Passing a 2D array to a C function

Question

I have a function which I want to take  as a parameter  a 2D array of variable size    So far I have this    void myFunction double   myArray        myArray x  y    5       etc        And I have declared an array elsewhere in my code   double anArray 10  10     However  calling myFunction anArray  gives me an error   I do not want to copy the array when I pass it in  Any changes made in myFunction should alter the state of anArray  If I understand correctly  I only want to pass in as an argument a pointer to a 2D array  The function needs to accept arrays of different sizes also  So for example   10  10  and  5  5   How can I do this

User · Answer

Single dimensional array decays to a pointer pointer pointing to the first element in the array. While a 2D array decays to a pointer pointing to first row. So, the function prototype should be -

void myFunction(double (*myArray) [10]);

I would prefer std::vector over raw arrays.

User · Answer

You can create a function template like this   template lt int R  int C gt  void myFunction double   amp myArray  R  C         myArray x  y    5      etc        Then you have both dimension sizes via R and C   A different function will be created for each array size  so if your function is large and you call it with a variety of different array sizes  this may be costly   You could use it as a wrapper over a function like this though   void myFunction double   arr  int R  int C        arr x   C   y    5      etc        It treats the array as one dimensional  and uses arithmetic to figure out the offsets of the indexes   In this case  you would define the template like this   template lt int C  int R gt  void myFunction double   amp myArray  R  C         myFunction  myArray  R  C

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Surprised that no one mentioned this yet  but you can simply template on anything 2D supporting      semantics   template  lt typename TwoD gt  void myFunction TwoD amp  myArray        myArray x  y    5       etc          call with double anArray 10  10   myFunction anArray     It works with any 2D  array-like  datastructure  such as std  vector lt std  vector lt T gt  gt   or a user defined type to maximize code reuse

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Fixed Size 1  Pass by reference template  lt size t rows  size t cols gt  void process 2d array template int   amp array  rows  cols         std  cout  lt  lt    func    lt  lt  std  endl      for  size t i   0  i  lt  rows    i                std  cout  lt  lt  i  lt  lt   quot    quot           for  size t j   0  j  lt  cols    j              std  cout  lt  lt  array i  j   lt  lt    t           std  cout  lt  lt  std  endl           In C   passing the array by reference without losing the dimension information is probably the safest  since one needn t worry about the caller passing an incorrect dimension  compiler flags when mismatching   However  this isn t possible with dynamic  freestore  arrays  it works for automatic  usually stack-living  arrays only i e  the dimensionality should be known at compile time  2  Pass by pointer void process 2d array pointer int   array  5  10         std  cout  lt  lt    func    lt  lt  std  endl      for  size t i   0  i  lt  5    i                std  cout  lt  lt  i  lt  lt   quot    quot           for  size t j   0  j  lt  10    j              std  cout  lt  lt    array  i  j   lt  lt    t           std  cout  lt  lt  std  endl               The C equivalent of the previous method is passing the array by pointer  This should not be confused with passing by the array s decayed pointer type  3   which is the common  popular method  albeit less safe than this one but more flexible  Like  1   use this method when all the dimensions of the array is fixed and known at compile-time  Note that when calling the function the array s address should be passed process 2d array pointer  amp a  and not the address of the first element by decay process 2d array pointer a   Variable Size These are inherited from C but are less safe  the compiler has no way of checking  guaranteeing that the caller is passing the required dimensions  The function only banks on what the caller passes in as the dimension s   These are more flexible than the above ones since arrays of different lengths can be passed to them invariably  It is to be remembered that there s no such thing as passing an array directly to a function in C  while in C   they can be passed as a reference  1     2  is passing a pointer to the array and not the array itself  Always passing an array as-is becomes a pointer-copy operation which is facilitated by array s nature of decaying into a pointer  3  Pass by  value  a pointer to the decayed type    int array   10  is just fancy notation for the same thing void process 2d array int   array  10   size t rows        std  cout  lt  lt    func    lt  lt  std  endl      for  size t i   0  i  lt  rows    i                std  cout  lt  lt  i  lt  lt   quot    quot           for  size t j   0  j  lt  10    j              std  cout  lt  lt  array i  j   lt  lt    t           std  cout  lt  lt  std  endl           Although int array   10  is allowed  I d not recommend it over the above syntax since the above syntax makes it clear that the identifier array is a single pointer to an array of 10 integers  while this syntax looks like it s a 2D array but is the same pointer to an array of 10 integers  Here we know the number of elements in a single row  i e  the column size  10 here  but the number of rows is unknown and hence to be passed as an argument  In this case there s some safety since the compiler can flag when a pointer to an array with second dimension not equal to 10 is passed  The first dimension is the varying part and can be omitted  See here for the rationale on why only the first dimension is allowed to be omitted  4  Pass by pointer to a pointer    int  array 10  is just fancy notation for the same thing void process pointer 2 pointer int   array  size t rows  size t cols        std  cout  lt  lt    func    lt  lt  std  endl      for  size t i   0  i  lt  rows    i                std  cout  lt  lt  i  lt  lt   quot    quot           for  size t j   0  j  lt  cols    j              std  cout  lt  lt  array i  j   lt  lt    t           std  cout  lt  lt  std  endl           Again there s an alternative syntax of int  array 10  which is the same as int   array  In this syntax the  10  is ignored as it decays into a pointer thereby becoming int   array  Perhaps it is just a cue to the caller that the passed array should have at least 10 columns  even then row count is required  In any case the compiler doesn t flag for any length size violations  it only checks if the type passed is a pointer to pointer   hence requiring both row and column counts as parameter makes sense here  Note   4  is the least safest option since it hardly has any type check and the most inconvenient  One cannot legitimately pass a 2D array to this function  C-FAQ condemns the usual workaround of doing int x 5  10   process pointer 2 pointer  int    amp x 0  0   5  10   as it may potentially lead to undefined behaviour due to array flattening  The right way of passing an array in this method brings us to the inconvenient part i e  we need an additional  surrogate  array of pointers with each of its element pointing to the respective row of the actual  to-be-passed array  this surrogate is then passed to the function  see below   all this for getting the same job done as the above methods which are more safer  cleaner and perhaps faster  Here s a driver program to test the above functions   include  lt iostream gt      copy above functions here  int main         int a 5  10                 process 2d array template a       process 2d array pointer  amp a          lt -- notice the unusual usage of addressof   amp   operator on an array     process 2d array a  5          works since a s first dimension decays into a pointer thereby becoming int     10       int  b 5       surrogate     for  size t i   0  i  lt  5    i                b i    a i                another popular way to define b  here the 2D arrays dims may be non-const  runtime var        int   b   new int  5          for  size t i   0  i  lt  5    i  b i    new int 10       process pointer 2 pointer b  5  10          process 2d array b  5          doesn t work since b s first dimension decays into a pointer thereby becoming int

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In the case you want to pass a dynamic sized 2-d array to a function  using some pointers could work for you   void func1 int  arr  int n  int m               int i j the element   arr i   m   j       use the idiom of i   m   j for arr i  j              void func2                int arr n  m               func1  amp  arr 0  0    n  m

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One important thing for passing multidimensional arrays is   First array dimension need not be specified  Second any any further dimension must be specified   1 When only second dimension is available globally  either as a macro or as a global constant  const int N   3   void print int arr   N   int m    int i  j  for  i   0  i  lt  m  i      for  j   0  j  lt  N  j        printf  quot  d  quot   arr i  j       int main     int arr   3      1  2  3    4  5  6    7  8  9    print arr  3   return 0     2 Using a single pointer  In this method we must typecast the 2D array when passing to function  void print int  arr  int m  int n    int i  j  for  i   0  i  lt  m  i      for  j   0  j  lt  n  j        printf  quot  d  quot      arr i n    j        int main     int arr   3      1  2  3    4  5  6    7  8  9    int m   3  n   3      We can also use  quot print  amp arr 0  0   m  n   quot  print  int   arr  m  n   return 0

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A modification to shengy s first suggestion  you can use templates to make the function accept a multi-dimensional array variable  instead of storing an array of pointers that have to be managed and deleted    template  lt size t size x  size t size y gt  void func double   amp arr  size x  size y         printf   p n    amp arr      int main         double a1 10  10       double a2 5  5        printf   p n p n n    amp a1   amp a2       func a1       func a2        return 0      The print statements are there to show that the arrays are getting passed by reference  by displaying the variables  addresses

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You can do something like this      include lt iostream gt   using namespace std     for changing values in 2D array void myFunc double  a int rows int cols       for int i 0 i lt rows i             for int j 0 j lt cols j                   a  i rows   j   10 0                       for printing 2D array similar to myFunc void printArray double  a int rows int cols       cout lt  lt  Printing your array    n       for int i 0 i lt rows i             for int j 0 j lt cols j                 cout lt  lt   a  i rows   j  lt  lt                     cout lt  lt   n            int main          declare and initialize your array     double a 2  2    1 5   2 5   3 5   4 5           the 1st argument is the address of the first row i e       the first 1D array       the 2nd argument is the no of rows of your array       the 3rd argument is the no of columns of your array     myFunc a 0  2 2          same way as myFunc     printArray a 0  2 2        return 0      Your output will be as follows     11 5  12 5 13 5  14 5

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We can use several ways to pass a 2D array to a function   Using single pointer we have to typecast the 2D array    include lt bits stdc   h gt   using namespace std     void func int  arr  int m  int n          for  int i 0  i lt m  i                   for  int j 0  j lt n  j                         cout lt  lt    arr i n    j  lt  lt  quot   quot                     cout lt  lt endl              int main           int m   3  n   3       int arr m  n      1  2  3    4  5  6    7  8  9         func  int   arr  m  n        return 0       Using double pointer In this way  we also typecast the 2d array   include lt bits stdc   h gt   using namespace std   void func int   arr  int row  int col       for  int i 0  i lt row  i               for int j 0   j lt col  j                    cout lt  lt arr i  j  lt  lt  quot   quot                 printf  quot  n quot            int main       int row  colum    cin gt  gt row gt  gt colum    int   arr   new int  row      for int i 0  i lt row  i             arr i    new int colum          for int i 0  i lt row  i              for int j 0  j lt colum  j                     cin gt  gt arr i  j                 func arr  row  colum      return 0

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There are three ways to pass a 2D array to a function    The parameter is a 2D array  int array 10  10   void passFunc int a   10                  passFunc array    The parameter is an array containing pointers  int  array 10   for int i   0  i  lt  10  i        array i    new int 10   void passFunc int  a 10     Array containing pointers                passFunc array    The parameter is a pointer to a pointer  int   array  array   new int   10   for int i   0  i  lt 10  i        array i    new int 10   void passFunc int   a                 passFunc array

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Here is a vector of vectors matrix example   include  lt iostream gt   include  lt vector gt  using namespace std   typedef vector lt  vector lt int gt   gt  Matrix   void print Matrix amp  m       int M m size       int N m 0  size       for int i 0  i lt M  i            for int j 0  j lt N  j             cout  lt  lt  m i  j   lt  lt             cout  lt  lt  endl          cout  lt  lt  endl      int main         Matrix m      1 2 3 4                     5 6 7 8                     9 1 2 3         print m          To initialize a 3 x 4 matrix with 0      Matrix n  3 vector lt int gt  4 0        print n       return 0      output   1 2 3 4 5 6 7 8 9 1 2 3  0 0 0 0 0 0 0 0 0 0 0 0

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You are allowed to omit the leftmost dimension and so you end up with two options   void f1 double a   2  3            void f2 double   a  2  3            double a 1  2  3    f1 a      ok f2 a      ok    This is the same with pointers      compilation error  cannot convert    double     2  3     to    double           double    p1   a      compilation error  cannot convert    double     2  3     to    double      3        double    p2  3    a   double   p3  2  3    a     ok     compilation error  array of pointers    pointer to array    double  p4 2  3    a   double   p5  3    a 0      ok  double  p6   a 0  1      ok   The decay of an N dimensional array to a pointer to N-1 dimensional array is allowed by C   standard  since you can lose the leftmost dimension and still being able to correctly access array elements with N-1 dimension information    Details in here  Though  arrays and pointers are not the same  an array can decay into a pointer  but a pointer doesn t carry state about the size configuration of the data to which it points   A char    is a pointer to a memory block containing character pointers  which themselves point to memory blocks of characters  A char      is a single memory block which contains characters  This has an impact on how the compiler translate the code and how the final performance will be   Source

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You can use template facility in C   to do this  I did something like this    template lt typename T  size t col gt  T process T a   col   size t row            the problem with this approach is that for every value of col which you provide  the a new function definition is instantiated using the template  so   int some mat 3  3   another mat 4 5   process some mat  3   process another mat  4     instantiates the template twice to produce 2 function definitions  one where col   3 and one where col   5

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If you want to pass int a 2  3  to void func int   pp  you need auxiliary steps as follows   int a 2  3   int  p 2     a 0  a 1    int   pp   p   func pp     As the first  2  can be implicitly specified   it can be simplified further as   int a   3   int  p      a 0  a 1    int   pp   p   func pp

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anArray 10  10  is not a pointer to a pointer  it is a contiguous chunk of memory suitable for storing 100 values of type double  which compiler knows how to address because you specified the dimensions  You need to pass it to a function as an array  You can omit the size of the initial dimension  as follows   void f double p   10         However  this will not let you pass arrays with the last dimension other than ten   The best solution in C   is to use std  vector lt std  vector lt double gt   gt   it is nearly as efficient  and significantly more convenient

[c++] Passing a 2D array to a C++ function

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