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Dynamically Allocated Multidimensional Array

This is a discussion on Dynamically Allocated Multidimensional Array within the C++ Programming forums, part of the General Programming Boards category; If I dynamically allocate a multidimensional array like this: Code: int* Example = new int[numInts][5]; do I still go about ...

  1. #1
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    Dynamically Allocated Multidimensional Array

    If I dynamically allocate a multidimensional array like this:

    Code:
    int* Example = new int[numInts][5];
    do I still go about freeing it like this?

    Code:
    delete[] Example;
    Also, how would I go about passing Example to another function?

  2. #2
    C++ Witch laserlight's Avatar
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    Quote Originally Posted by LyTning94
    If I dynamically allocate a multidimensional array like this:
    ... you get a compile error

    But yes, if you had written:
    Code:
    int (*Example)[5] = new int[numInts][5];
    then:
    Code:
    delete[] Example;
    would be appropriate.
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    You can't allocate a multidimensional array like that. See this: Multi-Dimensional Arrays - C++ Forum

    In short, to dynamically allocate a multidimensional array, you can do something like this:

    Code:
    	int **example = new int*[x];
    	for(int i = 0; i < x; ++i)
    		example[i] = new int[y];
    Then, to deallocate it, you can do something like this:

    Code:
    	  for (int i = 0; i < x; ++i)
    		delete [] example[i];
    	  delete [] example;

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    Thanks. How do I pass that array to another function though?

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    and the Hat of Guessing tabstop's Avatar
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    Ushakal's example? It is an int**, so you would pass it as such.

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    Click on the link that was provided. It answers that question, and also informs you that you should be using std::vector for this.
    Quote Originally Posted by anduril462 View Post
    Now, please, for the love of all things good and holy, think about what you're doing! Don't just run around willy-nilly, coding like a drunk two-year-old....
    Quote Originally Posted by quzah View Post
    ..... Just don't be surprised when I say you aren't using standard C anymore, and as such,are off in your own little universe that I will completely disregard.
    Warning: Some or all of my posted code may be non-standard and as such should not be used and in no case looked at.

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    Registered User whiteflags's Avatar
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    Quote Originally Posted by LyTning94 View Post
    Also, how would I go about passing Example to another function?
    If you did what laserlight showed you it's the same as any other array of arrays:

    void foo ( int Example[][5] );

    foo ( Example );

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    Quote Originally Posted by whiteflags View Post
    If you did what laserlight showed you it's the same as any other array of arrays:

    void foo ( int Example[][5] );

    foo ( Example );
    If I do this do I need to free the memory in the original function, the second function, or both???

  9. #9
    and the Hat of Guessing tabstop's Avatar
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    NEVER try to free memory more than once.

    You free it when you are done with it; not before.

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    C++まいる!Cをこわせ! Elysia's Avatar
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    Quote Originally Posted by Ushakal View Post
    You can't allocate a multidimensional array like that...
    Incorrect. It is possible, and laserlight just showed how to do it.

    Quote Originally Posted by LyTning94 View Post
    If I do this do I need to free the memory in the original function, the second function, or both???
    Is there a reason you won't use the vector-based approach?
    Quote Originally Posted by Adak View Post
    io.h certainly IS included in some modern compilers. It is no longer part of the standard for C, but it is nevertheless, included in the very latest Pelles C versions.
    Quote Originally Posted by Salem View Post
    You mean it's included as a crutch to help ancient programmers limp along without them having to relearn too much.

    Outside of your DOS world, your header file is meaningless.

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    Quote Originally Posted by Elysia View Post
    Incorrect. It is possible, and laserlight just showed how to do it.
    Yes, I know, I just meant that the way the OP had written it wouldn't compile.

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    Quote Originally Posted by Elysia View Post
    Is there a reason you won't use the vector-based approach?
    I'm still a sort of programming novice and have never used vectors before. From what I've heard though it's probably the better way to go, so I'll look into it.

    You can allocate vectors of variable size, correct? (Like in this example):

    Code:
    int size;
    
    cin>>size;
    
    vector<vector<int> > Example;
    
    Example.resize(size);
    for (i = 0; i < size; i++)
    {
           Example[i].resize(2);
    }
    And do I need to free the memory at the end with Example.erase()?
    Last edited by LyTning94; 08-06-2011 at 09:48 AM.

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    See Cprogramming.com STL Tutorial: Vector Class under the benefits heading (no you don't have to free them). Learning to use std::vector is not too difficult if you've already got the handle of arrays, and it's good that you're giving it a shot. Remember to look up full documentation for a list of functions which can conveniently modify the vectors you work with.

    EDIT: This was also interesting wrt the freeing question. http://www.velocityreviews.com/forum...eeing-mem.html

    Or just google "freeing std::vector", there are plenty of discussions on the internets.
    Last edited by Ocifer; 08-06-2011 at 10:09 AM.

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    Thanks. How do I use push_back() with a 2D vector? Say I have the array below and want to add a 7 to the red element, do I do something like Example[0].push_back(7)?

    [1][2][3][4][5][6][][][][]
    [3][4][2][5][7][4][][][][]
    Last edited by LyTning94; 08-06-2011 at 10:20 AM.

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    Umm...do you not click on any links? Multidimensional Arrays-C++ Click the thing in red, it has answered all these questions
    Quote Originally Posted by anduril462 View Post
    Now, please, for the love of all things good and holy, think about what you're doing! Don't just run around willy-nilly, coding like a drunk two-year-old....
    Quote Originally Posted by quzah View Post
    ..... Just don't be surprised when I say you aren't using standard C anymore, and as such,are off in your own little universe that I will completely disregard.
    Warning: Some or all of my posted code may be non-standard and as such should not be used and in no case looked at.

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