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Chapter 7 Pointers
C/C++ Language Programming
Wanxiang Che
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Addressing Method
• 如何读写内存中的数据？– 通过变量的地址访问变量所在的存
储单元• 两种寻址方式
– Direct Addressing• 直接按变量地址来存取变量内容
– Indirect Addressing• 通过指针变量来间接存取它所指
向的变量
0
┇
2000 3 变量 i
2002 6 变量 j
2004 9 变量 k
┇
3010 2000 变量 i_pointer
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• Pointer variables, simply called pointers
• Holding memory addresses as their values
• A kind of data type– Normally, a variable contains a specific value,
e.g., an integer, a floating-point value– However, a pointer contains the memory
address of a variable that in turn contains a specific value.
What is a Pointer?
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Declare a PointerLike any other variables, pointers must be declared before they can be used. To declare a pointer, use the following syntax:
dataType *pVarName;
For example, the following statement declares a pointer variable named pCount that can point to an int varaible.
int count;int *pCount;pCount = &count;
Address of pCount
Address of variable count Address of variable count
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pCount count
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DereferencingReferencing a value through a pointer is called indirection. The syntax for referencing a value from a pointer is
*pointer
For example, you can increase count using
count++; // direct reference
or
(*pCount)++; // indirect reference
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Test Pointer
#include <iostream>using namespace std;
int main() {
int count = 5; int *pCount = &count; cout << "The address of count is " << &count << endl; cout << "The address of count is " << pCount << endl; cout << "The value of count is " << count << endl; cout << "The value of count is " << *pCount << endl; return 0;
}
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Pointer Type
Must assign the address of the variable of the same type. It is a syntax error if the type of the variable does not match the type of the pointer. For example, the following code is wrong.
int area = 1;double *pArea = &area; // Wrong
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Initializing PointerLike a local variable, a local pointer is assigned an arbitrary value if you don’t initialize it. A pointer may be initialized to NULL, which is a special value for a pointer to indicate that the pointer points to nothing. You should always initialize pointers to prevent errors. Dereferencing a pointer that is not initialized could cause fatal runtime error or it could accidentally modify important data.
e.g.
int *p=NULL;
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CautionWe can declare two variables on the same line. For example, the following line declares two int variables:
int i, j;
Can we declare two pointer variables on the same line as follows?
int* pI, pJ;
No, it should be:
int *pI, *pJ;
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指针变量与其它类型变量的对比• 共性
– 在内存中占据一定大小的存储单元– 先定义，后使用
• 特殊性– 它的内容只能是地址，而不能是数据– 必须初始化后才能使用，否则指向不确定的存储单元– 只能指向同类型的变量– 可参与的运算：加、减一个整数，指针之间相减，自
增、自减、关系、赋值
指针运算（ 1/3 ）• 算术运算• int *p, a[10];
p = a;
p++; /*p 的值增加多少？ */• 指针的加减运算是以其指向的类型的字长为单位的
600060016002600360046005600660076008
p-1
p
p+111
指针运算（ 2/3 ）• int *p, *q, a[10];p = a;q = &a[5];– q - p – q = p + 3;
• 指针运算不能乱算– 一般只进行指针和整数的加减运算，同类型指针之间
的减法运算– 其它运算，比如乘法、除法、浮点运算、指针之间的
加法等，并无意义，所以也不支持
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指针运算（ 3/3 ）• 关系运算• 只有指向同一种数据类型的两个指针才能进行关系运
算。• 值为 1或 0
– p > q p < q p == q
• 指针不与非指针量进行比较，但可与 NULL(即 0值 ) 进行等或不等的关系运算– 判断 p 是否为空指针– p == NULL– p != NULL
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Passing Arguments by Reference with Pointers
There are three ways to pass arguments to a function in C++
•pass by value
•pass by reference with reference
•pass by reference with pointers.
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main(){ int a, b; a = 15; b = 8; swap(a, b); cout << a << “ “ << b;}
void Swap(int x, int y){ int temp; temp = x; x = y; y = temp;}
1515a b
88
x ya b
x y
Pass by Value
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简单变量作函数参数
Swap 函数调用前后参数变化的示意图
15 8 15 8 15 8
15
15 8 8 15 8 15
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a) 调用 Swap 函数
b) 执行 Swap 函数
c) 从 Swap 函数返回
temp
x y
b
main函数Swap函数
①
②
③
a a a
temp temp
x xy y
b b
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main(){ int a, b; a = 15; b = 8; swap(a, b); cout << a << “ “ << b;}
void Swap(int &x, int &y){ int temp; temp = x; x = y; y = temp;}
15a b
8
x ya b
x y
Pass by Reference with Reference Value
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main(){ int a, b; a = 15; b = 8; swap(&a, &b); cout << a << “ “ << b;}
void Swap(int *x, int *y){ int temp; temp = *x; *x = *y; *y = temp;}
&a&a &b&b
x ya b
x y15 a b
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Pass by Reference with Pointers
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指针变量作函数参数
用指针变量作函数参数实现两数互换函数的示意图
15 8 8 15
&a &b &a &b
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(a) 调用 Swap 函数
(b) 执行 Swap 函数
a
a
b
b
*x
*x
*y
*y
y
y
x
x
&a
&b
main函数
Swap函数
temp temp
②
① ③
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指针 swap 函数的几种错误形式（ 1 ）
• 参数单向传递void Swap(int *p1, int *p2)
{
int *p;
p = p1; /*p1,*p2 为内部变量 */
p1 = p2;
p2 = p;
}
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指针 swap 函数的几种错误形式（ 2 ）
• 指针 p 没有确切地址void Swap(int *p1, int *p2)
{
int *p; /* 指针 p 未初始化 */
*p = *p1;
*p1 = *p2;
*p2 = *p;
}
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Arrays and PointersRecall that an array name actually represents the starting address of the array. In this sense, an array variable is essentially a pointer. Suppose you declare an array of int value as follows:
int list[6] = {11, 12, 13, 14, 15, 16};
11 12 13 14 15 16
list[5]
list[4]
list[3]
list[2]
list[1]
list[0]
list+5list+4list+3list+2list+1list
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Array Pointer
#include <iostream>using namespace std;
int main() {int list[6] = {11, 12, 13, 14, 15, 16};for (int i = 0; i < 6; i++) cout << "address: " << (list + i) << " " << &list[i] << " value: " << *(list + i) << " " << list[i] << endl;return 0;
}
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Pointer with Index
#include <iostream>using namespace std;
int main() {int list[6] = {11, 12, 13, 14, 15, 16};int *pList = &list[0];for (int i = 0; i < 6; i++) cout << "address: " << (pList + i) << " value: " << *(pList + i) << " " << pList[i] << endl;return 0;
}
指针与数组• 数组名“可以”看作一个指针
– 只是不能修改这个指针的指向• 常指针 int* const
• 指针可当作数组名使用，反之亦然– int *p, a[10];p = a;p[1] = 0;*a = 0;
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Dynamic Memory Allocation
• Why?– Program Stack is Too Small– Cannot Know the Size of an Array Previously
• Syntax– int *pValue = new int;– int *list = new int[10];
• Allocating Memory from Heap– Reserved until explicitly free
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Free Memory
• delete– delete pValue;– delete []list;
• Caution Memory Leak – int *pValue = new int;– *pValue = 10;– int *pValue = new int;
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Characters and C-Strings
• A string is a sequence of characters
• There are two ways to process strings– Arrays of characters, pointer-based strings or
C-strings– Using the string class
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Character Test Functions Function Description Example
isdigit(c) Returns true if c is a digit. isdigit('7') is true
isdigit('a') is false
isalpha(c) Returns true if c is a letter. isalpha('7') is false
isalpha('a') is true
isalnum(c) Returns true if c is a letter or isalnum('7') is true
a digit. isalnum('a') is true
islower(c) Returns true if c is a lowercase islower('7') is false
letter. islower('a') is true
isupper(c) Returns true if c is an uppercase isupper('a') is false
letter. isupper('A') is true
isspace(c) Returns true if c is a whitespace isspace('\t') is true
character. isspace('A') is false
isprint(c) Returns true if c is a printable isprint(' ') is true
character including space ‘ ‘. isprint('A') is true
isgraph(c) Returns true if c is a printable isgraph(' ') is false
character excluding space ‘ ‘. isgraph('A') is true
ispunct(c) Returns true if c is a printable ispunct('*') is true
character other than a digit, ispunct(',') is true
letter, or space. ispunct('A') is false
iscntrl(c) Returns true if c is a control iscntrl('*') is false
character such as '\n', '\f', '\v', iscntrl('\n') is true
'\a', and '\b'. iscntrl('\f') is true
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Case Conversion Functions
Function Description Example
tolower(c) Returns the lowercase equivalent of tolower('A') returns 'a'
c, if c is an uppercase letter. tolower('a') returns 'a'
Otherwise, return c itself. tolower('\t') returns '\t'
toupper(c) Returns the uppercase equivalent of toupper('A') returns 'A'
c, if c is a lowercase letter. toupper('a') returns 'A'
Otherwise, return c itself. toupper('\t') returns '\t'
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Storing and Accessing StringsA pointer-based string is an array of characters ending in the null terminator ('\0'), which indicates where a string terminates in memory. A string can also be accessed via a pointer, which points to the first character in the string. So you can declare a string variable using an array or a pointer:
char city[7] = “Harbin"; // Option 1char *pCity = “Harbin"; // Option 2
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字符指针变量与字符数组的区别• 定义方法不同
char city[7];
char *pCity;• 赋值方法和含义不同
char city[7];
city = ”Harbin”; /* 错误 */
strcpy(str,”Harbin”); /* 正确 */
char *pCity;
pCity = ”Harbin”;
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Array and Pointer SyntaxYou can access city or pCity using the array syntax or pointer syntax. For example,
cout << city[1] << endl;cout << *(city + 1) << endl;cout << pCity[1] << endl;cout << *(pCity + 1) << endl;
each displays character a (the second element in the string).
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Reading Strings Read a string from the keyboard using the cin object:
char city[10];char *pCity;
cout << "Enter a city: ";cin >> city; // read to array citycin >> pCity; // Wrongcout << "You entered " << city << endl;
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Reading Strings Using getlineC++ provides the cin.getline function in the iostream header file, which reads a string into an array. The syntax of the function is:
cin.getline(char array[], int size, char delimitChar)
The function stops reading characters when the delimiter character is encountered or when the size - 1 number of characters are read. The last character in the array is reserved for the null terminator ('\0'). If the delimiter is encountered, it is read, but not stored in the array. The third argument delimitChar has a default value ('\n').
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String
Functions
Function Description
int strlen(char *s1)
Returns the length of the string, i.e., the number of the
characters before the null terminator.
char *strcpy(char *s1, const char *s2)
Copies the string s2 to string s1. The value in s1 is returned.
char *strncpy(char *s1, const char *s2, size_t n)
Copies at most n characters from string s2 to string s1. The
value in s1 is returned.
char *strcat(char *s1, const char *s2)
Appends string s2 to s1. The first character of s2 overwrites the
null terminator in s1. The value in s1 is returned.
char *strncat(char *s1, const char *s2, size_t n)
Appends at most n characters from string s2 to s1. The first
character of s2 overwrites the null terminator in s1 and appends
a null terminator to the result. The value in s1 is returned.
int *strcmp(char *s1, const char *s2)
Returns a value greater than 0, 0, or less than 0 if s1 is greater
than, equal to, or less than s2 based on the numeric code of the
characters.
int *strncmp(char *s1, const char *s2, size_t n)
Returns a value greater than 0, 0, or less than 0 if the n
characters in s1 is greater than, equal to, or less than the first
n characters in s2 based on the numeric code of the characters.
int atoi(char *s1)
Converts the string to an int value.
double atof(char *s1)
Converts the string to a double value.
long atol(char *s1)
Converts the string to a long value.
void itoa(int value, char *s1, int radix)
Converts the value to a string based on specified radix.
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String Copy with Char Arrayvoid MyStrcpy(char to[], char from[]){int i = 0; while (from[i] != '\0'){
to[i] = from[i]; i++; } to[i] = '\0';
}
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String Copy下标： 0 1 2 3 4 5 6 7 8 9 10 11
H e l l o C h i n a \0
H e l l o C h i n a \0
from[i]
to[i]
下标移动方向
to[i]=‘\0’
from
to
结束拷贝i
i++i
H e l l o C h i n a \0
H e l l o C h i n a \0
*from
*to
指针移动方向
指针移动方向*to=‘\0’to
from++ fromfrom
to++ to
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String Copy with Char Pointervoid MyStrcpy(char *to, char *from){while (*from != '\0'){
*to = *from; from++;
to++; }*to = '\0';
}
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String Length with Char Arrayunsigned int MyStrlen(char str[]){ int i;unsigned int len = 0; for (i=0; str[i]!='\0'; i++) {
len++; }return (len);
}
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String Length with Char Pointerunsigned int MyStrlen(char *pStr){ unsigned int len = 0; for (; *pStr!='\0'; pStr++) {
len++; }return (len);
}
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Checking Palindromes
A string is a palindrome if it reads the same forward and backward. The words “mom,” “dad,” and “noon,” for example, are all palindromes.
bool isPalindrome(char * s) { int low = 0; int high = strlen(s) - 1; while (low < high) { if (s[low] != s[high]) return false; low++; high--; } return true;}
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String Sort with Two-Dim Char Arraychar str[N][10] = {"Pascal","Basic","Fortran",
"Java","Visual C"};
for (i=0; i<N-1; i++)
{
for (j = i+1; j<N; j++)
{
if (strcmp(str[j], str[i]) < 0)
{
strcpy(temp,str[i]);
strcpy(str[i],str[j]);
strcpy(str[j],temp);
}
}
}
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String Sort with Two-Dim Char Array
name[0] P a s c a l \0 \0 \0 \0
name[1] B a s i c \0 \0 \0 \0 \0
name[2] F o r t r a n \0 \0 \0
name[3] J a v a \0 \0 \0 \0 \0 \0
name[4] V i s u a l C \0 \0
strstr
strstr
strstr
strstr
strstr
name[0] B a s i c \0 \0 \0 \0 \0
name[1] F o r t r a n \0 \0 \0
name[2] J a v a \0 \0 \0 \0 \0 \0
name[3] P a s c a l \0 \0 \0 \0
name[4] V i s u a l C \0 \0
strstr
strstr
strstr
strstr
strstr
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Array of Pointer
• The elements are pointer• Declare
datatype *array_name[len];• E.g.
char *pStr[5];
pStrpStr [5][5] ** charchar
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String Sort with Array of Pointer
char *ptr[N] = {"Pascal","Basic","Fortran", "Java","Visual C"};
for (i=0; i<N-1; i++) { for (j = i+1; j<N; j++) {
if (strcmp(ptr[j], ptr[i]) < 0) { char *temp = ptr[i]; ptr[i] = ptr[j]; ptr[j] = temp; }
} }
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指针数组 ptr 字符串排序前 Pascal\0
Basic\0
Fortran\0
Java\0
Visual C\0
ptr[0]
ptr[1]
ptr[2]
ptr[3]
ptr[4]
指针数组 ptr 字符串排序后 Pascal\0
Basic\0
Fortran\0
Java\0
Visual C\0
ptr[0]
ptr[1]
ptr[2]
ptr[3]
ptr[4]
String Sort with Array of Pointer
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Command-Line Arguments
• GUI 界面之前，计算机的操作界面都是字符式的命令行界面（ DOS 、 UNIX 、 Linux ）
• 通过设置命令行参数，用户可以决定程序干什么、怎么干
• int main(int argc, char* argv[])– 当你把 main 函数写成这样时– argc 的值为参数的数目 +1– argv[0] 为指向命令名的字符指针– argv[X](x>0) 为指向每个参数的字符指针
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Command-Line Arguments
int main(int argc, char *argv[]){ int i; cout << argc – 1 << " arguments" << endl; for (i = 1; i < argc; i++) cout << i << " " << argv[i];
return 0;}
argv[0]argv[0] echo.exe
argv[1]argv[1]
argv[2]argv[2]
argv[3]argv[3]
programming
is
fun
argv指针数组 字符串
String Matching
• Given a single keyword p (search string) and an input string s, answer “hit” if p occurs as a substring of s, or “lost” otherwise.
• E.g. – Input
• s = This is a long article ……
• p = article
– Output• hit!
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The Brute Force Algorithm
• Search string (pattern): p1p2…pm • Text string: s1s2…sn (usually n >> m)• Compare pattern with the m -character substring sksk+1…
sk+m-1 for each successive value of k from 1 to n - m +1.• The pattern either matches the substring entirely or finds
a position at which the corresponding characters from the pattern and the string don’t match
p1 … pi … pm
s1… sk … sj… sk+m-1 ...
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The Brute Force Algorithm
i=0
j=8
i=7
j=14
travel information
informingi=0
j=9
travel information
informing
Conceptually the pattern is shifted
to the right
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The Brute Force Algorithm
bool str_search(char s[], char p[]) { int i = 0; int j = 0; int m = strlen(p); int n = strlen(s);
while (i < m && j < n) { if (p[i] == s[j]) { i++; j++; } else { j = j - i + 1; i = 0; } }
return (i >= m) ? true : false;}
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The loop can stop
earlier: j n - m + 1
at mostn mtimes
The Algorithm Performance
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Worst case Best case Betweentext aaaaaaa abcdefg abababakeyword aab xab abccomparisons (n-m+ 1) * m n-m+ 1
15 5 11
mismatch foundat end of pattern
mismatch foundat beginning of pattern
