27 replies to this topic

[DMK741]

I need to create an array of pointers that point to int arrays. Can someone clarify the proper syntax for me?

[mnirahd]

Hi,

int array[100];

int **pointerToArray = & array;

I hope this helps!

Munir

[brownhead]

I think that this might be what your after.

int i[1000][1000]; // Creates a 2-dimensional array

But it sounds as if you don't want to have the array be contiguous in memory. Meaning if the array's dimensions were 1000x1000, you don't want a single chunk of 1,000,000 objects in your RAM cause it might not fit too great. If this is the case, take a look at the following code (if it isn't, ignore the following code).

int * array[1000]; // 1000 is the width of the array
for (int i = 0; i < 1000; ++i)
    array[i] = new int[1000]; // 1000 is also the height of the array

// Executed at some point..
for (int i = 0; i < 1000; ++i)
    delete [] array[i];

[DMK741]

Actually, what I'm trying to do is create a 3D array on the heap. and in order to do so, I have to mess around with pointers. Unfortunately, this got pretty confusing.

[brownhead]

Then perhaps this is closer to what your after.

int a[1000][1000][1000]; // 3-Dimensional array

And the non-contiguous version..

int *** a = new int ** [1000];
for (int i = 0; i < 1000; ++i) a[i] = new int*[1000];
for (int i = 0; i < 1000; ++i)
    for (int j = 0;  j < 1000; ++j)
        a[i][j] = new int[1000];

(You should of course be using vectors, but you did explicitly say array so I assumed you were talking about true arrays)

[DMK741]

Why are vectors any better to use?

[brownhead]

Vectors provide automation of various common tasks done with arrays with negligible overhead. A vector will also automatically deallocate itself which is immensely useful.
I will go ahead and say, always use vectors and never use arrays. There will be exceptions to this of course, but if you are a beginning programmer you don't have to worry about such exceptions.

Reference on Vectors

[DMK741]

I see... thank you for showing me this. However, it is possible that I may end up working on something with very limited space. Also, I'd like to learn to do it the hard way before I do it the easy way. I'll play around with these arrays some more, and if I need help I'll post back.

[brownhead]

Alright, however, using arrays over vectors does not save space. A vector will store the length of the array, which means it is storing sizeof(size_t) extra bytes of space, however, you'd need this information almost certainly anyways. Also, the size of an int which is usually 4 bytes, is extremely tiny compared to your array. Learning is fine, but if your not exploring how to work with arrays, use vectors, it is always the better choice. (Again, exceptions: yes, worry: no).

[dargueta]

FYI if you're working on something with "very limited space" you're probably going to want to use straight C. Since vectors are a C++ thing, you might have to use regular arrays anyway.

[DMK741]

byte*** bob = new byte**[4];

bob[0] = new byte*[5];

bob[0][0] = new byte[5];



I get an error over the first bracket here: "expected an expression". Any idea what I'm doing wrong?

bob[0][0] = {0, 0, 0, 0, 0};

 Untitled.png   3.36K   9 downloads

[dargueta]

byte*** bob = new byte**[4];

bob[0] = new byte*[5];

bob[0][0] = new byte[5];  [COLOR="blue"]/* why are you doing this? */[/COLOR]


[COLOR="blue"]/* Illegal unless you're doing it all at once at initialization */[/COLOR]

bob[0][0] = {0, 0, 0, 0, 0};