Return a 2d array from a function

Question

Hi I am a newbie to C    I am trying to return a 2d array from a function  It is something like this  int   MakeGridOfCounts int Grid   6       int cGrid 6  6      0      0      0      0      0      0         return cGrid

User · Answer

That code isn t going to work  and it s not going to help you learn proper C   if we fix it   It s better if you do something different   Raw arrays  especially multi-dimensional arrays  are difficult to pass correctly to and from functions   I think you ll be much better off starting with an object that represents an array but can be safely copied   Look up the documentation for std  vector   In your code  you could use vector lt vector lt int gt   gt  or you could simulate a 2-D array with a 36-element vector lt int gt

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This code returns a 2d array     include  lt cstdio gt          Returns a pointer to a newly created 2d array the array2D has size  height x width       int   create2DArray unsigned height  unsigned width              int   array2D   0        array2D   new int  height          for  int h   0  h  lt  height  h                        array2D h    new int width                for  int w   0  w  lt  width  w                                       fill in some initial values                       filling in zeros would be more logic  but this is just for the example                    array2D h  w    w   width   h                               return array2D             int main               printf  Creating a 2D array2D n          printf   n           int height   15        int width   10        int   my2DArray   create2DArray height  width         printf  Array sized   i  i  created  n n   height  width             print contents of the array2D       printf  Array contents   n           for  int h   0  h  lt  height  h                        for  int w   0  w  lt  width  w                                    printf   i    my2DArray h  w                              printf   n                          important  clean up memory           printf   n              printf  Cleaning up memory    n              for    h   0  h  lt  height  h                            delete    my2DArray h                         delete    my2DArray            my2DArray   0            printf  Ready  n           return 0

User · Answer

A better alternative to using pointers to pointers is to use std  vector  That takes care of the details of memory allocation and deallocation   std  vector lt std  vector lt int gt  gt  create2DArray unsigned height  unsigned width       return std  vector lt std  vector lt int gt  gt  height  std  vector lt int gt  width  0

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include  lt iostream gt  using namespace std    typedef int   Type  3  3     Type Demo function  Type      prototype  int main         cout  lt  lt    t t     Passing and returning 2D array from function      n       int array 3  3        Type recieve   ptr    amp array      recieve   Demo function  ptr          for   int i   0    i  lt  3   i               for   int j   0   j  lt  3   j                   cout  lt  lt     recieve  i  j   lt  lt                      cout  lt  lt  endl           return 0       Type Demo function  Type array     function definition         cout  lt  lt   Enter values    n        for  int i  0    i  lt  3   i             for   int j   0   j  lt  3   j                  cin  gt  gt    array  i  j         return array

User · Answer

What you are  trying to do  doing in your snippet is to return a local variable from the function  which is not at all recommended - nor is it allowed according to the standard   If you d like to create a int 6  6  from your function you ll either have to allocate memory for it on the free-store  ie  using new T malloc or similar function   or pass in an already allocated piece of memory to MakeGridOfCounts

User · Answer

returning an array of pointers pointing to starting elements of all rows is the only decent way of returning 2d array

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int   create2DArray unsigned height  unsigned width         int   array2D   0       array2D   new int  height         for  int h   0  h  lt  height  h                     array2D h    new int width              for  int w   0  w  lt  width  w                                  fill in some initial values                    filling in zeros would be more logic  but this is just for the example                 array2D h  w    w   width   h                           return array2D     int main           printf  Creating a 2D array2D n        printf   n         int height   15      int width   10      int   my2DArray   create2DArray height  width       printf  Array sized   i  i  created  n n   height  width           print contents of the array2D     printf  Array contents   n         for  int h   0  h  lt  height  h                   for  int w   0  w  lt  width  w                             printf   i    my2DArray h  w                        printf   n               return 0

User · Answer

I would suggest you Matrix library as an open source tool for c    its usage is like arrays in c    Here you can see documention   Matrix funcionName         Matrix lt int gt  arr 2  2        arr 0  0    5      arr 0  1    10      arr 1  0    0      arr 1  1    44       return arr

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The function returns a static 2D array const int N   6  int    MakeGridOfCounts     N     static int cGrid N  N      0      0      0      0      0      0       return cGrid     int main     int   arr  N   arr   MakeGridOfCounts       You need to make the array static since it will be having a block scope  when the function call ends  the array will be created and destroyed  Static scope variables last till the end of program

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Whatever changes you would make in function will persist So there is no need to return anything You can pass 2d array and change it whenever you will like     void MakeGridOfCounts int Grid   6               cGrid 6  6      0      0      0      0      0      0               or  void MakeGridOfCounts int Grid   6  int answerArray   6                    do the changes in the array as you like they will reflect in main

[c++] Return a 2d array from a function

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