Initializing array of structures [duplicate]

Question

Here's initialization I just found in somebody else's question.

my_data data[]={
    { .name = "Peter" },
    { .name = "James" },
    { .name = "John" },
    { .name = "Mike" }
};

I never saw something like this before and can't find explanation how is .name possible to be correct.
What I'm looking for is how step by step this process goes.

It looks like it gets:

1. data;
2. *data;
3. (*data).name;
4. (*data).name="Peter";

Or am I totally wrong?

Answer 1

my_data is a struct with name as a field and data[] is arry of structs, you are initializing each index. read following:

5.20 Designated Initializers:

In a structure initializer, specify the name of a field to initialize with .fieldname =' before the element value. For example, given the following structure,

struct point { int x, y; };

the following initialization

struct point p = { .y = yvalue, .x = xvalue };

is equivalent to

struct point p = { xvalue, yvalue };

Another syntax which has the same meaning, obsolete since GCC 2.5, is fieldname:', as shown here:

struct point p = { y: yvalue, x: xvalue };

You can also write:

my_data data[] = {
    { .name = "Peter" },
    { .name = "James" },
    { .name = "John" },
    { .name = "Mike" }
};

as:

my_data data[] = {
    [0] = { .name = "Peter" },
    [1] = { .name = "James" },
    [2] = { .name = "John" },
    [3] = { .name = "Mike" }
}; 

or:

my_data data[] = {
    [0].name = "Peter",
    [1].name = "James",
    [2].name = "John",
    [3].name = "Mike"
}; 

Second and third forms may be convenient as you don't need to write in order for example all of the above example are equivalent to:

my_data data[] = {
    [3].name = "Mike",
    [1].name = "James",
    [0].name = "Peter",
    [2].name = "John"
}; 

If you have multiple fields in your struct (for example, an int age), you can initialize all of them at once using the following:

my_data data[] = {
    [3].name = "Mike",
    [2].age = 40,
    [1].name = "James",
    [3].age = 23,
    [0].name = "Peter",
    [2].name = "John"
}; 

To understand array initialization read Strange initializer expression?

Additionally, you may also like to read @Shafik Yaghmour's answer for switch case: What is “…” in switch-case in C code

Answer 2

There are only two syntaxes at play here.

1. Plain old array initialisation:

   int x[] = {0, 0}; // x[0] = 0, x[1] = 0
   

2. A designated initialiser. See the accepted answer to this question: How to initialize a struct in accordance with C programming language standards

   The syntax is pretty self-explanatory though. You can initialise like this:

   struct X {
       int a;
       int b;
   }
   struct X foo = { 0, 1 }; // a = 0, b = 1
   

   or to use any ordering,

   struct X foo = { .b = 0, .a = 1 }; // a = 1, b = 0
   

Answer 3

It's called designated initializer which is introduced in C99. It's used to initialize struct or arrays, in this example, struct.

Given

struct point { 
    int x, y;
};

the following initialization

struct point p = { .y = 2, .x = 1 };

is equivalent to the C89-style

struct point p = { 1, 2 };

Answer 4

It's a designated initializer, introduced with the C99 standard; it allows you to initialize specific members of a struct or union object by name. my_data is obviously a typedef for a struct type that has a member name of type char * or char [N].

Answer 5

This is quite simple: my_data is a before defined structure type. So you want to declare an my_data-array of some elements, as you would do with

char a[] = { 'a', 'b', 'c', 'd' };

So the array would have 4 elements and you initialise them as

a[0] = 'a', a[1] = 'b', a[1] = 'c', a[1] ='d';

This is called a designated initializer (as i remember right).

and it just indicates that data has to be of type my_dat and has to be an array that needs to store so many my_data structures that there is a structure with each type member name Peter, James, John and Mike.