How do I declare a 2d array in C using new

Question

How do i declare a 2d array using new   Like  for a  normal  array I would   int  ary   new int Size    but  int   ary   new int sizeY  sizeX    a  doesn t work compile and b  doesn t accomplish what   int ary sizeY  sizeX     does

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I have left you with a solution which works the best for me, in certain cases. Especially if one knows [the size of?] one dimension of the array. Very useful for an array of chars, for instance if we need an array of varying size of arrays of char[20].

int  size = 1492;
char (*array)[20];

array = new char[size][20];
...
strcpy(array[5], "hola!");
...
delete [] array;

The key is the parentheses in the array declaration.

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Try doing this   int   ary   new int   sizeY   for  int i   0  i  lt  sizeY  i        ary i    new int sizeX

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The purpose of this answer is not to add anything new that the others don t already cover  but to extend  Kevin Loney s answer   You could use the lightweight declaration   int  ary   new int SizeX SizeY    and access syntax will be   ary i SizeY j         ary i  j    but this is cumbersome for most  and can lead to confusion  So  you can define a macro as follows    define ary i  j    ary  i  SizeY    j     Now you can access the array using the very similar syntax ary i  j      means ary i  j   This has the advantages of being simple and beautiful  and at the same time  using expressions in place of the indices is also simpler and less confusing   To access  say  ary 2 5  3 8   you can write ary 2 5  3 8  instead of the complex-looking ary  2 5  SizeY    3 8   i e  it saves parentheses and helps readability   Caveats    Although the syntax is very similar  it is NOT the same  In case you pass the array to other functions  SizeY has to be passed with the same name  or instead be declared as a global variable     Or  if you need to use the array in multiple functions  then you could add SizeY also as another parameter in the macro definition like so    define ary i  j  SizeY   ary  i   SizeY   j     You get the idea  Of course  this becomes too long to be useful  but it can still prevent the confusion of   and     This is not recommended definitely  and it will be condemned as bad practice by most experienced users  but I couldn t resist sharing it because of its elegance   Edit  If you want a portable solution that works for any number of arrays  you can use this syntax      define access ar  i  j  SizeY  ar  i   SizeY   j     and then you can pass on any array to the call  with any size using the access syntax     access ary  i  j  SizeY          ary i  j    P S   I ve tested these  and the same syntax works  as both an lvalue and an rvalue  on g  14 and g  11 compilers

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Below example may help   int main void        double   a2d   new double  5           initializing Number of rows  in this case 5 rows         for  int i   0  i  lt  5  i                  a2d i    new double 3      initializing Number of columns  in this case 3 columns               for  int i   0  i  lt  5  i                  for  int j   0  j  lt  3  j                          a2d i  j    1     Assigning value 1 to all elements                         for  int i   0  i  lt  5  i                  for  int j   0  j  lt  3  j                          cout  lt  lt  a2d i  j   lt  lt  endl      Printing all elements to verify all elements have been correctly assigned or not                         for  int i   0  i  lt  5  i            delete   a2d i        delete   a2d        return 0

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If you want an 2d array of integers  which elements are allocated sequentially in memory  you must declare it like   int   intPtr  n    new int x  n    where instead of x you can write any dimension  but n must be the same in two places  Example  int   intPtr  8    new int 75  8   intPtr 5  5    6  cout lt  lt intPtr 0  45  lt  lt endl    must print 6

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Here  I have two options  The first one shows the concept of an array of arrays or pointer of pointers  I prefer the second one because the addresses are contiguous  as you can see in the image      include  lt iostream gt   using namespace std    int main         int   arr 01   arr 02 i j rows 4 cols 5         Implementation 1     arr 01 new int  rows        for int i 0 i lt rows i            arr 01 i  new int cols        for i 0 i lt rows i             for j 0 j lt cols j                cout  lt  lt  arr 01 i  j  lt  lt                cout  lt  lt  endl              for int i 0 i lt rows i            delete   arr 01 i       delete   arr 01        cout  lt  lt  endl        Implementation 2     arr 02 new int  rows       arr 02 0  new int rows cols       for int i 1 i lt rows i            arr 02 i  arr 02 0  cols i       for int i 0 i lt rows i             for int j 0 j lt cols j                cout  lt  lt  arr 02 i  j  lt  lt                cout  lt  lt  endl             delete   arr 02 0       delete   arr 02        return 0

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Start by defining the array using pointers  Line 1    int   a   new int   x         x is the number of rows for int i   0  i  lt  x  i        a i    new int y         y is the number of columns

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I used this not elegant but FAST EASY and WORKING system  I do not see why can not work because the only way for the system to allow create a big size array and access parts is without cutting it in parts    define DIM 3  define WORMS 50000   gusanos  void halla centros V000 double CENW   DIM         CENW i  j                  int main         double  CENW MEM new double WORMS DIM       double   CENW  DIM       CENW  double     3    amp CENW MEM 0       halla centros V000 CENW       delete   CENW MEM

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I m using this when creating dynamic array  If you have a class or a struct  And this works  Example   struct Sprite       int x      int main         int num   50     Sprite   spritearray   a pointer to a pointer to an object from the Sprite class    spritearray   new Sprite   num      for  int n   0  n  lt  num  n             spritearray n    new Sprite         spritearray- gt x   n   3            delete from random position     for  int n   0  n  lt  num  n              if  spritearray n - gt x  lt  0          delete spritearray n         spritearray n    NULL                        delete the array     for  int n   0  n  lt  num  n            if  spritearray n     NULL            delete spritearray n            spritearray n    NULL                    delete   spritearray      spritearray   NULL      return 0

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If your project is CLI  Common Language Runtime Support   then   You can use the array class  not that one you get when you write    include  lt array gt  using namespace std    In other words  not the unmanaged array class you get when using the std namespace and when including the array header  not the unmanaged array class defined in the std namespace and in the array header  but the managed class array of the CLI   with this class  you can create an array of any rank you want   The following code below creates new two dimensional array of 2 rows and 3 columns and of type int  and I name it  arr    array lt int  2 gt   arr   gcnew array lt int  2 gt  2  3     Now you can access elements in the array  by name it and write only one squared parentheses     and inside them  add the row and column  and separate them with the comma     The following code below access an element in 2nd row and 1st column of the array I already created in previous code above   arr 0  1    writing only this line is to read the value in that cell  i e  get the value in this cell  but if you add the equal   sign  you are about to write the value in that cell  i e  set the value in this cell  You also can use the     -      and    operators of course  for numbers only  int  float  double    int16    int32    int64 and etc   but sure you know it already   If your project is not CLI  then you can use the unmanaged array class of the std namespace  if you  include  lt array gt   of course  but the problem is that this array class is different than the CLI array  Create array of this type is same like the CLI  except that you will have to remove the   sign and the gcnew keyword  But unfortunately the second int parameter in the  lt  gt  parentheses specifies the length  i e  size  of the array  not its rank   There is no way to specify rank in this kind of array  rank is CLI array s feature only    std array behaves like normal array in c    that you define with pointer  for example int  and then  new int size   or without pointer  int arr size   but unlike the normal array of the c    std array provides functions that you can use with the elements of the array  like fill  begin  end  size  and etc  but normal array provides nothing   But still std array are one dimensional array  like the normal c   arrays  But thanks to the solutions that the other guys suggest about how you can make the normal c   one dimensional array to two dimensional array  we can adapt the same ideas to std array  e g  according to Mehrdad Afshari s idea  we can write the following code   array lt array lt int  3 gt   2 gt  array2d   array lt array lt int  3 gt   2 gt       This line of code creates a  jugged array   which is an one dimensional array that each of its cells is or points to another one dimensional array   If all one dimensional arrays in one dimensional array are equal in their length size  then you can treat the array2d variable as a real two dimensional array  plus you can use the special methods to treat rows or columns  depends on how you view it in mind  in the 2D array  that std array supports   You also can use Kevin Loney s solution   int  ary   new int sizeX sizeY       ary i  j  is then rewritten as ary i sizeY j    but if you use std array  the code must look different   array lt int  sizeX sizeY gt  ary   array lt int  sizeX sizeY gt     ary at i sizeY j     And still have the unique functions of the std array   Note that you still can access the elements of the std array using the    parentheses  and you don t have to call the at function  You also can define and assign new int variable that will calculate and keep the total number of elements in the std array  and use its value  instead of repeating sizeX sizeY  You can define your own two dimensional array generic class  and define the constructor of the two dimensional array class to receive two integers to specify the number of rows and columns in the new two dimensional array  and define get function that receive two parameters of integer that access an element in the two dimensional array and returns its value  and set function that receives three parameters  that the two first are integers that specify the row and column in the two dimensional array  and the third parameter is the new value of the element  Its type depends on the type you chose in the generic class   You will be able to implement all this by using either the normal c   array  pointers or without  or the std array and use one of the ideas that other people suggested  and make it easy to use like the cli array  or like the two dimensional array that you can define  assign and use in C

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I don t know for sure if the following answer wasn t provided but I decided to add some local optimizations to the allocation of 2d array  e g   a square matrix is done through only one allocation    int   mat   new int  n   mat 0    new int  n   n     However  deletion goes like this because of linearity of the allocation above   delete    mat 0   delete    mat

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How to allocate a contiguous multidimensional array in GNU C     There s a GNU extension that allows the  standard  syntax to work   It seems the problem come from operator new     Make sure you use operator new instead    double    in  n  n    new  double m  n  n        GNU extension   And that s all   you get a C-compatible multidimensional array

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I presume from your static array example that you want a rectangular array  and not a jagged one  You can use the following   int  ary   new int sizeX   sizeY     Then you can access elements as   ary y sizeX   x    Don t forget to use delete   on ary

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typedef is your friend  After going back and looking at many of the other answers I found that a deeper explanation is in order  as many of the other answers either suffer from performance problems or force you to use unusual or burdensome syntax to declare the array  or access the array elements   or all the above     First off  this answer assumes you know the dimensions of the array at compile time   If you do  then this is the best solution as it will both give the best performance and allows you to use standard array syntax to access the array elements    The reason this gives the best performance is because it allocates all of the arrays as a contiguous block of memory meaning that you are likely to have less page misses and better spacial locality   Allocating in a loop may cause the individual arrays to end up scattered on multiple non-contiguous pages through the virtual memory space as the allocation loop could be interrupted   possibly multiple times   by other threads or processes  or simply due to the discretion of the allocator filling in small  empty memory blocks it happens to have available     The other benefits are a simple declaration syntax and standard array access syntax   In C   using new    include  lt stdio h gt   include  lt stdlib h gt   int main int argc  char   argv     typedef double  array5k t  5000    array5k t  array5k   new array5k t 5000    array5k 4999  4999    10  printf  array5k 4999  4999      f n   array5k 4999  4999     return 0      Or C style using calloc    include  lt stdio h gt   include  lt stdlib h gt   int main int argc  char   argv     typedef double   array5k t  5000    array5k t array5k   calloc 5000  sizeof double  5000    array5k 4999  4999    10  printf  array5k 4999  4999      f n   array5k 4999  4999     return 0

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declaring 2D array dynamically        include lt iostream gt      using namespace std      int main                 int x   3  y   3           int   ptr   new int   x            for int i   0  i lt y  i                          ptr i    new int y                     srand time 0             for int j   0  j lt x  j                          for int k   0  k lt y  k                                  int a   rand   10                  ptr j  k    a                  cout lt  lt ptr j  k  lt  lt                                cout lt  lt endl                    Now in the above code we took a double pointer and assigned it a dynamic memory and gave a value of the columns  Here the memory allocated is only for the columns  now for the rows we just need a for loop and assign the value for every row a dynamic memory  Now we can use the pointer just the way we use a 2D array  In the above example we then assigned random numbers to our 2D array pointer  Its all about DMA of 2D array

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This problem has bothered me for 15 years  and all the solutions supplied weren t satisfactory for me  How do you create a dynamic multidimensional array contiguously in memory  Today I finally found the answer  Using the following code  you can do just that    include  lt iostream gt   int main int argc  char   argv        if  argc    3                std  cerr  lt  lt   You have to specify the two array dimensions   lt  lt  std  endl          return -1             int sizeX  sizeY       sizeX   std  stoi argv 1        sizeY   std  stoi argv 2         if  sizeX  lt   0                std  cerr  lt  lt   Invalid dimension x   lt  lt  std  endl          return -1            if  sizeY  lt   0                std  cerr  lt  lt   Invalid dimension y   lt  lt  std  endl          return -1                       Create a two dimensional dynamic array in continuous memory                      - Define the pointer holding the array        - Allocate memory for the array  linear         - Allocate memory for the pointers inside the array        - Assign the pointers inside the array the corresponding addresses          in the linear array                                                                                          The resulting array     unsigned int   array2d          Linear memory allocation     unsigned int  temp   new unsigned int sizeX   sizeY           These are the important steps         Allocate the pointers inside the array         which will be used to index the linear memory     array2d   new unsigned int  sizeY           Let the pointers inside the array point to the correct memory addresses     for  int i   0  i  lt  sizeY    i                array2d i     temp   i   sizeX                   Fill the array with ascending numbers     for  int y   0  y  lt  sizeY    y                for  int x   0  x  lt  sizeX    x                        array2d y  x    x   y   sizeX                            Code for testing        Print the addresses     for  int y   0  y  lt  sizeY    y                for  int x   0  x  lt  sizeX    x                        std  cout  lt  lt  std  hex  lt  lt   amp  array2d y  x    lt  lt                           std  cout  lt  lt    n n           Print the array     for  int y   0  y  lt  sizeY    y                std  cout  lt  lt  std  hex  lt  lt   amp  array2d y  0    lt  lt  std  dec          std  cout  lt  lt                for  int x   0  x  lt  sizeX    x                        std  cout  lt  lt  array2d y  x   lt  lt                         std  cout  lt  lt  std  endl                  Free memory     delete   array2d 0       delete   array2d      array2d   nullptr       return 0      When you invoke the program with the values sizeX 20 and sizeY 15  the output will be the following   0x603010 0x603014 0x603018 0x60301c 0x603020 0x603024 0x603028 0x60302c 0x603030 0x603034 0x603038 0x60303c 0x603040 0x603044 0x603048 0x60304c 0x603050 0x603054 0x603058 0x60305c 0x603060 0x603064 0x603068 0x60306c 0x603070 0x603074 0x603078 0x60307c 0x603080 0x603084 0x603088 0x60308c 0x603090 0x603094 0x603098 0x60309c 0x6030a0 0x6030a4 0x6030a8 0x6030ac 0x6030b0 0x6030b4 0x6030b8 0x6030bc 0x6030c0 0x6030c4 0x6030c8 0x6030cc 0x6030d0 0x6030d4 0x6030d8 0x6030dc 0x6030e0 0x6030e4 0x6030e8 0x6030ec 0x6030f0 0x6030f4 0x6030f8 0x6030fc 0x603100 0x603104 0x603108 0x60310c 0x603110 0x603114 0x603118 0x60311c 0x603120 0x603124 0x603128 0x60312c 0x603130 0x603134 0x603138 0x60313c 0x603140 0x603144 0x603148 0x60314c 0x603150 0x603154 0x603158 0x60315c 0x603160 0x603164 0x603168 0x60316c 0x603170 0x603174 0x603178 0x60317c 0x603180 0x603184 0x603188 0x60318c 0x603190 0x603194 0x603198 0x60319c 0x6031a0 0x6031a4 0x6031a8 0x6031ac 0x6031b0 0x6031b4 0x6031b8 0x6031bc 0x6031c0 0x6031c4 0x6031c8 0x6031cc 0x6031d0 0x6031d4 0x6031d8 0x6031dc 0x6031e0 0x6031e4 0x6031e8 0x6031ec 0x6031f0 0x6031f4 0x6031f8 0x6031fc 0x603200 0x603204 0x603208 0x60320c 0x603210 0x603214 0x603218 0x60321c 0x603220 0x603224 0x603228 0x60322c 0x603230 0x603234 0x603238 0x60323c 0x603240 0x603244 0x603248 0x60324c 0x603250 0x603254 0x603258 0x60325c 0x603260 0x603264 0x603268 0x60326c 0x603270 0x603274 0x603278 0x60327c 0x603280 0x603284 0x603288 0x60328c 0x603290 0x603294 0x603298 0x60329c 0x6032a0 0x6032a4 0x6032a8 0x6032ac 0x6032b0 0x6032b4 0x6032b8 0x6032bc 0x6032c0 0x6032c4 0x6032c8 0x6032cc 0x6032d0 0x6032d4 0x6032d8 0x6032dc 0x6032e0 0x6032e4 0x6032e8 0x6032ec 0x6032f0 0x6032f4 0x6032f8 0x6032fc 0x603300 0x603304 0x603308 0x60330c 0x603310 0x603314 0x603318 0x60331c 0x603320 0x603324 0x603328 0x60332c 0x603330 0x603334 0x603338 0x60333c 0x603340 0x603344 0x603348 0x60334c 0x603350 0x603354 0x603358 0x60335c 0x603360 0x603364 0x603368 0x60336c 0x603370 0x603374 0x603378 0x60337c 0x603380 0x603384 0x603388 0x60338c 0x603390 0x603394 0x603398 0x60339c 0x6033a0 0x6033a4 0x6033a8 0x6033ac 0x6033b0 0x6033b4 0x6033b8 0x6033bc 0x6033c0 0x6033c4 0x6033c8 0x6033cc 0x6033d0 0x6033d4 0x6033d8 0x6033dc 0x6033e0 0x6033e4 0x6033e8 0x6033ec 0x6033f0 0x6033f4 0x6033f8 0x6033fc 0x603400 0x603404 0x603408 0x60340c 0x603410 0x603414 0x603418 0x60341c 0x603420 0x603424 0x603428 0x60342c 0x603430 0x603434 0x603438 0x60343c 0x603440 0x603444 0x603448 0x60344c 0x603450 0x603454 0x603458 0x60345c 0x603460 0x603464 0x603468 0x60346c 0x603470 0x603474 0x603478 0x60347c 0x603480 0x603484 0x603488 0x60348c 0x603490 0x603494 0x603498 0x60349c 0x6034a0 0x6034a4 0x6034a8 0x6034ac 0x6034b0 0x6034b4 0x6034b8 0x6034bc   0x603010  0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19  0x603060  20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39  0x6030b0  40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59  0x603100  60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79  0x603150  80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99  0x6031a0  100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119  0x6031f0  120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139  0x603240  140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159  0x603290  160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179  0x6032e0  180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199  0x603330  200 201 202 203 204 205 206 207 208 209 210 211 212 213 214 215 216 217 218 219  0x603380  220 221 222 223 224 225 226 227 228 229 230 231 232 233 234 235 236 237 238 239  0x6033d0  240 241 242 243 244 245 246 247 248 249 250 251 252 253 254 255 256 257 258 259  0x603420  260 261 262 263 264 265 266 267 268 269 270 271 272 273 274 275 276 277 278 279  0x603470  280 281 282 283 284 285 286 287 288 289 290 291 292 293 294 295 296 297 298 299   As you can see  the multidimensional array lies contiguously in memory  and no two memory addresses are overlapping  Even the routine for freeing the array is simpler than the standard way of dynamically allocating memory for every single column  or row  depending on how you view the array   Since the array basically consists of two linear arrays  only these two have to be  and can be  freed   This method can be extended for more than two dimensions with the same concept  I won t do it here  but when you get the idea behind it  it is a simple task   I hope this code will help you as much as it helped me

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There are two general techniques that I would recommend for this in C  11 and above  one for compile time dimensions and one for run time  Both answers assume you want uniform  two-dimensional arrays  not jagged ones    Compile time dimensions  Use a std  array of std  array and then use new to put it on the heap      the alias helps cut down on the noise  using grid   std  array lt std  array lt int  sizeX gt   sizeY gt   grid   ary   new grid    Again  this only works if the sizes of the dimensions are known at compile time   Run time dimensions  The best way to accomplish a 2 dimensional array with sizes only known at runtime is to wrap it into a class  The class will allocate a 1d array and then overload operator    to provide indexing for the first dimension   This works because in C   a 2D array is row-major      Taken from http   eli thegreenplace net 2015 memory-layout-of-multi-dimensional-arrays    A contiguous sequence of memory is good for performance reasons and is also easy to clean up  Here s an example class that omits a lot of useful methods but shows the basic idea    include  lt memory gt   class Grid     size t  rows    size t  columns    std  unique ptr lt int   gt  data   public    Grid size t rows  size t columns           rows rows            columns columns           data std  make unique lt int   gt  rows   columns         size t rows   const   return  rows       size t columns   const   return  columns       int  operator   size t row    return row    columns   data get         int  amp operator   size t row  size t column        return data row    columns   column           So we create an array with std  make unique lt int   gt  rows   columns  entries  We overload operator    which will index the row for us  It returns an int   which points to the beginning of the row  which can then be dereferenced as normal for the column  Note that make unique first ships in C  14 but you can polyfill it in C  11 if necessary   It s also common for these types of structures to overload operator   as well     int  amp operator   size t row  size t column        return data row    columns   column         Technically I haven t used new here  but it s trivial to move from std  unique ptr lt int   gt  to int   and use new delete

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int   ary   new int sizeY  sizeX    should be   int   ary   new int  sizeY   for int i   0  i  lt  sizeY    i        ary i    new int sizeX       and then clean up would be   for int i   0  i  lt  sizeY    i        delete    ary i     delete    ary    EDIT  as Dietrich Epp pointed out in the comments this is not exactly a light weight solution  An alternative approach would be to use one large block of memory   int  ary   new int sizeX sizeY       ary i  j  is then rewritten as ary i sizeY j

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In C  11 it is possible   auto array   new double M  N      This way  the memory is not initialized  To initialize it do this instead   auto array   new double M  N       Sample program  compile with  g   -std c  11      include  lt iostream gt   include  lt utility gt   include  lt type traits gt   include  lt typeinfo gt   include  lt cxxabi h gt  using namespace std   int main         const auto M   2      const auto N   2          allocate  no initializatoin      auto array   new double M  N           pollute the memory     array 0  0    2      array 1  0    3      array 0  1    4      array 1  1    5          re-allocate  probably will fetch the same memory block  not portable      delete   array      array   new double M  N           show that memory is not initialized     for int r   0  r  lt  M  r                  for int c   0  c  lt  N  c                cout  lt  lt  array r  c   lt  lt               cout  lt  lt  endl            cout  lt  lt  endl       delete   array          the proper way to zero-initialize the array     array   new double M  N             show the memory is initialized     for int r   0  r  lt  M  r                  for int c   0  c  lt  N  c                cout  lt  lt  array r  c   lt  lt               cout  lt  lt  endl             int info      cout  lt  lt  abi    cxa demangle typeid array  name   0 0  amp info   lt  lt  endl       return 0      Output   2 4  3 5   0 0  0 0  double      2

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Why not use STL vector  So easy  and you don t need to delete the vector   int rows   100  int cols   200  vector lt  vector lt int gt   gt  f rows  vector lt int gt  cols    f rows - 1  cols - 1    0     use it like arrays   You can also initialise the  arrays   just give it a default value  const int DEFAULT   1234  vector lt  vector lt int gt   gt  f rows  vector lt int gt  cols  DEFAULT      Source  How to Create 2  3  or Multi  Dimensional Arrays in C C

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Although this popular answer will give you your desired indexing syntax  it is doubly inefficient  big and slow both in space and time  There s a better way   Why That Answer is Big and Slow  The proposed solution is to create a dynamic array of pointers  then initializing each pointer to its own  independent dynamic array  The advantage of this approach is that it gives you the indexing syntax you re used to  so if you want to find the value of the matrix at position x y  you say   int val   matrix  x    y      This works because matrix x  returns a pointer to an array  which is then indexed with  y   Breaking it down   int  row   matrix  x    int  val   row  y      Convenient  yes  We like our   x    y   syntax    But the solution has a big disadvantage  which is that it is both fat and slow    Why   The reason that it s both fat and slow is actually the same  Each  row  in the matrix is a separately allocated dynamic array  Making a heap allocation is expensive both in time and space  The allocator takes time to make the allocation  sometimes running O n  algorithms to do it  And the allocator  pads  each of your row arrays with extra bytes for bookkeeping and alignment  That extra space costs   well   extra space  The deallocator will also take extra time when you go to deallocate the matrix  painstakingly free-ing up each individual row allocation  Gets me in a sweat just thinking about it   There s another reason it s slow  These separate allocations tend to live in discontinuous parts of memory  One row may be at address 1 000  another at address 100 000   you get the idea  This means that when you re traversing the matrix  you re leaping through memory like a wild person  This tends to result in cache misses that vastly slow down your processing time   So  if you absolute must have your cute  x  y  indexing syntax  use that solution  If you want quickness and smallness  and if you don t care about those  why are you working in C      you need a different solution   A Different Solution  The better solution is to allocate your whole matrix as a single dynamic array  then use  slightly  clever indexing math of your own to access cells  The indexing math is only very slightly clever  nah  it s not clever at all  it s obvious   class Matrix               size t index  int x  int y   const   return x   m width   y         Given this index   function  which I m imagining is a member of a class because it needs to know the m width of your matrix   you can access cells within your matrix array  The matrix array is allocated like this   array   new int  width   height      So the equivalent of this in the slow  fat solution   array  x    y        is this in the quick  small solution   array  index  x  y      Sad  I know  But you ll get used to it  And your CPU will thank you

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A 2D array is basically a 1D array of pointers  where every pointer is pointing to a 1D array  which will hold the actual data   Here N is row and M is column   dynamic allocation  int   ary   new int  N     for int i   0  i  lt  N  i          ary i    new int M     fill  for int i   0  i  lt  N  i        for int j   0  j  lt  M  j          ary i  j    i    print  for int i   0  i  lt  N  i        for int j   0  j  lt  M  j          std  cout  lt  lt  ary i  j   lt  lt    n     free  for int i   0  i  lt  N  i        delete    ary i   delete    ary

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If your row length is a compile time constant  C  11 allows auto arr2d   new int  nrows  CONSTANT    See this answer   Compilers like gcc that allow variable-length arrays as an extension to C   can use new as shown here to get fully runtime-variable array dimension functionality like C99 allows  but portable ISO C   is limited to only the first dimension being variable  Another efficient option is to do the 2d indexing manually into a big 1d array  as another answer shows  allowing the same compiler optimizations as a real 2D array  e g  proving or checking that arrays don t alias each other   overlap    Otherwise  you can use an array of pointers to arrays to allow 2D syntax like contiguous 2D arrays  even though it s not an efficient single large allocation  You can initialize it using a loop  like this  int   a   new int  rowCount   for int i   0  i  lt  rowCount    i      a i    new int colCount    The above  for colCount  5 and rowCount   4  would produce the following   Don t forget to delete each row separately with a loop  before deleting the array of pointers   Example in another answer

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This question was bugging me - it s a common enough problem that a good solution should already exist  something better than the vector of vectors or rolling your own array indexing   When something ought to exist in C   but doesn t  the first place to look is boost org  There I found the Boost Multidimensional Array Library  multi array  It even includes a multi array ref class that can be used to wrap your own one-dimensional array buffer

[c++] How do I declare a 2d array in C++ using new?

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