C ++ STL basics

1. Reasons for choosing C ++ brush algorithm

1. C ++ is fast (C is not faster, java is too slow)
2. C ++ has STL (what is STL)-very easy to use class library
3. How to use STL for efficient brushing algorithm
4. Benefits: Brushing algorithm, learning cost is extremely low
5. How to go from C to C ++ (only basic grammar to brush algorithm level)

俗话说：磨刀不误砍柴工
不会c++仍然可以做，但是效率低

2. Input and output

C ++ retains C's scanf and printf, and adds additional cin and cout

example

2.1. Input and output in C program

int a;
scanf("%d",&a);
printf("%d",a);

2.2. C ++ input and output

int a;
cin>>a;
cout<<a;

2.3. Continuous input and output variables

int a,b,c;
cin>>a>>b>>c;
cout<<a<<b<<c;

2.4. Wrap gracefully

cout<<1;
cout<<endl;
cout<<2;
cout<<3<<endl<<endl;

benefit:

1. Write a lot less

2. Continuous input and output variables

3. Elegant line breaks

注意：cin、cout比scanf、printf慢，有时候刷算法超时，可能因为使用了cin、cout

输入输出的数量(>1000)特别多，刷算法用cin，cout容易超时

3. STL (Standard Template Library) and algorithm header files

STL is a collection of "containers". These "containers" are list, vector, set, map, etc. STL is also a collection of algorithms and other components.

algorithm is some algorithm functions inherited from the container

sort function

Concept: iterator-understood as a pointer

#include <iostream>
#include <algorithm>

using namespace std;

int main()
{
    int a[]={2,1,5,0,-1,5,9};
    sort(a,a+7);
    for(int i=0;i<7;i++)
        cout<<a[i]<<" ";
    cout<<endl;
    system("pause");
    return 0;
}

4、STL——string(*)

Concept: equivalent to the encapsulation of char *, understood as a string

4.1. Simple to use

/**C中定义字符串以及打印*/
char *ch="asdkajbf";
for(int i=0;ch[i]!='\0';i++) cout<<*(ch+i);
/**C++中*/
string s="ssadaffw";
cout<<s<<endl;

4.2. Get a line of strings

I want to get a line of string

hello world

scanf("%s",ch);//1.仅获取一个单词，空格结束 2.ch[100]得设置初始大小

C ++:

string s;
getline(cin,s);//获取一行数据
cout<<s;

4.3. + = Operator

+ = For strings, characters are valid, numbers will be converted to asc codes

string s;
s+="hello";
s+=" world";
s+='5';
s+=10;//10对应的asc码是换行
int a=5;//想把a加入字符串
s+=(a+'0');
cout<<s;

4.4. Sorting (using algorithm)

string s="5418340";
sort(s.begin(),s.end());
cout<<s;

4.5.erase function

/**begin是头迭代器，end是尾迭代器*/
string s="5418340";
s.erase(s.begin());//删除第一个
s.erase(--s.end());//删除最后一个
cout<<s;

4.6. Substr function

/**begin是头迭代器，end是尾迭代器*/
string s="5418340";
s=s.substr(1,3);//取418,取索引为1，往后截断3个
s=s.substr(1,-1);//索引为1，截断到最后
cout<<s;

4.7. Loop (3 types)

1.for loop

string s="5418340";
for(int i=0;i<s.length();i++) cout<<s[i];

2. Iterator

for(string::iterator it=s.begin();it!=s.end();it++) cout<<*it;

3. Iterator simplification

for(auto it=s.begin();it!=s.end();it++) cout<<*it;

4. Use C ++ 11 new features for loop

for(auto x:s) cout<<x;

5、STL——vector(*)

Concept: vector is equivalent to an array, and the template type is equivalent to the stored content

1.vector construction

vector<int> v;//定义一个空vector
vector<int> v2(4);//定义一个4个大小的vector，初始为0
vector<int> v3(4,6);//定义一个4个大小的vector，初始为6
vector<int> v{1,2,3,4,5};//定义一个vector，数字为1,2，3,4,5
for(auto x:v3) cout<<x;

2. Use at or [] to get the element

vector<int> v{1,2,3,4,5};
cout<<v[1];//取索引为1的
cout<<v.at(2);//取索引为2的

3. Method

push_back additional content

vector<int> v;
v.push_back(5);
v.push_back(5);
v.push_back(5);
v.push_back(5);
v.push_back(6);
for(auto x:v) cout<<x;

resize

v.resize(10);//不赋值默认为0

erase delete element, complexity is O (n)

v.erase(v.begin());//删除第一个元素
v.erase(--v.end());//删除最后一个元素

Get the first element, get the last element

/**获取第一个元素*/
cout<<v.front();
cout<<v[0];
cout<<*v.begin();
/**获取最后一个元素*/
cout<<v.back();
cout<<v[v.size()-1];//size是获取大小
cout<<*--v.end();

4. Sort

The third parameter is the comparator, not written, the default is less ()

vector<int> v{5,1,2,5,4,0,-1};
sort(v.begin(),v.end(),less<int>());//从小到大
sort(v.begin(),v.end(),greater<int>());//从大到小排序
for(auto x:v) cout<<x;

5. Cycle

vector<int> v{5,1,2,5,4,0,-1};
for(int i=0;i<v.size();i++) cout<<v[i];//for循环
cout<<endl;
for(vector<int>::iterator it=v.begin();it!=v.end();it++) cout<<*it;//迭代器循环
cout<<endl;
for(auto it=v.begin();it!=v.end();it++) cout<<*it;//迭代器简化循环
cout<<endl;
for(auto x:v) cout<<x;//c++11新特性

6、STL——stack(*)

Concept: stack

structure

stack<int> s;

push、pop、size、empty
push an element onto the stack
pop pops an element, pop has no return value
top Take the top element of the stack
size View the number of elements

s.push(2);
s.push(3);
cout<<s.top()<<endl;
s.pop();
cout<<s.top()<<endl;
cout<<s.size()<<endl;

Hex conversion (decimal to binary)

int itob(int decimal){
    stack<int> s;int res=0;
    while(decimal!=0){
        s.push(decimal%2);
        decimal/=2;
    }
    while(!s.empty()){
        res=res*10+s.top();
        s.pop();
    }
    return res;
}

Reverse words (expand sstream, stoi, itoa)

Enter a string of characters and print the strings in reverse order

输入：hello world my name is steve yu

输出：yu steve is name my world hello

#include <iostream>
#include <stack>
#include <sstream>

using namespace std;

int main(){
    string str;
    stack<string> s;
    getline(cin,str);
    stringstream ss;
    ss<<str;
    while(ss>>str)
        s.push(str);
    while(!s.empty()){
        cout<<s.top();
        s.pop();
        if(s.size()!=0) cout<<" ";
    }
    return 0;
}

String to number

method 1:

 string s="1234";
 int i;
 stringstream ss;
 ss<<s;
 ss>>i;
 cout<<i;

Method 2:

string s="1234";
int i=stoi(s);
cout<<i;

Number to string

method 1:

int a=1234;
string out;
stringstream ss;
ss<<a;
ss>>out;
cout<<out<<endl;

Method 2: (c ++ 11)

int a=1234;
cout<<to_string(a)<<endl;

7、STL——queue

Concept: Queue

structure

queue<int> q;

push、back

q.push(5);
q.push(6);
cout<<q.front()<<endl;
q.pop();
cout<<q.front()<<endl;
cout<<q.size()<<endl;

8、STL——map(unordered_map pair)

Concept: mapping (map is a tree table, unorderedmap is a hash table)

map<int,int> m;//有序的，树状结构（底层）
m[6]=3;
m[5]=8;
m[4]=9;
for(auto it=m.begin();it!=m.end();it++)
    cout<<it->first<<" "<<it->second<<endl;
for(auto tmp:m){
    cout<<tmp.first<<" "<<tmp.second<<endl;
}

unordered_map

unordered_map<int,int> m;//无序的，哈希结构（底层）
m[6]=3;
m[5]=8;
m[4]=9;
for(auto it=m.begin();it!=m.end();it++)
    cout<<it->first<<" "<<it->second<<endl;
for(auto tmp:m){
    cout<<tmp.first<<" "<<tmp.second<<endl;
}

Pair usage (map to vector for sorting)

bool cmp(pair<int,int> a,pair<int,int> b){
    return a.first>b.first;
}
int main(){
    unordered_map<int,int> m;//无序的，哈希结构（底层）
    m[6]=3;
    m[5]=8;
    m[4]=9;
    vector<pair<int,int>> v(m.begin(),m.end());
    sort(v.begin(),v.end(),cmp);
    for(auto tmp:v){
        cout<<tmp.first<<tmp.second<<endl;
    }
    return 0;
}

9、set(unordered_set)

Concept: collection

Application counting, deduplication

set<int> s;//树状结构，有序
unordered_set<int> s2;//哈希结构，无序，快
s.insert(3);
s.insert(4);
s.insert(4);
s.insert(4);
cout<<s.size()<<endl;
for(auto tmp:s)
    cout<<tmp<<" ";
cout<<endl;
for(auto it=s.begin();it!=s.end();it++)
    cout<<*it<<" ";
cout<<endl;

10, STL it -

Concept: double-ended queue

deque<int> d;
// 4 9 1 2
d.push_back(1);
d.push_back(2);
d.push_front(9);
d.push_front(4);
d.pop_back();
d.pop_front();
for(auto tmp:d) cout<<tmp<<endl;
for(auto it=d.begin();it!=d.end();it++) cout<<*it<<endl;

Sort

sort(d.begin(),d.end(),greater<int>());

11、STL——list

Concept: doubly linked list

list<int> li;
li.push_back(6);
li.push_front(5);
li.emplace_front(9);
li.emplace_back(10);
li.insert(++li.begin(),2);
for(auto tmp:li) cout<<tmp<<endl;
for(auto it=li.begin();it!=li.end();it++) cout<<*it<<endl;

12. Documentation

English:

http://www.cplusplus.com/reference/stl/

Chinese:

http://c.biancheng.net/stl/map/

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