dynamic 2D array handling ?

This is a discussion on dynamic 2D array handling ? within the C Programming forums, part of the General Programming Boards category; I am very new to the programming. Please don't be too harsh on me.^^ I allocate two dimensional array, get ...

  1. #1
    Registered User (*)BLUE's Avatar
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    Question dynamic 2D array handling ?

    I am very new to the programming. Please don't be too harsh on me.^^

    I allocate two dimensional array, get an input and want to printf the data.
    Could you tell me what may be wrong with this code, other than not freeing memory?

    The code compiled ok. Two scanfs work fine.
    Error occurs at the last printf on main, when I print out the math data.
    The error message says "access violation", which I don't understand why.
    BTW, I am using VC6.0.

    Code:
    #include<stdio.h> 
    #include<stdlib.h> 
    
    void getdata(int **a){ 
        int num_data=0,i; 
        printf("Input the number : "); 
        scanf("%d",&num_data); 
    
        /* allocacate two D. */
        a=(int **)malloc(num_data*sizeof(int*)); 
        for(i=0 ; i<num_data ; i++){ 
            a[i]=(int*)malloc(3*sizeof(int)); 
        } 
    
        printf("Math: "); 
        scanf("%d",&a[0][0]); 
    
        /* I know I have to delocate the memory somewhere. */
    } 
    
    int main(){ 
        int **a; 
        getdata(a); 
        printf("%d\n",a[0][0]); /* error */
        return 0;
    }

  2. #2
    Registered User (*)BLUE's Avatar
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    Lightbulb Oh.....

    Oh.... I see.

    So if I want to use &a,
    I shoud have used
    void getdata(int *** a){/*...*/}

    Am I right?

  3. #3
    Programming Sex-God Polymorphic OOP's Avatar
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    Also you're better of allocating your entire array in one block rather than as an array of pointers. So, for example, for a 10 x 3 Array of ints you'd be better off doing:

    int Array[30]; // 10 x 3

    // And then access elements by

    Array[ RowToAccess * NumCols + ColToAccess ];

    This is actually how multidimensional arrays are stored and accessed internally by the compiler when you don't dynamically create one. It shortens your construction time, shortens your destruction time, takes up less memory, and prevents memory fragmentation. And if you really don't like accessing like that, you can always put it in a function.

  4. #4
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    Thank you all.

    I will keep that in mind.

  5. #5
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    Later on, watch for precedence
    (*a)[i]=malloc(3*sizeof(int));
    Oh, I could've make that error too.
    Thank you Salem.

  6. #6
    Programming Sex-God Polymorphic OOP's Avatar
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    Originally posted by Salem
    int (*arr)[3] = malloc ( x * sizeof(*arr) );
    Very true, however you can use that if and only if you know the value of the last one or more dimensions of the array you are allocating at compile time which isn't always the case. It's good to get a full understanding of array representation and access for more varying circumstances.

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