C declaration and initialization and assignment

c Declaration of data structure of language and initialization

In a word, summarize , In data type When making a statement , If you assign a value to this variable at the same time , This time is called initialization .
If at the time of declaration , No initialization is provided for variables . No matter what happens later, it cannot be called initialization , You can only assign values to this variable .

Typical are strings and data , structure .

Such as common data types

int a =3; // Declare variables a At the same time , Provide initialization operation .

int b;
b = 2; // This is called assignment , Not called initialization .

Such as arrays

int a[] = {1, 2, 3, 4}; // Declaration array a, And provide initialization .

int b[10];
b[1] = 233; This is called assignment .

Arrays and strings , There is something here

char a[] = {‘a’, ‘b’, ‘c’, ‘d’}; // Declaration character array a, Provide initialization , Notice the single quotation marks inside . As if c There are only three places in language that use single quotation marks , The first is char Define single character ;

char b[] = “hello world”; // Declaration character array b, Provide initialization . Notice that it looks like “hello world” Is a string constant , But in the expression initialized here , Put the “hello world” Break up , A character by character loading character array b.

char b[10]; // If initialization is not provided , Just specify the length , Whether explicit or implicit .
b[2] = ‘x’; // to b The second of assignment ; Another single quotation mark is used here . If it is a double quotation mark, it means two bits , By default, the last one is 、\0... Single quotation marks represent only one bit .

Here the array can only be operated one position at a time , Such as assignment , Replacement, etc . You cannot assign an array directly to another array , No matter what type the array is .

There are two ways to copy arrays , One is to use for Loop to assign values to each position . The second way is ： use 《string.h》 Inside memcpy(a, b, sizeof(a));

For structures Is the same

struct {
int a;
char b;
} ss = {a=2, b=‘x’}; // Declare and initialize ; For this structure without structure label , This is the declared variable , There is no way to use this structure to declare variables later . Because there is no structural label .

struct p {
int c;
char *d;
} ss = {c = 12, d=“helllo”}; // Declare and initialize , This one has a structural label p The structure of the body , Subsequently, you can continue to define variables independently . If the structure is newly defined , Take it with you struct… They are together , Such as ：
struct p s3; // Note that a type is defined here as struct p The structure of the body , The name is s3, But we didn't initialize , Only variables are defined
struct p s4 = {c = 33, d=“world”}; // Statement s4 Variable , Initialization is also provided . Notice here and the top variable ss … These two are the same to some extent , Both define new variables and initialize them at the same time .

meanwhile ：
s3.c = 23;
s3.d = “new value”;
You can give it like this s3 The members in the structure are assigned .

Structures can be assigned to each other , This is better than arrays , An array cannot be assigned to another array , But the structure can .

So the top s3=s4 It's also OK Of .

At the same time, if you want to assign two arrays to each other , You can put two arrays into the structure first , Then assign values to the two structures , You can deform and realize the assignment of two arrays ;

Such as ：
struct Arr {
int a[10];
} a1, a2;
Zeke a1=a2;