Quick transition from C language to C++

One, vector container

1. What is a vector container

Vector is a sequence container (Sequence Container) that encapsulates a dynamic size array. Like any other type of container, it can store various types of objects. It can be simply considered that a vector is a dynamic array that can store any type.

2. Commonly used functions

1), constructor (initialization)

vector<int> a;      //创建一个空vector

vector<int> a(10);  //初始化了10个默认值为0的元素

vector<int> a(10, 1);   //初始化了10个值为1的元素

//通过复制a初始化
vector<int> a(5, 1);
vector<int> b(a);

int a[5] = {
    
    1, 2, 3, 4, 5};
//通过数组a的地址初始化，注意地址是从0到5（左闭右开区间）
vector<int> b(a, a+5);

2), add delete function

vector<int> a;
//在末尾加入元素1
a.push_back(1);
//在下标1指向的元素前插入一个5
a.insert(1, 5);
//在下标1指向元素前插入3个5
a.insert(1, 3, 6);
//将b的所有元素插入到a中
int b[5] = {
    
    1, 2, 3, 4, 5};
a.insert(a.begin(), b, b+5);
//删除末尾元素
a.pop_back();

3. Remove duplicate elements from vector array

When writing the leetcode title "Sum of Three Numbers", it is necessary to de-duplicate the vector two-dimensional array. The set data structure is mainly used here.

#include<set>
#include<vector>

//这里声明一个二维vector数组
vector<vector<int>> res;

//...中间操作省略

set<vector<int>> s(res.begin(), res.end());
res.assign(s.begin(), s.end());

More specific reference documentation rookie tutorial

Two, string class

1. Character array in C language style

C-style strings originated from the C language and continue to be supported in C++. The string is actually a one-dimensional character array terminated with the null character'\0'. Therefore, a string ending in null contains the characters that make up the string.
There are a large number of functions in C++ to manipulate null-terminated strings, summarized as follows:

#include <iostream>
#include <cstring>
using namespace std;
 
int main ()
{
    
    
   char str1[11] = "Hello";
   char str2[11] = "World";
   char str3[11];
   int  len ;
   // 复制 str1 到 str3
   strcpy( str3, str1);
   cout << "strcpy( str3, str1) : " << str3 << endl;
   // 连接 str1 和 str2 结果在str1
   strcat( str1, str2);
   cout << "strcat( str1, str2): " << str1 << endl;
   // 连接后，str1 的总长度
   len = strlen(str1);
   cout << "strlen(str1) : " << len << endl;
   //比较str1,str2是否相等,相同则返回0；如果<返回值小于0,如果s1>s2返回值大于0。
   int res = strcmp(str1, str2);
   cout << "strcmp(str1, str2): " << res <<endl;
   //返回一个指针，指向字符串 s1 中字符 ch 的第一次出现的位置
   char ch = 'e';
   int *a = strchr(s1, ch);
   //返回一个指针，指向字符串 s1 中字符串 s2 的第一次出现的位置。
   int *b = strstr(s1, s2);
   return 0;
}

2. Commonly used special functions

1), string to int, long, float, double type

string s = "123.5";
//string 转 int;
cout << stoi(s) << endl;
//string 转 long
cout << stol(s) << endl;
//string 转 float
cout << stof(s) << endl;
//string 转 double
cout << stod(s) << endl;

For more details, please refer to the C language Chinese website