Using malloc for allocation of multi-dimensional arrays with different row lengths

Question

I have the following C code    int  a  size t size   2000 sizeof int   a    int    malloc size     which works fine  But if I have the following    char   b   malloc 2000 sizeof  b     where every element of b has different length   How is it possible to do the same thing for b as i did for a  i e  the following code would hold correct   char  c  size t size   2000 sizeof char     c    char    malloc size

User · Answer

The other approach would be to allocate one contiguous chunk of memory comprising header block for pointers to rows as well as body block to store actual data in rows. Then just mark up memory by assigning addresses of memory in body to the pointers in header on per-row basis. It would look like follows:

int** 2dAlloc(int rows, int* columns) {    
    int header = rows * sizeof(int*);

    int body = 0;
    for(int i=0; i<rows; body+=columnSizes[i++]) {  
    }
    body*=sizeof(int);

    int** rowptr = (int**)malloc(header + body);

    int* buf  = (int*)(rowptr + rows);
    rowptr[0] = buf;
    int k;
    for(k = 1; k < rows; ++k) {
        rowptr[k] = rowptr[k-1] + columns[k-1];
    }
    return rowptr;
}

int main() {
    // specifying column amount on per-row basis
    int columns[] = {1,2,3};
    int rows = sizeof(columns)/sizeof(int);
    int** matrix = 2dAlloc(rows, &columns);

    // using allocated array
    for(int i = 0; i<rows; ++i) {
        for(int j = 0; j<columns[i]; ++j) {
            cout<<matrix[i][j]<<", ";
        }   
            cout<<endl;
    }

    // now it is time to get rid of allocated 
    // memory in only one call to "free"
    free matrix;
}

The advantage of this approach is elegant freeing of memory and ability to use array-like notation to access elements of the resulting 2D array.

User · Answer

The typical form for dynamically allocating an NxM array of type T is  T   a   malloc sizeof  a   N   if  a      for  i   0  i  lt  N  i            a i    malloc sizeof  a i    M           If each element of the array has a different length  then replace M with the appropriate length for that element  for example  T   a   malloc sizeof  a   N   if  a      for  i   0  i  lt  N  i            a i    malloc sizeof  a i    length for this element

User · Answer

malloc does not allocate on specific boundaries  so it must be assumed that it allocates on a byte boundary   The returned pointer can then not be used if converted to any other type  since accessing that pointer will probably produce a memory access violation by the CPU  and the application will be immediately shut down

User · Answer

If every element in b has different lengths  then you need to do something like   int totalLength   0  for every element in b       totalLength    length of this b in bytes    return  char    malloc totalLength

User · Answer

First  you need to allocate array of pointers like char   c   malloc  N   sizeof  char      then allocate each row with a separate call to malloc  probably in the loop       N is the number of rows        note  c is char      if    c   malloc  N sizeof  char         NULL        error       for   i   0  i  lt  N  i            x i here is the size of given row  no need to      multiply by sizeof  char    it s always 1         if    c i    malloc  x i       NULL          error            probably init the row here          access matrix elements  c i  give you a pointer    to the row array  c i  j  indexes an element     c i  j     a     If you know the total number of elements  e g  N M  you can do this in a single allocation

User · Answer

2-D Array Dynamic Memory Allocation  int   a i      for any number of rows  amp  columns this will work a    int    malloc rows sizeof int      for i 0 i lt rows i          a i     int   malloc cols sizeof int

User · Answer

I think a 2 step approach is best  because c 2-d arrays are just and array of arrays   The first step is to allocate a single array  then loop through it allocating arrays for each column as you go   This article gives good detail

User · Answer

Equivalent memory allocation for char a 10  20  would be as follows   char   a   a  char     malloc 10 sizeof char       for i 0 i lt 10 i        a i   char    malloc 20 sizeof char      I hope this looks simple to understand

[c] Using malloc for allocation of multi-dimensional arrays with different row lengths

Examples related to c

Examples related to arrays

Examples related to malloc