need help in C++
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Thread: need help in C++

  1. #1
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    need help in C++

    Hi friend, I know how to specify a dynamic 1-dimension array. For example,
    int k = 20;
    char *s=new char[k];
    But I don't know that for the 2-dimension array.
    Anybody knows, please help me.
    And, when I can specify the dynamic 2-dimension array, how can I call it in a function. For example, I have a 2-dimension array named S2[][], and I want to refer to it in a funtion's variable, for instance: lexico(char S2[][k], int n). Because the number k here is only known when running the program, the compiler will announce the error: unbound array (or something like that). Anyone knows the way to solve this, please help.
    By the way, if in the process of debugging, it is announced that: 0xC0000005:Access violation; what can I do then? Could you help me the way out of the error?
    Thanks
    ILCF

  2. #2
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    What compiler are you using? C/C++ does not support arrays with variable sizes. Your code
    int k = 20;
    char *s=new char[k];
    will and should generate an error. If it doesn't there's something seriously wrong with your compiler and I wouldn't use it to compile anything because I don't think random crashes are what you expect from your program. However, i could have misunderstood your question, in which case please clarify further.
    Cheers,
    cgkanchi
    Buy the Snakes of India book, support research and education (sorry the website has been discontinued)
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  3. #3
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    You need to declare a first array of double (char) pointers to second dimension arrays:
    ex:

    int k = 20;
    //The first dimension is in fact an array of pointers
    char **s = new (char*)[k]; //Not sure about the "new" notation for for a char* array... (I'm used to malloc which would be "char **s = (char**)malloc( sizeof(char**) * k ) )

    for( int i = 0; i<k; i++ )
    {
    s* = new char[k];
    }


    After which you can refere to that 2-dimension array (matrix) as
    s[x][y]

    Ammo
    Credit travels up, blame travels down -- The Boss

  4. #4
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    I think it might have something to do with how the compiler actually interprets arrays. I believe (though am not certain) that in a static array such as: foo[ xSize ][ ySize ], the compiler sees it as foo[ xSize * ySize ]. For a two dimensional array of say 2 * 2. the declaration: char* foo = new char[ xSize * ySize ]; would create a dynamic "two-dimensional" array. You can access array scalars with a simple calculation say you wish to access the char held in "foo[ 0 ][ 1 ]" you could code: foo[ 0 * xSize + 1 ]; where xSize is the number of rows in the array. Or, if you wish to access "foo[ 1 ][ 1 ]", the code: foo[ 1 * rows + 1 ] should retrieve the char stored there. I believe that is how it is done, unforunatly it's been a while, and i do not have a compiler on this comp.

    *unfortunatly. And i think i meant to say XSize represents the size of the columns, not rows.
    \"All truth passes through three stages. First, it is ridiculed. Second, it is violently opposed. Third, it is accepted as being self evident.\"
    —Arthur Schopenhauer

  5. #5
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    Greetings,
    /*
    std::cin >> size ;
    //The 2D array is a pointer to a pointer to an int.
    int ** magicSquare ;
    // Allocate an array of pointers to int and make magicSquare a pointer to the base element of the array.
    magicSquare = new int * [size] ;
    // For each row i of magicSquare (magicSquare[i]), allocate an array of int's and make magicSquare[i] a pointer to the base element of the array.
    for ( int i=0; i < size; i++ ) magicSquare[i] = new int[size] ;
    */

    can you could pass the 2d array to a function as a pointer to the pointer,
    void function( int **magicSquare, int something_ whatever ) {/*magicSquare[ x ][ y ]?*/}
    Hmm...theres something a little peculiar here. Oh i see what it is! the sentence is talking about itself! do you see that? what do you mean? sentences can\'t talk! No, but they REFER to things, and this one refers directly-unambigeously-unmistakably-to the very sentence which it is!

  6. #6
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    Besides a small mistake (wrote it quickly.. sorry) from my part (s* = new char[k]; should be s[i] = new char[k]; ), that answer looks pretty much like mine doesn't it...

    Besides, agranam, it would work, but it's not the "accepted" way of doing it...

    Ammo
    Credit travels up, blame travels down -- The Boss

  7. #7
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    yup, the exact same lest the new[] thing, i wrapped it with different english words though, as another explanation for the record of this thread...
    Hmm...theres something a little peculiar here. Oh i see what it is! the sentence is talking about itself! do you see that? what do you mean? sentences can\'t talk! No, but they REFER to things, and this one refers directly-unambigeously-unmistakably-to the very sentence which it is!

  8. #8
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    To cgkanchi,
    Nope, man. What I mentioned is compiled well by MS Visual C++. No error announced.

    To ammo,
    Your code seems to be alright when compiling, but when running, it cause error and stop. I tried to debug, the error is "access violation". That means something wrong with the array we declared. Can you try it again man
    Thanks very much for your replies
    ILCF

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