c - Are variable array parameters allowed to have as index variables outside the function? -

my book states: "the expression used specify length of" variable array "could refer variables outside function". guess book means:

int main(void) {    int xternal = 3;    int variable_array[xternal];    function(variable_array); }  void function(int variable_array[xternal]) {    ... } 

i understood variables outside function invisible it. wrong?

i presume referring variable-length-array parameters of c99.

i think textbook trying can use length specifier not other arguments of function, e.g.:

void f(int array_len, char array[array_len]); 

but outside still visible, global variable:

int some_value;  void f(char array[some_vaue]); 

keep in mind - not mean compiler array bounds checks you. useful when use multidimensional arrays navigate on inner dimensions.

so instead of code:

 int *arr = malloc(sizeof(int)*nrows*ncols);   void f(int *a)  {       int row, int column;       ...calculate row, column      int value = a[row*ncols+column];      ...  } 

you write:

 int arr[nrows][ncols];   int get_array_cols()  {      return ncols;  }  int array_rows=nrows;//or calculate...   void f(int a[array_rows][get_array_cols()])  {       int row, int column;       ...calculate row, column       int value=a[row][column];       ...  } 

as can see couldbe there.

basically, when compiler knows how value dimensions, can automatically prepare expression: a[row*ncols+column]; (row*ncols). example in last case, having dimensions hint, automatically compile a[row][col] into: a[row*get_array_cols()+col].