Initializing entire 2D array with one value

Question

With the following declaration   int array ROW  COLUMN   0     I get the array with all zeroes but with the following one  int array ROW  COLUMN   1     I don   t get the array with all one value  The default value is still 0   Why this behavior and how can I initialize with all 1   EDIT  I have just understood that using memset with value as 1  will set each byte as 1 and hence the actual value of each array cell wont be 1 but 16843009  How do I set it to 1

User · Answer

Use vector array instead   vector lt vector lt int gt  gt  array ROW  vector lt int gt  COLUMN  1

User · Answer

int array ROW  COLUMN   1     This initialises only the first element to 1  Everything else gets a 0   In the first instance  you re doing the same - initialising the first element to 0  and the rest defaults to 0   The reason is straightforward  for an array  the compiler will initialise every value you don t specify with 0   With a char array you could use memset to set every byte  but this will not generally work with an int array  though it s fine for 0    A general for loop will do this quickly   for  int i   0  i  lt  ROW  i      for  int j   0  j  lt  COLUMN  j        array i  j    1    Or possibly quicker  depending on the compiler   for  int i   0  i  lt  ROW COLUMN  i         int  a   i    1

User · Answer

You get this behavior  because int array  ROW  COLUMN     1   does not mean  set all items to one   Let me try to explain how this works step by step   The explicit  overly clear way of initializing your array would be like this    define ROW 2  define COLUMN 2  int array  ROW  COLUMN         0  0      0  0       However  C allows you to leave out some of the items in an array  or struct union   You could for example write   int array  ROW  COLUMN         1  2       This means  initialize the first elements to 1 and 2  and the rest of the elements  as if they had static storage duration   There is a rule in C saying that all objects of static storage duration  that are not explicitly initialized by the programmer  must be set to zero   So in the above example  the first row gets set to 1 2 and the next to 0 0 since we didn t give them any explicit values   Next  there is a rule in C allowing lax brace style  The first example could as well be written as  int array  ROW  COLUMN     0  0  0  0     although of course this is poor style  it is harder to read and understand  But this rule is convenient  because it allows us to write  int array  ROW  COLUMN     0     which means   initialize the very first column in the first row to 0  and all other items as if they had static storage duration  ie set them to zero    therefore  if you attempt  int array  ROW  COLUMN     1     it means  initialize the very first column in the first row to 1 and set all other items to zero

User · Answer

char grid row  col   memset grid       sizeof grid     That s for initializing char array elements to space characters

User · Answer

Note that GCC has an extension to the designated initializer notation which is very useful for the context   It is also allowed by clang without comment  in part because it tries to be compatible with GCC      The extension notation allows you to use     to designate a range of elements to be initialized with the following value   For example    include  lt stdio h gt   enum   ROW   5  COLUMN   10     int array ROW  COLUMN       0     ROW-1       0     COLUMN-1    1       int main void        for  int i   0  i  lt  ROW  i                  for  int j   0  j  lt  COLUMN  j                printf   2d   array i  j            putchar   n              return 0      The output is  unsurprisingly    1 1 1 1 1 1 1 1 1 1  1 1 1 1 1 1 1 1 1 1  1 1 1 1 1 1 1 1 1 1  1 1 1 1 1 1 1 1 1 1  1 1 1 1 1 1 1 1 1 1   Note that Fortran 66  Fortran IV  had repeat counts for initializers for arrays  it s always struck me as odd that C didn t get them when designated initializers were added to the language   And Pascal uses the 0  9 notation to designate the range from 0 to 9 inclusive  but C doesn t use    as a token  so it is not surprising that was not used   Note that the spaces around the     notation are essentially mandatory  if they re attached to numbers  then the number is interpreted as a floating point number   For example  0   9 would be tokenized as 0       9  and floating point numbers aren t allowed as array subscripts  With the named constants     ROW-1 would not cause trouble  but it is better to get into the safe habits     Addenda   I note in passing that GCC 7 3 0 rejects   int array ROW  COLUMN       0     ROW-1       0     COLUMN-1      1          where there s an extra set of braces around the scalar initializer 1  error  braces around scalar initializer  -Werror     I m not sure that s correct given that you can normally specify braces around a scalar in int a     1     which is explicitly allowed by the standard   I m not certain it s incorrect  either   I also wonder if a better notation would be  0     9      that is unambiguous  cannot be confused with any other valid syntax  and avoids confusion with floating point numbers   int array ROW  COLUMN       0     4       0     9    1        Maybe the standards committee would consider that

User · Answer

To initialize 2d array with zero use the below method  int arr n  m         NOTE   The above method will only work to initialize with 0

User · Answer

If you  want to initialize the array  to -1 then you can use the following   memset array  -1  sizeof array 0  0     row   count    But this will work 0 and -1 only

[c] Initializing entire 2D array with one value

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