multi dimensional array at runtime

This is a discussion on multi dimensional array at runtime within the C++ Programming forums, part of the General Programming Boards category; Hi I have 2 question: 1-)I know vector class in stl but i want to make a multi-dimensional array.While user ...

  1. #1
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    multi dimensional array at runtime

    Hi
    I have 2 question:
    1-)I know vector class in stl but i want to make a multi-dimensional array.While user enter some parameters its lengh will grow.
    How can i do this with "pure c".Only way linked list?Or are there another ways?

    2-)In c++
    i try :
    Code:
    #include <iostream>
     using namespace std; 
    int main() 
    { 
    int x,y;
     cout << "first number";
     cin >> x; cout << "second nunmber";
     cin >> y; 
    int * a[][] = new int[x][y];
     delete[] a;
     return 0;
     }
    It gives error.
    How can i declare multi dimensional array with new in c++

    thanks.

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    You could make a single dimensional array with size x*y
    Cell i,j would be accessed by the index i * y + j

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    Code:
    //Like This//
    //Change//
    int * a[][] = new int[x][y];
    //To//
    int a[x][y];
    Last edited by unkownname; 05-10-2006 at 05:17 PM.

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    Hi, "sawer".

    In pure "C", I think your only option for creating dynamic arrays is malloc. You input the matrix or vector dimension, then use malloc to allocate the appropriate amount of space, do your computations, and then deallocate the memory before exiting the program.

    In C++, you could do something similar using "new" and "free"; however, the STL's <vector> class is the best way to go, even for multidimenional arrays.

    I have posted three versions of the same small program off the following page:

    Dynamic Arrays Intro

    Versions 1 and 2 are the ones you should focus on, unless you happen to be using an older comiler. They should give you the basic concepts for building your own programs.

    Regards,


    David

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    new returns a pointer, so in order to allocate a 2D array, first allocate an array of pointers:
    Code:
    int a(2);
    int** arr = new int* [a];
    Then you can allocate each sub-array seperately:
    Code:
    int b(3);
    for(int i(0); i!=a; ++i)
       arr[i] = new int[b];
    the STL is definitely the best way, otherwise I believe 'matrix' libraries exist which do this sort of thing.
    If you insist on creating 2D arrays from scratch, give this page a read
    http://www.parashift.com/c++-faq-lit...html#faq-16.16

  6. #6
    Devil's Advocate SlyMaelstrom's Avatar
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    Quote Originally Posted by unkownname
    Code:
    //Like This//
    //Change//
    int * a[][] = new int[x][y];
    //To//
    int a[x][y];
    Dynamic means it's created at runtime and stored on the freestore. Your example of:
    Code:
    int a[x][y];
    is created at compile time.
    Sent from my iPadŽ

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    Alright Dont see much Difference besides the way its created but oh well I tried.

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    >> Dont see much Difference besides the way its created but oh well I tried.
    The difference is that your version is illegal in C++ unless x and y are const (although it is legal in C and will work on at least one C++ compiler as an extension).

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    Are you sure of that? Like really sure Because I tried it out on DevC++ and it worked.

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    Dev-C++ uses the one C++ compiler that allows it as an extension (gcc).

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    oh didnt see the at least on one c++ compiler And I know devc++ is just a GUI for gcc c++ compiler.

  12. #12
    Registered User whiteflags's Avatar
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    Yes, if x and y are variables,
    Code:
    int a[x][y];
    is very illegal.

  13. #13
    semi-colon generator ChaosEngine's Avatar
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    Quote Originally Posted by unkownname
    Are you sure of that? Like really sure Because I tried it out on DevC++ and it worked.
    I'm surprised at that, but Daved is correct. It is non-standard C++.
    "I saw a sign that said 'Drink Canada Dry', so I started"
    -- Brendan Behan

    Free Compiler: Visual C++ 2005 Express
    If you program in C++, you need Boost. You should also know how to use the Standard Library (STL). Want to make games? After reading this, I don't like WxWidgets anymore. Want to add some scripting to your App?

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    Alright I get your point it just seemed to work Dont get all flamy at me. your forgetting the reason for this post.

    One more thing I know its illegal first of all But its being decalred after it has x and y so wouldnt that make it legal?
    Last edited by unkownname; 05-10-2006 at 05:53 PM.

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    Quote Originally Posted by unkownname
    Alright I get your point it just seemed to work Dont get all flamy at me. your forgetting the reason for this post.

    One more thing I know its illegal first of all But its being decalred after it has x and y so wouldnt that make it legal?
    The reason that the standard disallows you from doing this is a bit like a stupidity protection system - the compiler has no guarantee that the non-const variable will not be changed afterwards.

    The "stupidity protection" is for people who might otherwise try something like this:
    Code:
    int y=5;
    int x[y];
    // code to populate x goes here..
    // "oh, i want to resize my array.. i'll just do this..."
    y=7;
    x[6] = 42;
    // Bad, bad, bad..
    There may actually be other good reasons why the standard was written this way, but this is a good enough reason IMHO
    Last edited by Bench82; 05-10-2006 at 06:15 PM.

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