	const int len = 4; // this fixes everything
	fmpz_t test[len]; 

Your first random_code actually passed the array by reference and it's a good thing to know how to do that since pointer and size is pretty much the worst possible interface a function that works on an array can have. (array by reference isn't perfect either, the best interfaces are iterator pair and array view - see ISO C++ Core Guidelines item I.13: https://github.com/isocpp/CppCoreGuidelines/blob/master/CppCoreGuidelines.md#-i13-do-not-pass-an-array-as-a-single-pointer