Declaration of an array without highest dimension

arrays, c

Solution

Somewhat to my surprise, your declaration:

int arr[][2]={1,2,3};

appears to be legal. You can legally omit inner braces in initializers. For example, if you wanted to define the bounds of `arr` explicitly and initialize all its elements, you could write either:

int arr[2][2] = { { 1, 2}, { 3, 4 } };

or, equivalently:

int arr[2][2] = { 1, 2, 3, 4 };

You can also omit trailing elements, and they'll be implicitly set to zero. For a one-dimensional array, this is straightforward; this:

int arr[4] = { 1, 2, 3 };

is equivalent to:

int arr[4] = { 1, 2, 3, 0 };

And if you omit a dimension, it can be inferred from the initializer:

int arr[] = { 1, 2, 3, 4 }; // 4 elements

Since you're defining `arr` as an array of 2-element arrays of `int`, this:

int arr[][2]={1,2,3};

is equivalent to this:

int arr[][2] = { { 1, 2 }, { 3 } };

You have two elements in the initializer (each of which initializes a 2-element array), so that's equivalent to:

int arr[2][2] = { { 1, 2 }, { 3 } };

Finally, you've omitted the last initializer sub-element, so it's implicitly zero:

int arr[2][2] = { { 1, 2 }, { 3, 0 } };

In general, omitting dimensions and trailing initializers is useful because it lets the compiler figure out certain things. If I write:

char message[] = "hello, world";

I don't have to count the characters in the string (and add 1 for the terminating `'\0'`). Computers are really good at counting things; making a human do that job would be silly.

Similarly, omitting some initializers lets you provide only the information you need. With C99's addition of designated initializers, you can even initialize a specified member of a structure and let the compiler take care of everything else:

struct foo obj = { .something = 42 };

As for flattening initializers by omitting inner curly braces, I personally don't see much use for that except for the special case of using a `{ 0 }` initializer to initialize and entire array or structure to zero. In particular, for multidimensional arrays I find it much clearer to show the entire structure of the array.

Problem

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