Detailed explanation of C array

C language programming: array

array

define array

• <type> variable name[number of elements];
  • int grades[100];
  • double weight[20];
• The number of elements must be an integer
• Before C99: the number of elements must be a literal value determined at compile time

what is an array

An array is a container with the following characteristics:

Containers are a very important concept in modern programming, or modern programming languages ​​should provide some form of containers, or the capabilities of containers provided by languages ​​are important criteria for judging language capabilities.

• where all elements have the same data type
• Once created, cannot change size
• In memory, the elements of an array are arranged in close order

elements of the array

• Each cell of the array is a variable of the array type

• The number placed in the array when using the array []is called a subscript or index, and the subscript counts from 0:

  The array subscript counting from 0 was originally created by the C language, because it can simplify a lot of operations for the C language compiler.

  • grades[0]
  • grades[99]
  • average[5]

valid subscript range

• Neither the compiler nor the runtime environment will check whether the array subscript is out of bounds, whether it is reading or writing to the array unit
• Once the program is running, an out-of-bounds array can cause problems, causing the program to crash
  • segmentation fault
• But it may also be lucky that there are no serious consequences
• Therefore, it is necessary for the programmer to ensure that the program only uses valid subscript values: [0, the size of the array - 1]

Array of length 0?

• int a[0];
• can exist but is useless

Array operations

Integrated initialization of arrays

int a[] = {2,4,2,31,312,31,31,231,21,31,3}

• Directly give the initial value of all elements of the array

• There is no need to give the size of the array, the compiler will automatically judge

• If the size of the array is given, but the number of subsequent initial values ​​is insufficient, the subsequent elements will be initialized to 0

  • int[10] = {
            
            2, 3};
    PRINT: 2 3 0 0 0 0 0 0 0 0 0
    

Positioning during integration initialization

int a[10] = {
    
    
	[0] = 2, [2] = 3, 6 ,
};

• Use [n] to give the position in the initialization data
• Data without positioning is appended to the previous position
• Values ​​in other positions are zero-padded
• You can also not give the array size, let the compiler
• It is especially suitable for arrays with sparse initial data

the size of the array

• sizeofWill give the size of the content occupied by the array, in bytes
• Thus the length of the array can siezof(a)/sizeof(a[0])be used to obtain

When an array is used as a function parameter:

• cannot give size of array in []
• Can no longer be used sizeofto calculate the number of elements in an array!

Array assignment

int a[] = {2,34,4,14,14,1,};

int b[] = a;

• Array variables themselves cannot be assigned
• To pass all the elements of one array to another array, traversal must be used

for(int i = 0; i<length; i++){
    
    
	b[i] = a[i];
}

Example: judging prime numbers

Determine whether it is divisible by a known prime number <X

int isPrime(int x, int list[], int lenght);

int main(){
    
    
	int i = 3, x, count = 1, list[100] = {
    
    2};
	scanf("%d", &x);

	while(i < x){
    
    
		if( isPrime(i, list, count) ){
    
    
			list[count++] = i;
		}
		i++;
	}
	if( isPrime(x, list, count) ){
    
    
		printf("%d是素数", x);
	}else{
    
    
		printf("%d不是素数", x);
	}
} 

int isPrime(int x, int list[], int lenght){
    
    
	for(int i = 0; i < lenght; i++){
    
    
		if(x % list[i] == 0){
    
    
			return 0;
		}
	}
	return 1;
}

Construct a prime number table

• Construct a table of prime numbers within N

1. Let x be 2
2. Mark numbers from 2x3x4x up to ax<n as non-prime
3. Let x be the next number that is not marked as non-prime, repeat 2; until all books have been tried

int main(){
    
    
	int n;
	scanf("%d", &n);
	int list[n];

	for(int i = 0; i < n; i++){
    
    
		list[i] = 1;
	}

	for(int x = 2; x < n; x++){
    
    
		if( list[x] ){
    
    
			for(int i = 2; x*i<n; i++){
    
    
				list[x*i] = 0;
			}
		}
	}
}