Initializing entire 2D array with one value

c arrays initialization
Kraken picture Kraken · Mar 20, 2013 · Viewed 242.1k times · Source

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?

Answer

Lundin picture Lundin · Mar 20, 2013

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".

Related questions

How to initialize all members of an array to the same value?

I have a large array in C (not C++ if that makes a difference). I want to initialize all members of the same value. I could swear I once knew a simple way to do this. I could use memset() …

Read More
C char array initialization

I'm not sure what will be in the char array after initialization in the following ways. 1.char buf[10] = ""; 2. char buf[10] = " "; 3. char buf[10] = "a"; For case 2, I think buf[0] should be ' ', buf[1] should be '\0', and from buf[2] …

Read More
How to initialize array to 0 in C?

I need a big null array in C as a global. Is there any way to do this besides typing out char ZEROARRAY[1024] = {0, 0, 0, /* ... 1021 more times... */ }; ?

Read More