Introduction to C language Learning (iii) sizeof function, memory address, array

sizeof can be used to calculate the number of bytes of memory that a variable or a constant, a data type occupies.

sizeof(ten);

char c = ' a ';

sizeof(c);

sizeof(float);

Address

1. The memory in the computer is the storage space in bytes . Each byte of memory has a unique number, which is called an address .

Every program and data stored in memory has an address, that is, a function has its own memory address

2. When a variable is defined, the system allocates a storage unit with a unique address to store the variable. Like what:

Char A = ' A ' ; The ASCII value of a is 65

int b[] = {66,33};

In the 16bit compiler environment, the system allocates 1 bytes, 2 bytes of storage unit for a and B respectively. The address of the first byte of a variable storage unit is the address of the variable .

As you can see, the address of variable A is FFC3, and the address of variable B is ffc1. 2 binary data is stored in memory

One-dimensional arrays

The form of the definition is: type array name [number of elements]

int a[5];

int [] A; It is wrong to write differently than Java writes before or after a variable name, and defines the number of elements to be specified.

Initialization

int a[2] = {8,ten};

is actually equivalent to:

int a[2];

a[0] = 8;

a[1] = ten;

The element value list can be the initial value of all elements of an array, or it can be the initial value of a previous element

int a[4] = {2,5};  This assigns the value to the a[0]=2;a[1]=5; a[2]=a[3]= Default value 0

You can not specify the number of elements when defining and initializing directly

int a[] = {2,3,5};

Array initialization can only be used for the definition of the array, after the definition can only one element of an element to be assigned value

int a[3];

a = {1,2,3};  Error. Because the array name a represents the address of a variable, it is a constant, assigning a value to a constant is wrong.

When an array is an argument, because an address is passed, the parameter changes and the argument changes

Two-dimensional arrays

Definition form: type array name [number of rows] [number of columns]

int a[2][3]; 2 rows, 3 columns, 6 elements, two one-dimensional array with three elements

In-memory storage, such as int a[2][2]

( Note:a[0], a[1] is also an array, is a one-dimensional array, and a[0], a[1] is the name of the array, so a[0], a[1] represents the address of the one-dimensional array )

The address of the 1> array A is FFC1, and the address of the array a[0] is also ffc1, i.e. a = a[0];

The address of the 2> element a[0][0] is FFC1, so the address of the array a[0] is the same as the address of the element a[0][0], i.e. a[0] = &a[0][0];

3> can finally come to a conclusion: a = a[0] = &a[0][0], and so on, can be derived a[1] = &a[1][0]

printf ("%p\n%p\n%p", A, a[0], &a[0] [0]);

Initialization of two-dimensional arrays

* Initialize by line

int a[2[3] = {{2,2,3}, {3,4,5}};

* Initialize in order of storage (1th row, then 2nd row)

int a[2[3] = {2,2,3,3,4,5};

* Initialization of some elements

int  a[2 ][3 ] = {{2 }, {3 , 4 }};

int b[3[3] = {{}, {,,2}, {1,2,3}};

*  if only part of the element is initialized, the number of rows can be omitted, but the number of columns cannot be omitted

int  a[][3 ] = {1 , 2 , 3 , 4 , 5 , 6 };

int a[][3] = {{1,2,3}, {3,5}, {}};

Why not just omit the number of columns? Because the int a[2][]={1,2,3,4,5,6} does not determine which row the element is in.

Introduction to C language Learning (iii) sizeof function, memory address, array