c++ -- returning an array from function

Ok i have a small problem where i have a function that generates an array of characters. However, if i try to return that array i get a mess of errors. I think that i need to use a pointer to do it however, all the examples i saw used integers and did not work for me. This project i am working on is over my head, but that is how i learned all my other programing languages so please understand that my errors may just be newbie errors. Thanks! ~p4plus2~

...and if it helps here is the function:
i know i could combine the separate for loops but i left em apart since its was for a debugging reason and will not be there in my final product.
This also may not be the best way that i can accomplish what i am doing, but i think it is probably a better learning experience to do it the long way, however feel free to suggest another way to do this so if i make a new version maybe i will try a method suggested.

Code:

---

char start_rubik(){
        char cube[6][3][3];
        char color[] = {'b', 'o', 'g', 'r', 'w', 'y'};
        for(int i = 0; i != 6; i++){       
                for(int b = 0; b != 3; b++){
                        for(int a = 0; a !=3; a++){
                                cube[i][b][a] = color[i];
                        }
                }
        }
        for(int i = 0; i != 6; i++){       
                for(int b = 0; b != 3; b++){
                        for(int a = 0; a !=3; a++){
                                std::cout << cube[i][b][a];
                        }
                        std::cout << "\n";
                }
        }
        return cube;  //this is that nasty array i can't figure out.
}

---

For anybody who didn't guess, this script (once combined with all its other functions) will be a rubiks cube command line.....as stupid of an idea as that is i think it is a great learning experiance

I think I posed the same thread when I signed up to these forums… but, what I ended up doing was passing an array to the function by reference… so something like this

Code:

---

void start_rubik(char cube){
        char color[] = {'b', 'o', 'g', 'r', 'w', 'y'};
        for(int i = 0; i != 6; i++){       
                for(int b = 0; b != 3; b++){
                        for(int a = 0; a !=3; a++){
                                cube[i][b][a] = color[i];
                        }
                }
        }
        for(int i = 0; i != 6; i++){       
                for(int b = 0; b != 3; b++){
                        for(int a = 0; a !=3; a++){
                                std::cout << cube[i][b][a];
                        }
                        std::cout << "\n";
                }
        }
}

---

edit: i just found my old thread... and i was told there that arrays are passed by reference automatically... so that should work... :)

i get the error:
tutorial.cpp: In function ‘void start_rubik(char)’:
tutorial.cpp:11: error: invalid types ‘char[int]’ for array subscript
tutorial.cpp:18: error: invalid types ‘char[int]’ for array subscript
from g++ when trying to compile like that :|
lines 11 and 18 are the [i][b][a] lines

edit:
after reobserving your code i come to question why would i use cube as an arguement when that should be my result...

The function is returning a single char. To return the array you need to return a char *. Also, you need to dynamically allocate the room for the array with new. Otherwise, when the function ends the array goes out of scope and gets trashed.

Quote:

---

Originally Posted by Loic

I think I posed the same thread when I signed up to these forums… but, what I ended up doing was passing an array to the function by reference… so something like this

Code:

---

void start_rubik(char cube){
        char color[] = {'b', 'o', 'g', 'r', 'w', 'y'};
        for(int i = 0; i != 6; i++){       
                for(int b = 0; b != 3; b++){
                        for(int a = 0; a !=3; a++){
                                cube[i][b][a] = color[i];
                        }
                }
        }
        for(int i = 0; i != 6; i++){       
                for(int b = 0; b != 3; b++){
                        for(int a = 0; a !=3; a++){
                                std::cout << cube[i][b][a];
                        }
                        std::cout << "\n";
                }
        }
}

---

edit: i just found my old thread... and i was told there that arrays are passed by reference automatically... so that should work... :)

---

What array? You're passing a single char by value to the function.
This is how you pass an array by reference:

Code:

---

void start_rubik( char*** cube )
or
void start_rubik( char cube[6][3][3] )

---

@kernel how do i dynamically allocate more room for the array(remember i am a newbie :|)


@cpjust
my function made an array IN the function, that funtion was a suggested one i should try and i couldn't make it work(if i understand what you said...) and yes i tried adding [6][3][3] to that version too

I do not want to add the array as a reference

You cannot return an array by value. You can return an array by reference, but to do that you must first store the array somewhere.

Since this is C++, the best way is to instead of using a function, use an object that holds an array inside it, and have those functions that would modify the array directly be methods of that class.

Quote:

---

@kernel how do i dynamically allocate more room for the array(remember i am a newbie :|)

---

use "new" and "delete"

at this stage, a reference would be a better solution IMHO.

The problem with returning a pointer to an array allocated on the stack (char asdf[10]) is that the memory is deallocated when the function returns, and you end up accessing random things that happen to be in memory where the array used to be.

I have not got to classes yet sadly...I have been trying to get a grasp on lower content first....to an extent i have it all down but pointers keep dragging me down.....i decided to make this script just to practice what i already know....and since other languages like php are able to return arrays and it is written in c++ i just figured it would be possible...

well thanks for all input i suppose i will trash this scrip(for now untill i am better)


EDIT:
would it be possible to write all the contents of the array to a file to use in another function? i don't really know how i would do it.....

Quote:

---

Originally Posted by p4plus2

@kernel how do i dynamically allocate more room for the array(remember i am a newbie :|)

---

In this case the syntax is:

Code:

---

char (*cube)[3][3]= new char[6][3][3];

---

Also the return value would be

Code:

---

char (*start_rubik())[3][3]{
  ...
  return cube;
}

---

Notice that since the return type surrounds the function name and arguements, in exactly the same way that is surrounds the variable name above.

Or alternatively:

Code:

---

char (&start_rubik())[6][3][3]{
  ...
  return *reinterpret_cast<char(*)[6][3[3]>(cube);
}

---

Here you are returning an array reference. The advantage of this is that the 6 is preserved in the type.
EDIT: This one requires a cast.

But you don't want to do that because generally functions are not expected to make the user de-allocate the object as required here.

>> would it be possible to write all the contents of the array to a file to use in another function? i don't really know how i would do it

That's possible but laughably slow.

Do you have a book you can look at? You need to learn about C++ functions and arrays as parameters.

Code:

---

char *start_rubik(){
        char *cube = new char[6*3*3];
        char color[] = {'b', 'o', 'g', 'r', 'w', 'y'};
        for(int i = 0; i != 6; i++){       
                for(int b = 0; b != 3; b++){
                        for(int a = 0; a !=3; a++){
                                cube[i][b][a] = color[i];
                        }
                }
        }
        for(int i = 0; i != 6; i++){       
                for(int b = 0; b != 3; b++){
                        for(int a = 0; a !=3; a++){
                                std::cout << cube[i][b][a];
                        }
                        std::cout << "\n";
                }
        }
        return cube;  //this is that nasty array i can't figure out.
}

---

At some point you need to call delete cube[] (or whatever you store the return value in) to return the memory to the heap

**EDIT**
was typing when King Mir posted

Quote:

---

Originally Posted by cpjust

What array? You're passing a single char by value to the function.
This is how you pass an array by reference:

Code:

---

void start_rubik( char*** cube )
or
void start_rubik( char cube[6][3][3] )

---

---

I'd recommend:

Code:

---

void start_rubik( char (&cube)[6][3][3] )

---

So that you cannot pass an array with a different first dimension.

The char *** version is uses when the array is not in continuous memory; it gets allocated differently than I showed above. In this case I think it makes more sense to use continuous memory.

Another option is to make the array in the function static. Then you can return it by reference without allocating memory for it. It won't be multi-thread safe, but since you're just starting out, you probably won't be using threads any time soon anyways.

Quote:

---

Originally Posted by citizen

>> would it be possible to write all the contents of the array to a file to use in another function? i don't really know how i would do it

That's possible but laughably slow.

Do you have a book you can look at? You need to learn about C++ functions and arrays as parameters.

---

i do not have a book, just using various internet resources....

@everybody else i will be looking at your posts, need time to acctually think through and test each one...

You really should get a book.

Quote:

---

Originally Posted by cpjust

What array? You're passing a single char by value to the function.
This is how you pass an array by reference:

Code:

---

void start_rubik( char*** cube )
or
void start_rubik( char cube[6][3][3] )

---

---

lol... sorry, stupid mistake :)

ok i came to the conclusion for now this project is officially being scraped, i am running into thousands of problems....especially once i start trying to make a function to turn the cube...I will need to look for something a bit simpler perhaps.

Quote:

---

Originally Posted by King Mir

You really should get a book.

---

my parents will not buy me one....I went to library they had like 6 million books on PHP but i am already very good with php so thats not what i need... for now i will need to stick to the internet.

Take a look at our book recommendation thread. For novices I would point out Accelerated C++ - check if it is hiding among the many PHP books in your local library.

Quote:

---

other languages like php are able to return arrays and it is written in c++ i just figured it would be possible...

---

They are completely different languages. There are things that can be done in one and not in the other, or at least as easily.

In this particular case, though, it is possible. A PHP array is more like a std::vector in C++. A C++ array is a lot more primitive. It is essentially just a continuous block in memory, and doesn't know about it's own length and such.

I know I will be disagreeing with many people here by saying this, but I find the internet just as helpful as books given you find the right tutorials written by people who know what they are doing. I have never read a book on C++. Not saying I know much about C++, I don't. But I think I do know enough to know how to Google for specific topics that I need. For instance, I was doing SDL a while ago, and had done a few simple games. I am currently learning Qt, and can follow through the tutorials quite comfortably, knowing what I am doing at each stage. I do have a (what I think is) weird way of following through tutorials, though - I read through the section once, see the end result, and close everything except my editor and a command prompt for compilation, and try to reimplement what I just saw, myself, from scratch. And then I would compare my code to the tutorial code, and ponder on the differences. I think it's quite effective.

That said, I seem to have an endless supply of passion for programming, so I can and do spend A LOT of time on it, mostly on Google.

There are a few books available online.
Try here or this.

I haven't read these books myself, so I can't vouch for their quality.

well, once again thanks for the input, I will be keeping my eyes out for the books...i may grab a e book of c++ to read....*pops google up*

Quote:

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Originally Posted by laserlight

Take a look at our book recommendation thread. For novices I would point out Accelerated C++ - check if it is hiding among the many PHP books in your local library.

---

Also, most libraries allow cross-lending from one library to another, so if you ASK the staff nicely to look up this book:

Quote:

---

Accelerated C++: Practical Programming by Example
by Andrew Koenig (Author), Barbara E. Moo (Author)
ISBN: 020170353X

---

If they can't locate it, perhaps you can find a university library or some such that can lend it to you?

--
Mats

>> I will need to look for something a bit simpler perhaps.

void start_rubik( char cube[6][3][3] )

That solution is simple and is probably the best one for this problem anyway. Hopefully you will learn about dynamic memory later in your C++ studies, but I don't think there's any reason to use new/delete here.