C/C++ initialization of a normal array with one default value

Question

http://www.fredosaurus.com/notes-cpp/arrayptr/array-initialization.html

1: Page above has a nice list over initialization of arrays.

So I have a

int array[100] = {-1};

expecting it to be full with -1's but its not, only first value is and the rest are 0's mixed with random values.

The code

int array[100] = {0};

works just fine and sets each element to 0.

What am I missing here.. Can't one initialize it if the value isn't zero ?

2: Is the default initialization (as above ) faster than the usual loop through the whole array and assign a value or does it do the same thing?

Answer 1

With {} you assign the elements as they are declared; the rest is initialized with 0.

If there is no = {} to initalize, the content is undefined.

Answer 2

using the syntax that you used:

int array[100] = {-1};

says "set the first element to -1 and the rest to 0" since all omitted elements are set to 0 if you specify at least one.

to set them all to -1, you can use something like std::fill_n

std::fill_n(array, 100, -1);

Answer 3

The page you linked states

If an explicit array size is specified, but an shorter initiliazation list is specified, the unspecified elements are set to zero.

Speed issue: Any differences would be negligible for arrays this small. If you work with large arrays and speed is much more important than size, you can have a const array of the default values (initialized at compile time) and then memcpy them to the modifiable array.

Answer 4

1) When you use an initializer, for a struct or an array like that, the unspecified values are essentially default constructed. In the case of a primitive type like ints, that means they will be zeroed. Note that this applies recursively: you could have an array of structs containing arrays and if you specify just the first field of the first struct, then all the rest will be initialized with zeros and default constructors.

2) The compiler will probably generate initializer code that is at least as good as you could do by hand. I tend to prefer to let the compiler do the initialization for me, when possible.

Answer 5

The page you linked to already gave the answer to the first part:

If an explicit array size is specified, but an shorter initiliazation list is specified, the unspecified elements are set to zero.

There is no built-in way to initialize the entire array to some non-zero value.

As for which is faster, the usual rule applies: "The method that gives the compiler the most freedom is probably faster".

int array[100] = {0};

simply tells the compiler "set these 100 ints to zero", which the compiler can optimize freely.

for (int i = 0; i < 100; ++i){
  array[i] = 0;
}

is a lot more specific. It tells the compiler to create an iteration variable i, it tells it the order in which the elements should be initialized, and so on. Of course, the compiler is likely to optimize that away, but the point is that here you are overspecifying the problem, forcing the compiler to work harder to get to the same result.

Finally, if you want to set the array to a non-zero value, you should (in C++, at least) use std::fill:

std::fill(array, array+100, 42); // sets every value in the array to 42

Again, you could do the same with an array, but this is more concise, and gives the compiler more freedom. You're just saying that you want the entire array filled with the value 42. You don't say anything about in which order it should be done, or anything else.

Answer 6

There is an extension to the gcc compiler which allows the syntax:

int array[100] = { [0 ... 99] = -1 };

This would set all of the elements to -1.

This is known as "Designated Initializers" see here for further information.

Note this isn't implemented for the gcc c++ compiler.

Answer 7

For the case of an array of single-byte elements, you can use memset to set all elements to the same value.

There's an example here.