ANSI C - initializing an array

Question

What I was always doing is:

int arr[] = {2, 3, 4};

and it always worked.

I've heard of a better way to initialize new array, using the pointer:

int *arr = {2, 3, 4};

However, it doesn't work in any IDE, it throws some errors like int differs in levels of indirection from int , or too many initializers . How should I do this?

Answer 1

int arr[] = {2, 3, 4};

is just fine, and completely correct. No need to change.

Answer 2

That initialization seems to work for me, on gcc, but not correctly.

int *arr = {2, 3, 4}; //weird behaviour, stores first value `2` as read-only

int arr[] = {2, 3, 4}; //array decl

The former isn't a correct way to initialize an array.

For a char* , it makes more sense

char* arr = "abcde"; //Pointer to a read-only char array in memory

char[] arr = "abcde"; //Normal char array

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The difference:

The former is written to the Rodata (constant, read-only data) section of the assembly, while the latter, resides in the read/write Data-Segment . Any attempt to change the former, might result in a segmentation-fault .

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The places where the values are stored are different.

char* arr = "abcde";
arr[1] = 'f'; //(undefined behavior)

char[] arr2 = "abcde";
arr2[1] = 'f'; //no issue

Answer 3

If you want to "initialize an array", you have to initialize an array , not a pointer.

Anyway, in C99 compound literals can be used and a pointer can be initialized as

int *arr = (int []) {2, 3, 4};

which is close to what you were trying to do. Although the term "ANSI C" is often used to refer to C89/90, where this feature is not available.

There's nothing "better" about this approach. It just gives you a pointer instead of an array, so it is really a question of what you need.

Answer 4

Why would the second version be better than the first?

The first version at least is explicit: you define an int array, with the given elements. Let the compiler determine how to do this optimally.

Answer 5

Going from your comment to Evan Li ("string is also kind of an array, and it is initialized with a pointer. Thus, arrays should be also initialized this way."). If the instructor actually told you this, I'd seriously think about finding a new instructor, because he's confused about things.

A string literal is an array expression; the literal "Hello" is a 6-element array of char ( const char in C++). String literals are stored in such a way that their memory is allocated over the lifetime of the program; this memory may or may not be read-only, depending on the platform. The behavior on attempting to modify the contents of a string literal is undefined , meaning you may get a segfault, or the string may get modified, or something else happens.

When an array expression appears in a context other than as an operand to the sizeof or unary & operators, or is a string literal being used to initialize another array in a declaration, then the type of the expression is converted ("decays") from "N-element array of T " to "pointer to T ", and the value of the expression is the address of the first element of the array.

That's why you can write something like

char *foo = "This is a test";

The string literal "This is a test" is an array expression of type "15-element array of char "; since it isn't the operand of the sizeof or & operators, and isn't being used to initialize another array of char , the type of the expression becomes "pointer to char ", and the address of the first character is assigned to foo . By comparison,

char foo[] = "This is a test";

declares foo as an array of char ; the size is computed from the size of the initializer string (15 characters), and the contents of the string literal are copied to foo .

A string is an array expression; a brace-enclosed list of values is not .

int *foo = {1, 2, 3};

won't create a 3-element array of int and assign the address of the first element to foo ; instead, this should be a constraint violation if I'm reading this right:

2 No initializer shall attempt to provide a value for an object not contained within the entity being initialized.

As of C99, you can use what are known as compound literals , like so:

int *foo = (int []) {1, 2, 3};

The cast-expression (int []) is required. This does create a new 3-element array of int and assigns the address of the first element to foo . Unlike string literals, compound literals like this only exist for the duration of the enclosing block ¹ ; IOW, if you did something like

int *foo = NULL;
if (condition())
{
  foo = (int []){1, 2, 3};
  // do stuff
}
// do more stuff

the anonymous array object pointed to by foo only exists within the if block; once the if block exits, the array no longer exists, and the value of foo is no longer valid.

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^{1. If the compound literal is defined at file scope (outside of any function), then it has static duration and exists for the lifetime of the program.}

Answer 6

int arr[] = {2, 3, 4}; is ok.

If you want to use pointer, you will have to allocate memory with for example malloc. Something like that :

  int *arr = malloc(sizeof(int)*4);

  arr[0]=1;
  arr[1]=2;
  arr[2]=3;
  arr[3]=4;

  //display
  printf("%d %d %d %d\n",arr[0],arr[1],arr[2],arr[3]);

  free(arr);