Initializing array of structures [duplicate]

Answer 1

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.

Answer 2

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 3

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 4

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 5

There's no "step-by-step" here. When initialization is performed with constant expressions, the process is essentially performed at compile time. Of course, if the array is declared as a local object, it is allocated locally and initialized at run-time, but that can be still thought of as a single-step process that cannot be meaningfully subdivided.

Designated initializers allow you to supply an initializer for a specific member of struct object (or a specific element of an array). All other members get zero-initialized. So, if my_data is declared as

typedef struct my_data {
  int a;
  const char *name;
  double x;
} my_data;

then your

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

is simply a more compact form of

my_data data[4]={
    { 0, "Peter", 0 },
    { 0, "James", 0 },
    { 0, "John", 0 },
    { 0, "Mike", 0 }
};

I hope you know what the latter does.

Answer 6

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].