第 16.3 節

string container

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string container

The basic concept of a string.

Essence:

A string in C++ is a string in the C++ style, and string is fundamentally a class.

*Difference between string and char :

char * is a pointer.
A string is a class that encapsulates a char* internally, managing the string and serving as a container of type char*.

Features:

The String class internally encapsulates many member methods.

例如：查找 find，拷贝 copy，删除 delete，替换 replace，插入 insert

String manages the memory allocated for char*, eliminating concerns about copying overflows or value retrieval overflows, as the class handles this internally.

string constructor

Constructor Prototype:

string(); //Create an empty string, for example: string str; string(const char* s); //Initialize with string s
string(const string& str); //Initialize one string object with another
string(int n, char c); // Initialize with n characters c

Example:

#include <string>
//string构造
void test01()
{
    string s1; //创建空字符串，调用无参构造函数
    cout << "str1 = " << s1 << endl;

    const char* str = "hello world";
    string s2(str); //把c_string转换成了string

    cout << "str2 = " << s2 << endl;

    string s3(s2); //调用拷贝构造函数
    cout << "str3 = " << s3 << endl;

    string s4(10, 'a');
    cout << "str3 = " << s3 << endl;
}

int main() {

    test01();


    return 0;
}

Running/Observation Results: After running, the corresponding content will be printed according to the output statements. The variable values can be inferred based on the order of initialization, assignment, and function calls.

Summary: The various ways to construct a string aren't comparable—just use whichever works best.

string assignment operation

Functional description:

给string字符串进行赋值

Function prototype for assignment:

string& operator=(const char* s); // Assigning a char* type string to the current string
string& operator=(const string &s); //assign string s to the current string
string& operator=(char c); //Assign character to current string
string& assign(const char *s); //assign string s to the current string
string& assign(const char *s, int n); //assigns the first n characters of string s to the current string
string& assign(const string &s); //Assign string s to the current string
string& assign(int n, char c); //assign n characters c to the current string

Example:

//赋值
void test01()
{
    string str1;
    str1 = "hello world";
    cout << "str1 = " << str1 << endl;

    string str2;
    str2 = str1;
    cout << "str2 = " << str2 << endl;

    string str3;
    str3 = 'a';
    cout << "str3 = " << str3 << endl;

    string str4;
    str4.assign("hello c++");
    cout << "str4 = " << str4 << endl;

    string str5;
    str5.assign("hello c++",5);
    cout << "str5 = " << str5 << endl;

    string str6;
    str6.assign(str5);
    cout << "str6 = " << str6 << endl;

    string str7;
    str7.assign(5, 'x');
    cout << "str7 = " << str7 << endl;
}

int main() {

    test01();


    return 0;
}

Summary:

There are many ways to assign strings, and operator= is quite practical.

string concatenation

Description:

Implement appending a string to the end of a string.

Function prototype:

string& operator+=(const char* str); //Overload += operator
string& operator+=(const char c); //Overload += operator
string& operator+=(const string& str); //Overload += operator
string& append(const char *s); //Concatenate string s to the end of the current string
string& append(const char *s, int n); //Concatenate the first n characters of string s to the end of the current string
string& append(const string &s); //same as operator+=(const string& str)
string& append(const string &s, int pos, int n);//Append n characters starting from position pos in string s to the end of the string

Example:

//字符串拼接
void test01()
{
    string str1 = "我";

    str1 += "爱玩游戏";

    cout << "str1 = " << str1 << endl;
    
    str1 += ':';

    cout << "str1 = " << str1 << endl;

    string str2 = "LOL DNF";

    str1 += str2;

    cout << "str1 = " << str1 << endl;

    string str3 = "I";
    str3.append(" love ");
    str3.append("game abcde", 4);
    //str3.append(str2);
    str3.append(str2, 4, 3); // 从下标4位置开始 ，截取3个字符，拼接到字符串末尾
    cout << "str3 = " << str3 << endl;
}
int main() {

    test01();


    return 0;
}

Summary: There are many overloaded versions of string concatenation. In the initial stage of learning, just remember a few.

String search and replace

Description:

Search for the specified string
Replace: Replace the string at the specified position

Function prototype:

int find(const string& str, int pos = 0) const; //Finds the first occurrence of str starting from pos
int find(const char* s, int pos = 0) const; //Find the first occurrence of 's', starting the search from position 'pos'
int find(const char* s, int pos, int n) const; //Find the first occurrence of the first n characters of s starting from pos
int find(const char c, int pos = 0) const; // Find the first occurrence of character c
int rfind(const string& str, int pos = npos) const; //Find the last occurrence of a string, starting the search from pos
int rfind(const char* s, int pos = npos) const; // Find the last occurrence position of 's', starting search from pos
int rfind(const char* s, int pos, int n) const; //Find the last position of the first n characters of string s starting from pos
int rfind(const char c, int pos = 0) const; //Find the last occurrence position of character c
string& replace(int pos, int n, const string& str); //replace n characters starting from pos with string str
string& replace(int pos, int n,const char* s); //Replace n characters starting at position pos with string s

Example:

//查找和替换
void test01()
{
    //查找
    string str1 = "abcdefgde";

    int pos = str1.find("de");

    if (pos == -1)
    {
        cout << "未找到" << endl;
    }
    else
    {
        cout << "pos = " << pos << endl;
    }
    

    pos = str1.rfind("de");

    cout << "pos = " << pos << endl;

}

void test02()
{
    //替换
    string str1 = "abcdefgde";
    str1.replace(1, 3, "1111");

    cout << "str1 = " << str1 << endl;
}

int main() {

    //test01();
    //test02();


    return 0;
}

Summary:

find searches from left to right, rfind from right to left
The find function returns the position of the first character when a string is found, and returns -1 if it is not found.
When using replace, you need to specify the starting position, the number of characters, and the string to replace them with.

String comparison

Description:

String comparison

Comparison Method:

String comparison is performed based on the ASCII codes of the characters.

= return 0

Back 1

< return -1

Function prototype:

int compare(const string &s) const; //compare with string s
int compare(const char *s) const; //compare with string s

Example:

//字符串比较
void test01()
{

    string s1 = "hello";
    string s2 = "aello";

    int ret = s1.compare(s2);

    if (ret == 0) {
        cout << "s1 等于 s2" << endl;
    }
    else if (ret > 0)
    {
        cout << "s1 大于 s2" << endl;
    }
    else
    {
        cout << "s1 小于 s2" << endl;
    }

}

int main() {

    test01();


    return 0;
}

Summary: String comparison is primarily used to check whether two strings are equal, as determining which is greater or smaller isn’t particularly meaningful.

string character access

There are two ways to access a single character in a string:

Using the square bracket operator []: This provides direct access to the character at the specified index. For example, str[index].
Using the at() method: This also accesses the character at a given index but includes bounds checking. If the index is out of range, it throws an out_of_range exception. For example, str.at(index).

char& operator[](int n); //take characters using method
char& at(int n); //Using the at method to get the character

Example:

void test01()
{
    string str = "hello world";

    for (int i = 0; i < str.size(); i++)
    {
        cout << str[i] << " ";
    }
    cout << endl;

    for (int i = 0; i < str.size(); i++)
    {
        cout << str.at(i) << " ";
    }
    cout << endl;

    //字符修改
    str[0] = 'x';
    str.at(1) = 'x';
    cout << str << endl;
    
}

int main() {

    test01();


    return 0;
}

Summary: There are two ways to access individual characters in a string: using brackets or the at method.

String Insertion and Deletion

Description:

Perform insertion and deletion operations on a string

Function prototype:

string& insert(int pos, const char* s); //insert string
string& insert(int pos, const string& str); //insert string
string& insert(int pos, int n, char c); //Insert n characters 'c' at the specified position.
string& erase(int pos, int n = npos); // Delete n characters starting from Pos

Example:

//字符串插入和删除
void test01()
{
    string str = "hello";
    str.insert(1, "111");
    cout << str << endl;

    str.erase(1, 3);  //从1号位置开始3个字符
    cout << str << endl;
}

int main() {

    test01();


    return 0;
}

Summary: Both insertion and deletion start indexing from 0.

string substring

Description:

Extracting the desired substring from a string.

Function prototype:

return the string composed of n characters starting from pos

Example:

//子串
void test01()
{

    string str = "abcdefg";
    string subStr = str.substr(1, 3);
    cout << "subStr = " << subStr << endl;

    string email = "hello@sina.com";
    int pos = email.find("@");
    string username = email.substr(0, pos);
    cout << "username: " << username << endl;

}

int main() {

    test01();


    return 0;
}

Summary: Flexible use of substring function can extract useful information in practical development.