How do I work with dynamic multi-dimensional arrays in C

Question

Does someone know how I can use dynamically allocated multi-dimensional arrays using C  Is that possible

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use new instead of malloc as using malloc leads to memory leaks   enter code here      int   adj list   new int  rowsize              for int i   0  i  lt  rowsize    i                     adj list i    new int colsize

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Since C99  C has 2D arrays with dynamical bounds  If you want to avoid that such beast are allocated on the stack  which you should   you can allocate them easily in one go as the following  double   A  n    malloc sizeof double n  n       and that s it  You can then easily use it as you are used for 2D arrays with something like A i  j   And don t forget that one at the end  free A     Randy Meyers wrote series of articles explaining variable length arrays  VLAs

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Basics  Arrays in c are declared and accessed using the    operator  So that  int ary1 5     declares an array of 5 integers  Elements are numbered from zero so ary1 0  is the first element  and ary1 4  is the last element  Note1  There is no default initialization  so the memory occupied by the array may initially contain anything  Note2  ary1 5  accesses memory in an undefined state  which may not even be accessible to you   so don t do it   Multi-dimensional arrays are implemented as an array of arrays  of arrays  of          So  float ary2 3  5     declares an array of 3 one-dimensional arrays of 5 floating point numbers each  Now ary2 0  0  is the first element of the first array  ary2 0  4  is the last element of the first array  and ary2 2  4  is the last element of the last array  The  89 standard requires this data to be contiguous  sec  A8 6 2 on page 216 of my K amp R 2nd  ed   but seems to be agnostic on padding   Trying to go dynamic in more than one dimension  If you don t know the size of the array at compile time  you ll want to dynamically allocate the array  It is tempting to try  double  buf3  buf3   malloc 3 5 sizeof double       error checking goes here      which should work if the compiler does not pad the allocation  stick extra space between the one-dimensional arrays   It might be safer to go with   double  buf4  buf4   malloc sizeof double 3  5        error checking      but either way the trick comes at dereferencing time  You can t write buf i  j  because buf has the wrong type  Nor can you use  double   hdl4    double   buf  hdl4 2  3    0     Wrong       because the compiler expects hdl4 to be the address of an address of a double  Nor can you use double incomplete ary4      because this is an error   So what can you do    Do the row and column arithmetic yourself Allocate and do the work in a function Use an array of pointers  the mechanism qrdl is talking about    Do the math yourself  Simply compute memory offset to each element like this     for  i 0  i lt 3    i        for j 0  j lt 3    j           buf3 i   5   j    someValue i j      Don t need to worry about                                               padding in this case                 Allocate and do the work in a function  Define a function that takes the needed size as an argument and proceed as normal  void dary int x  int y     double ary4 x  y     ary4 2  3    5      Of course  in this case ary4 is a local variable and you can not return it  all the work with the array must be done in the function you call of in functions that it calls   An array of pointers  Consider this   double   hdl5   malloc 3 sizeof double        Error checking    for  i 0  i lt 3    i      hdl5 i    malloc 5 sizeof double         Error checking        Now hdl5 points to an array of pointers each of which points to an array of doubles  The cool bit is that you can use the two-dimensional array notation to access this structure---hdl5 0  2  gets the middle element of the first row---but this is none-the-less a different kind of object than a two-dimensional array declared by double ary 3  5     This structure is more flexible then a two dimensional array  because the rows need not be the same length   but accessing it will generally be slower and it requires more memory  you need a place to hold the intermediate pointers    Note that since I haven t setup any guards you ll have to keep track of the size of all the arrays yourself    Arithmetic  c provides no support for vector  matrix or tensor math  you ll have to implement it yourself  or bring in a library   Multiplication by a scaler and addition and subtraction of arrays of the same rank are easy  just loop over the elements and perform the operation as you go  Inner products are similarly straight forward   Outer products mean more loops

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Here is working code that defines a subroutine make 3d array to allocate a multidimensional 3D array with N1  N2 and N3 elements in each dimension  and then populates it with random numbers  You can use the notation A i  j  k  to access its elements    include  lt stdio h gt   include  lt stdlib h gt   include  lt time h gt       Method to allocate a 2D array of floats float    make 3d array int nx  int ny  int nz        float    arr      int i j       arr    float      malloc nx sizeof float           for  i   0  i  lt  nx  i              arr i     float     malloc ny sizeof float              for j   0  j  lt  ny  j                  arr i  j     float    malloc nz   sizeof float                         return arr        int main int argc  char  argv          int i  j  k      size t N1 10 N2 20 N3 5          allocates 3D array     float    ran   make 3d array N1  N2  N3           initialize pseudo-random number generator     srand time NULL             populates the array with random numbers     for  i   0  i  lt  N1  i             for  j 0  j lt N2  j                  for  k 0  k lt N3  k                      ran i  j  k      float rand    float  RAND MAX                                         prints values     for  i 0  i lt N1  i              for  j 0  j lt N2  j                  for  k 0  k lt N3  k                      printf  A  d   d   d     f  n   i j k ran i  j  k                                       free ran

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If you know the number of columns at compile time  it s pretty simple    define COLS         size t rows     get number of rows T   ap  COLS    malloc sizeof  ap   rows      ap is a  pointer to an array  of T   You can treat ap like any 2D array   ap i  j    x    When you re done you deallocate it as  free ap     If you don t know the number of columns at compile time  but you re working with a C99 compiler or a C2011 compiler that supports variable-length arrays  it s still pretty simple   size t rows  size t cols     get rows and cols T   ap  cols    malloc sizeof  ap   rows       ap i  j    x      free ap     If you don t know the number of columns at compile time and you re working with a version of C that doesn t support variable-length arrays  then you ll need to do something different   If you need all of the elements to be allocated in a contiguous chunk  like a regular array   then you can allocate the memory as a 1D array  and compute a 1D offset   size t rows  cols     get rows and columns T  ap   malloc sizeof  ap   rows   cols       ap i   rows   j    x      free ap     If you don t need the memory to be contiguous  you can follow a two-step allocation method   size t rows  cols     get rows and cols T   ap   malloc sizeof  ap   rows   if  ap      size t i   0    for  i   0  i  lt  cols  i            ap i    malloc sizeof  ap i    cols          ap i  j    x    Since allocation was a two-step process  deallocation also needs to be a two-step process   for  i   0  i  lt  cols  i      free ap i    free ap

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With dynamic allocation  using malloc   int   x   x   malloc dimension1 max   sizeof  x    for  int i   0  i  lt  dimension1 max  i        x i    malloc dimension2 max   sizeof x 0          Writing values x 0   dimension1 max-1   0   dimension2 max-1     Value          for  int i   0  i  lt  dimension1 max  i        free x i      free x     This allocates an 2D array of size dimension1 max   dimension2 max  So  for example  if you want a 640 480 array  f e  pixels of an image   use dimension1 max   640  dimension2 max   480  You can then access the array using x d1  d2  where d1   0  639  d2   0  479   But a search on SO or Google also reveals other possibilities  for example in this SO question  Note that your array won t allocate a contiguous region of memory  640 480 bytes  in that case which could give problems with functions that assume this  So to get the array satisfy the condition  replace the malloc block above with this   int   x  int  temp   x   malloc dimension1 max   sizeof  x    temp   malloc dimension1 max   dimension2 max   sizeof x 0     for  int i   0  i  lt  dimension1 max  i        x i    temp    i   dimension2 max             free temp   free x

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int rows  columns     initialize rows and columns to the desired value         arr    int   malloc rows sizeof int             for i 0 i lt rows i                          arr i     int  malloc cols sizeof int

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There s no way to allocate the whole thing in one go  Instead  create an array of pointers  then  for each pointer  create the memory for it  For example   int   array  array    int   malloc sizeof int     50   for int i   0  i  lt  50  i        array i     int  malloc sizeof int    50     Of course  you can also declare the array as int  array 50  and skip the first malloc  but the second set is needed in order to dynamically allocate the required storage   It is possible to hack a way to allocate it in a single step  but it would require a custom lookup function  but writing that in such a way that it will always work can be annoying  An example could be L arr x y max x  arr  y   max x     x    then malloc a block of 50 50 ints or whatever and access using that L macro  e g    define L arr x y max x  arr  y   max x     x    int dim x   50  int dim y   50   int  array   malloc dim x dim y sizeof int     int foo   L array  4  6  dim x     But that s much nastier unless you know the effects of what you re doing with the preprocessor macro

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malloc will do    int rows   20   int cols   20   int  array     array   malloc rows   cols   sizeof int      Refer the below article for help -  http   courses cs vt edu  cs2704 spring00 mcquain Notes 4up Managing2DArrays pdf

[c] How do I work with dynamic multi-dimensional arrays in C?

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