Transcript Pointer

Pointers
Data Structure 補充單元
2015/7/21
1
Introduction
• If you want to be proficient in the writing of
code in the C programming language, you
must have a thorough working knowledge
of how to use pointers
– Unfortunately, C pointers appear to be rather
unfriendly
– Here, let try to introduce for the first time what
pointer is all about …
2015/7/21
2
get a feeling on C’s variables …
• A variable in a program is something with
a name, the value of which can vary.
• When we declare a variable we inform the
compiler of two things,
– the name of the variable
– the type of the variable.
2015/7/21
3
get a feeling on C’s variables …
• The way the compiler and linker handles
this is that it assigns a specific block of
memory within the computer to hold the
value of that variable
– The variable size of that block depends on the
range over which the is allowed to vary
2015/7/21
4
Assigning specific blocks of
memory to variables
i
j
int i;
short j;
i=100;
2015/7/21
5
Size of Integer
2015/7/21
6
get a feeling on C’s variables …
• For example, we declare a variable of type
integer with the name k by writing:
– int k
• On seeing the "int" part of this statement the
compiler
– sets aside 4 bytes of memory (on a PC) to hold the
value of the integer.
– It also sets up a symbol table.
• In that table it adds the symbol k and the relative address in
memory where those 4 bytes were set aside.
2015/7/21
7
get a feeling on C’s variables …
• Int k, m;
k = 2;
• m = k;
• There are two "values" associated with the
object k.
– One is the value of the integer stored there (2
in the above example) and
– the other the "value" of the memory location,
i.e., the address of k.
2015/7/21
8
get a feeling on C’s variables …
• int j, k;
k = 2;
j = 7; <-- line 1
k = j; <-- line 2
• In line 1, j is interpreted as the “address”
of j
• In line 2, j is interpreted as the “value”
stored in address of j
2015/7/21
9
Example
• 我如何將 variable (變數) 的 address (位址)
列印出來?
– Int i;
printf(“%p”,&i)
2015/7/21
10
2015/7/21
11
32-bit vs 64-bit
#include <stdio.h>
int main()
{
int i;
long j;
printf("size of int = %d\n", sizeof(i));
printf("size of long = %d\n", sizeof(j));
printf(“Address of variable I = %p\n", &i);
printf(“Address of variable J = %p\n", &j);
}
2015/7/21
12
32-bit vs 64-bit
2015/7/21
13
32-bit vs 64-bit
%gcc -m32 -o out32 c.c
%gcc -m64 -o out64 c.c
2015/7/21
14
32-bit vs 64-bit
%./out32
size of int = 4
size of long = 4
address of variable I = 0xffffdc10
address of variable J = 0xffffdc0c
%./out64
size of int = 4
size of long = 8
address of variable I = 0x7fffffffeaec
address of variable J = 0x7fffffffeae0
2015/7/21
15
思考一下 …
• 所以, 當我們看到一個 C 的變數時, 它有時
代表的意義是該變數的 “位址”,有時則代
表該變數內存的 “值”
– 通常當變數出現在 “=” 左邊時,其意義是該變
數的 “位址”
– 而通常當變數出現在 “=” 右邊時,其意義是該
變數的 “值”
• 看來, 有點自由心證味道! 那, 我們可不可以
有個變數, 它很明確地就是儲存 “位址”?
2015/7/21
16
儲存 “位址” 的變數
• Yes! Such a variable is called a pointer variable
– In C when we define a pointer variable we do so by
preceding its name with an asterisk (*).
– In C we also give our pointer a type which, in this
case, refers to the type of data stored at the
address we will be storing in our pointer.
• For example, consider the variable declaration:
– int *ptr;
– ptr : A pointer variable that point to address of an
integer
– *ptr : value stores in the memory pointed to by ptr
2015/7/21
17
Example
• Int *ptr;
int k;
ptr = &k;  ptr stores address of k
*ptr = 7;  *ptr store its “value”
• 可以這樣做類比
– ptr  &k
– *ptr  k
• 注意 : ptr 如果沒有給初值, 預設值是 null (or 0)
– if (ptr == NULL) …
2015/7/21
18
Assigning specific blocks of
memory to variables
1004
7
ptr
k
int *ptr;
int k;
ptr = &k;
*ptr=7;
2015/7/21
19
Example
2015/7/21
20
為什麼需要宣告 pointer 的 type?
因為事先宣告 ptr 指向的是 integer,
因此當 ptr + 1 時, 我們知道它的位
址值 + 4 byte
2015/7/21
21
Void pointer?
• void *ptr;
Void : 空的
– 空的 pointer?
– 較好的解釋是 : generic pointer (不是某種特定
型態, 例如 integer or character, 的 pointer)
• You cannot compare an integer pointer to
a character pointer
– But you can compare them to a void pointer
2015/7/21
22
Dynamic Allocation of Memory
• There are times when it is convenient to allocate
memory at run time using malloc(), calloc()
• Using this approach permits
– postponing the decision on the size of the memory
block need to store an array, for example, until run
time.
– Or it permits using a section of memory for the
storage of an array of integers at one point in time,
and then when that memory is no longer needed it
can be freed up for other uses, such as the storage of
an array of structures.
2015/7/21
23
Pointers and Dynamic Allocation of
Memory
• When memory is allocated, the allocating
function (such as malloc(), calloc(), etc.)
returns a pointer.
2015/7/21
24
Example
• void *ptr;
ptr = (void *)malloc(10 * sizeof(int));
if (ptr == NULL)
{
……….
}
– 動態地 allocates 10 個 integers 的空間
– <重要>動態建立的 memory 不用時, 記得要
set it “free”
• free (ptr);
2015/7/21
25
Dynamic Array 動態陣列
#include <stdio.h>
#include <stdlib.h>
int main()
{
int *ptr, size,i;
printf("Please input the array size:");
scanf("%d", &size);
ptr = (int *)malloc(sizeof(int)*size);
for (i=0; i<size; i++) ptr[i]=i; // *(ptr+i) = i;
for (i=0; i<size; i++) printf("ptr[%d]=%d\n", i,ptr[i]);
}
2015/7/21
26
Pointers and Structures
• 繼續討論之前, 先 “醞釀” 一下
– C 語言之主程式/副程式呼叫(另一)副程式時參
數的傳遞是採 call by value (傳值)
– 假設, 我們需要在副程式呼叫時傳遞一個很大
的 structure (e.g., 100 bytes 那麼大), 且頻率很
高
•  BAD IDEA!
– 那 … 怎麼辦?
• 何不傳遞一個指向特定 structure 的 pointer!
2015/7/21
27
Int maximum();
main()
{
int max, x[10000],y[10000];
….
max = maximum(x, y);
}
Int maximum(int x, int y)
{
int max;
if (x>= y)
max = x;
else
max=y;
2015/7/21
return (max);
28
But, what is a structure?
• Let’s see array first … To represent
somebody’s name no more than 20
characters …
– char name[20];
• How about representing 50 students’
name in a class?
– char name[50][20];
2015/7/21
29
Now, what about representing 50
students’ demographics?
• Include name, ID, gender, address, etc
– char name[50][20];
– char ID[50][10];
– int gender[50];
– char address[50][100];
• A bit awkward, right?
2015/7/21
30
How about this …
• Define a new data types, like “student_type”,
which aggregate name, ID, gender & address
together?
 This is what structure does
• For example:
struct student_type {
char name[20];
char ID[10];
int gender
char address[100];
}
2015/7/21
31
Then …
• In 宣告：
– struct student_type student[50];
• To access:
– student[0]->name
– student[10]->ID
– student[49]->gender
– etc
2015/7/21
32
Structure Data Type
2015/7/21
33
#include <stdlib.h>
#include <stdio.h>
typedef struct node {
int data;
struct node * next; } NODE;
NODE *next;
void main() {
NODE * curr, * head;
int i;
head = NULL;
for(i=1;i<=10;i++) {
curr = (NODE *)malloc(sizeof(item));
curr->data = i;
curr->next = head;
head = curr;
}
curr = head;
while(curr) {
printf("%d\n", curr->data);
curr = curr->next ;
}
}
2015/7/21
34
2015/7/21
35
Sample Code & Explanation
定義一個新的 data
type, 名稱為 NODE,
包含一整數及一指標
2015/7/21
36
程式執行前
2015/7/21
37
Sample Code & Explanation
定義一個新的 data type, 名稱為 NODE,
包含一整數及一指標
這段程式 不會 在記憶體新增佔用空間
2015/7/21
38
NODE 宣告完後
NO Change
2015/7/21
39
Sample Code & Explanation
宣告 curr 及 head 為指向某一 NODE 的
指標
這段程式 會 在記憶體新增佔用空間
2015/7/21
40
NODE *curr, *head 宣告完後
curr
head
佔用空間多大? Why?
2015/7/21
41
如果是這樣宣告 …
NODE curr, head;
curr
head
佔用空間多大? Why?
2015/7/21
42
Sample Code & Explanation
動態地向系統要一個 NODE 的記憶空間,
並將此記憶空間的位置儲存在 curr 中
這段程式 會 在記憶體新增佔用空間
2015/7/21
43
malloc 宣告完後
NODE 的大小(佔的記憶
空間)
1016
= 4 bytes (int) + 4 bytes
(address)
2015/7/21
44
Sample Code & Explanation
For loop 之 i=0 iteration 執行完後 …
2015/7/21
45
For loop 之 i=0 執行完後 …
curr
head
將 0 儲存至透過 malloc新獲得
之NODE記憶位置的 data 欄
位
讓 head 也指向此新獲得之記
憶位置, 換句話說, head 所在
位置儲存的資料內容是 1016,
也就是新增 NODE 之 address
2015/7/21
(Next Slide)
46
For loop 之 i=0 執行完後 …
curr
1016
1016
head
0
2015/7/21
47
For loop 之 i=0 執行完後 …
Address
curr
1016
1016
head
0
2015/7/21
48
Sample Code & Explanation
For loop 之 i=1 iteration 執行過程 …
head = curr 執行前
2015/7/21
49
For loop 之 i=1 執行過程 …
2004
1016
curr
head
0
1
1016
2015/7/21
50
For loop 之 i=1 執行完後 …
2004
2004
curr
head
0
1
1016
2015/7/21
51
用熟悉的表示方式 …
(i=1)
curr
0
head
1
2015/7/21
52