c array - expression must have a constant value

Question

I get an error when I try to create an array from the variables I declared    int row   8  int col  8  int  row  col     Why do I get this error       expression must have a constant value

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When creating an array like that  its size must be constant   If you want a dynamically sized array  you need to allocate memory for it on the heap and you ll also need to free it with delete when you re done     allocate the array int   arr   new int  row   for int i   0  i  lt  row  i        arr i    new int col       use the array    deallocate the array for int i   0  i  lt  row  i        delete   arr i   delete   arr    If you want a fixed size  then they must be declared const   const int row   8  const int col   8  int arr row  col     Also    int  row  col     doesn t even provide a variable name

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No it doesn t need to be constant  the reason why his code above is wrong is because he needs to include a variable name before the declaration   int row   8  int col  8  int x row  col     In Xcode that will compile and run without any issues  in M  C   compiler in  NET it won t compile  it will complain that you cannot use a non const literal to initialize array  the size needs to be known at compile time

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The standard requires the array length to be a value that is computable at compile time so that the compiler is able to allocate enough space on the stack  In your case  you are trying to set the array length to a value that is unknown at compile time  Yes  i know that it seems obvious that it should be known to the compiler  but this is not the case here  The compiler cannot make any assumptions about the contents of non-constant variables  So go with   const int row   8  const int col  8  int a row  col     UPD  some compilers will actually allow you to pull this off  IIRC  g   has this feature  However  never use it because your code will become un-portable across compilers

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one could also use a fixed lengths vector and access it with indexing  int Lcs string a  string b         int x   a size     1      int y   b size     1       vector lt vector lt int gt  gt  L x  vector lt int gt  y         for  int i   1  i  lt  x  i                  for  int j   1  j  lt  y  j                          L i  j    a i - 1     b j - 1                    L i - 1  j - 1    1                   max L i - 1  j   L i  j - 1                         return L a size    b size

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Using the GCC compiler which I did locally  and an online compiler link   a code similar to the one in the question runs provided a variable name   include lt iostream gt   int main        int r  2  c 3      int arr r  c       arr 0  1   3      std  cout lt  lt arr 0  1      Output  3 Though I quote cplusplus com   NOTE  The elements field within square brackets     representing the number of elements in the array  must be a constant expression since arrays are blocks of static memory whose size must be determined at compile-time before the program runs   It seems like the present compilers have been changed in this regard  This is a C99 standard which the maintainers of the standard compilers added to their compilers  We need not have a const declaration  During compilation  the compiler automatically substitutes the value  This question goes around the topic  NOTE  This is not the standard coding practice  so some compilers might still throw an error

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You can use  define as an alternative solution  which do not introduce vector and malloc  and  you are still using the same syntax when defining an array    define row 8  define col 8  int main     int array name row  col

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C   doesn t allow non-constant values for the size of an array  That s just the way it was designed   C99 allows the size of an array to be a variable  but I m not sure it is allowed for two dimensions  Some C   compilers  gcc  will allow this as an extension  but you may need to turn on a compiler option to allow it   And I almost missed it - you need to declare a variable name  not just the array dimensions

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When you declare a variable as here  int a 10  10     you are telling the C   compiler that you want 100 consecutive integers allocated in the program s memory at runtime  The compiler will then provide for your program to have that much memory available and all is well with the world    If however you tell the compiler  int x   9001  int y   5  int a x  y     the compiler has no way of knowing how much memory you are actually going to need at run time without doing a lot of very complex analysis to track down every last place where the values of x and y changed  if any   Rather than support such variable size arrays  C   and C strongly suggest if not outright demand that you use malloc   to manually allocate the space you want   TL DR  int x   5  int y   5  int   a   malloc x sizeof int     for int i   0  i  lt  y  i          a i    malloc sizeof int   y       a is now a 2D array of size 5x5 and will behave the same as int a 5  5   Because you have manually allocated memory  C   and C demand that you delete it by hand too     for int i   0  i  lt  x  i          free a i       delete the 2nd dimension array   free a      delete a itself

[c++] c++ array - expression must have a constant value

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