Haste makes waste

Nano01(自動運転)-【C++】-Lesson12-Vectors and Iterators容器与迭代器

Posted on Jan 01, 2019 By lijun

1. Vectors Introduction

C++中有很多很好用的容器类，比如vector:

  1. The vector has size 0 when we instantiate it.
  2. We use ‘resize’ to change the size of the vector.
// constructing vectors
#include <iostream>
#include <vector>  //Need to include the vector library!


int main ()
{
  //creating a vector of integers
  std::vector<int> vectorInts;  
  std::cout<<"vectorInts has "<<vectorInts.size()<<" elements\n";
  
  //Changing the size of vectorInts to 6
  vectorInts.resize(6);
  std::cout<<"\n\nvectorInts now has "<<vectorInts.size()<<" elements\n";
 
  return 0;
}

vectorInts has 0 elements
vectorInts now has 6 elements

4. Vectors and Iterators

vector比array更通用，可以在运行时修改其长度，也可以插入元素到vector。

// constructing vectors
// constructing vectors
#include <iostream>
#include <vector>


int main ()
{
  //creating a vector of integers
  std::vector<int> vectorInts;  
  //creating an iterator for the vector
  std::vector<int>::iterator it;
  
  std::cout<<"vectorInts has "<<vectorInts.size()<<" elements\n";
  
  std::cout<<"\n\nAdding four elements to the vector\n";
  //assigning the value 3 to 4 elements of the vector
  vectorInts.assign(4,3);
  std::cout<<"vectorInts has "<<vectorInts.size()<<" elements\n";
  
  //printing the contents of vectorInts
  std::cout<<"VectorInts has these elements:\n";
  for (it = vectorInts.begin(); it != vectorInts.end(); ++it)
    std::cout<<*it<<" ";

  return 0;
}


vectorInts has 0 elements

Adding four elements to the vector
vectorInts has 4 elements
VectorInts has these elements:
3 3 3 3

5. Vectors and Its Prog Ans

// constructing vectors
// constructing vectors
#include <iostream>
#include <vector>

using namespace std;

void printVector(vector<float> vIn);

 void printVector(vector<float> vIn)
 {//printing the contents of vIns
    //TODO: Complete the function 
    vector<float>::iterator it;
    for (it = vIn.begin(); it != vIn.end(); ++it)
        cout<<*it<<" ";
 }


// Goal: pracitce constructing vectors
// and their iterators
// Call the vector vFloat
// Call the iterator it

//TODO: Add the necessary libraries
#include "main.hpp"

int main ()
{
  //TODO: create a vector of floats
   vector<float> vFloat;

  std::cout<<"vFloat has "<<vFloat.size()<<" elements\n";

  //TODO: add elements to the library

  std::cout<<"\n\nAdding 10 elements to the vector\n";

  //TODO: assign the value 8.8 to 10 elements of the vector
   vFloat.assign(10, 8.8);
   std::cout<<"vFloat has "<<vFloat.size()<<" elements\n"; 


  //TODO: Complete the Print function in main.hpp 
  // Call the function here to print out each element of vFloat
  printVector(vFloat);

  return 0;
}

输出：

vFloat has 0 elements

Adding 10 elements to the vector
vFloat has 10 elements
8.8 8.8 8.8 8.8 8.8 8.8 8.8 8.8 8.8 8.8

7. assign Elements

参考下面的代码，通过assign赋值的时候，会把之前的vector覆盖：

// constructing vectors
// constructing vectors
#include <iostream>
#include <vector>


int main ()
{
  //creating a vector of integers
  std::vector<int> vectorInts;  
  //creating an iterator for the vector
  std::vector<int>::iterator it;
  
  std::cout<<"vectorInts has "<<vectorInts.size()<<" elements\n";
  
  std::cout<<"Adding 23 to the vector\n";
  vectorInts.assign(1,23);
  for (it = vectorInts.begin(); it != vectorInts.end(); ++it)
    std::cout<<*it<<" ";
  
  std::cout<<"\n\nAdding four elements to the vector\n";
  //assigning the value 3 to 4 elements of the vector
  vectorInts.assign(4,3);
  std::cout<<"vectorInts has "<<vectorInts.size()<<" elements\n";
  
  //printing the contents of vectorInts
  std::cout<<"VectorInts has these elements:\n";
  for (it = vectorInts.begin(); it != vectorInts.end(); ++it)
    std::cout<<*it<<" ";
 
  return 0;
}

输出：

vectorInts has 0 elements
Adding 23 to the vector
23 

Adding four elements to the vector
vectorInts has 4 elements
VectorInts has these elements:
3 3 3 3

8. push_back Elements

有时我们想全覆盖的形式，那可以用上面的assign，但是有时想从末尾追加的话：

vectorInts.push_back(24);
vectorInts.push_back(25);
vectorInts.push_back(26);
vectorInts.push_back(27);

修改上面的示例代码：

// constructing vectors
// constructing vectors
#include <iostream>
#include <vector>


int main ()
{
  //creating a vector of integers
  std::vector<int> vectorInts;  
  //creating an iterator for the vector
  std::vector<int>::iterator it;
  
  std::cout<<"vectorInts has "<<vectorInts.size()<<" elements\n";
  
  std::cout<<"Adding 23 to the vector\n";
  vectorInts.assign(1,23);
  for (it = vectorInts.begin(); it != vectorInts.end(); ++it)
    std::cout<<*it<<" ";
  

  std::cout<<"\n\nAdding four elements to the vector\n";
  //assigning the value 3 to 4 elements of the vector
  vectorInts.push_back(24);
  vectorInts.push_back(25);
  vectorInts.push_back(26);
  vectorInts.push_back(27);
  std::cout<<"vectorInts has "<<vectorInts.size()<<" elements\n";
  
  //printing the contents of vectorInts
  std::cout<<"VectorInts has these elements:\n";
  for (it = vectorInts.begin(); it != vectorInts.end(); ++it)
    std::cout<<*it<<" ";
 
  return 0;
}

vectorInts has 0 elements
Adding 23 to the vector
23 

Adding four elements to the vector
vectorInts has 5 elements
VectorInts has these elements:
23 24 25 26 27

9. insert elements

通过insert，可以在任意我们想插入的地方添加新的value，比如：

it  = vectorInts.begin() + 1;
vectorInts.insert(it, -1); // 在第一个元素后添加-1

比如：

// constructing vectors
// constructing vectors
#include <iostream>
#include <vector>


int main ()
{
  //creating a vector of integers
  std::vector<int> vectorInts;  
  //creating an iterator for the vector
  std::vector<int>::iterator it;

  vectorInts.push_back(0);
  vectorInts.push_back(1);
  vectorInts.push_back(2);
  vectorInts.push_back(3);
  vectorInts.push_back(4);
  vectorInts.push_back(5);
  vectorInts.push_back(6);
  vectorInts.push_back(7);
  
  std::cout<<"vectorInts has "<<vectorInts.size()<<" elements\n";
  
  //printing the contents of vectorInts
  std::cout<<"VectorInts has these elements:\n";
  for (it = vectorInts.begin(); it != vectorInts.end(); ++it)
    std::cout<<*it<<" ";
 
  //insert an element after the first element
  it  = vectorInts.begin() + 1;
  vectorInts.insert(it, -1);
  std::cout<<"\n\nAfter the insert\n";
for (it = vectorInts.begin(); it != vectorInts.end(); ++it)
    std::cout<<*it<<" ";
    
  //insert an element after the third element
  it  = vectorInts.begin();
  vectorInts.insert(it + 3, -2);
  std::cout<<"\n\nAfter the insert\n";
for (it = vectorInts.begin(); it != vectorInts.end(); ++it)
    std::cout<<*it<<" ";

  //insert an element after the third element
  it  = vectorInts.begin();
  vectorInts.insert(it + 5, -3);
  std::cout<<"\n\nAfter the insert\n";
for (it = vectorInts.begin(); it != vectorInts.end(); ++it)
    std::cout<<*it<<" ";
    
  return 0;
}

vectorInts has 8 elements
VectorInts has these elements:
0 1 2 3 4 5 6 7 

After the insert
0 -1 1 2 3 4 5 6 7 

After the insert
0 -1 1 -2 2 3 4 5 6 7 

After the insert
0 -1 1 -2 2 -3 3 4 5 6 7

10. emplace Elements

C++11中，针对顺序容器(如vector、deque、list)，新标准引入了三个新成员： emplace_front、emplace和emplace_back，这些操作构造而不是拷贝元素。这些操作分别对应push_front、insert和push_back，允许我们将元素放置在容器头部、一个指定位置之前或容器尾部。

当调用push或insert成员函数时，我们将元素类型的对象传递给它们，这些对象被拷贝到容器中。而当我们调用一个emplace成员函数时，则是将参数传递给元素类型的构造函数。

The difference makes emplace more efficient than insert in special cases.

  • insert: copies objects into the vector.
  • emplace: construct them inside of the vector.

如下代码：

struct Foo
{
  Foo(int n, double x);
};

std::vector<Foo> v;
v.emplace(someIterator, 42, 3.1416);
v.insert(someIterator, Foo(42, 3.1416));

11. Add Elements Prog. Quiz

In this programming quiz, use these member functions to complete the functions in the header file.

  • assign
  • push_back
  • insert
  • emplace
// constructing vectors
// constructing vectors
#include <iostream>
#include <vector>

using namespace std;

  void printVector(vector<int> vIn);
  void assignFunction(vector<int> vInts, int in);
  void pushBackFunction(vector<int> vInts, int in);
  void emplaceFunction(vector<int> vInts, int loc, int in);
  
 void printVector(vector<int> vIn)
 {//printing the contents of vIns
  //TODO: Finish the function
  vector<int>::iterator it;
  for (it=vIn.begin();it!=vIn.end();++it){
      cout << *it;
  }
 }
 
  void assignFunction(vector<int> vInts, int in)
  {
      cout<<"\nassigning "<<in<<" and printing the vector\n";
      //TODO: Finish the function
      int iSize = vInts.size();
      vInts.assign(iSize,in);
      
      printVector(vInts);
  }
  void pushBackFunction(vector<int> vInts, int in)
  {
      cout<<"\npush back "<<in<<" and printing the vector\n";
      //TODO: Finish the function
      vInts.push_back(in);
      printVector(vInts);
  }
  void emplaceFunction(vector<int> vInts,  int loc, int in)
  {
      vector<int>::iterator it;
      cout<<"\nemplacing "<<in<<" and printing the vector\n";
      //TODO: Finish the function
      it = vInts.begin() + loc;
      vInts.emplace(it,in);
      printVector(vInts);
  }

// Goal: pracitce constructing vectors
// and their iterators
// Call the vector vFloat
// Call the iterator it


#include "main.hpp"

int main ()
{
  vector<int> vInts; 
  vInts.assign(10, 5);
  printVector(vInts);
  assignFunction(vInts, 1);
  pushBackFunction(vInts, 2);
  emplaceFunction(vInts,1, 3);

  return 0;
}

输出：

5 5 5 5 5 5 5 5 5 5 
assigning 1 and printing the vector
1 1 1 1 1 1 1 1 1 1 
push back 2 and printing the vector
5 5 5 5 5 5 5 5 5 5 2 
emplacing 3 and printing the vector
5 3 5 5 5 5 5 5 5 5 5

上面的每一次输出，均不是基于前一次的结果，因为传递进入的值本身没有被改变。

13. clear Vector

We can remove all elements from the vector at the same time using the method ::clear.

// constructing vectors
// constructing vectors
#include <iostream>
#include <vector>


int main ()
{
  //creating a vector of integers
  std::vector<int> vectorInts;  
  //creating an iterator for the vector
  std::vector<int>::iterator it;

  vectorInts.push_back(0);
  vectorInts.push_back(1);
  vectorInts.push_back(2);
  vectorInts.push_back(3);
  vectorInts.push_back(4);
  vectorInts.push_back(5);
  vectorInts.push_back(6);
  vectorInts.push_back(7);
  
  std::cout<<"vectorInts has "<<vectorInts.size()<<" elements\n";
  
  //printing the contents of vectorInts
  std::cout<<"VectorInts has these elements:\n";
  for (it = vectorInts.begin(); it != vectorInts.end(); ++it)
    std::cout<<*it<<" ";
 
  //clear the vector
  vectorInts.clear();
  std::cout<<"\nAfter clear, the vector has these elements:\n";
  for (it = vectorInts.begin(); it != vectorInts.end(); ++it)
    std::cout<<*it<<" ";

  return 0;
}

vectorInts has 8 elements
VectorInts has these elements:
0 1 2 3 4 5 6 7 
After clear, the vector has these elements:

14. erase Vector

  1. delete a single element from a vector For example:
//erase the 5th element in the vector
vectorInts.erase(vectorInts.begin()+4);
  1. delete a range of elements form a vector For example:
//erase a range of elements in the vector
vectorInts.erase(vectorInts.begin()+1, vectorInts.begin()+3);

示例代码如下：

// constructing vectors
// constructing vectors
#include <iostream>
#include <vector>


int main ()
{
  //creating a vector of integers
  std::vector<int> vectorInts;  
  //creating an iterator for the vector
  std::vector<int>::iterator it;

  vectorInts.push_back(0);
  vectorInts.push_back(1);
  vectorInts.push_back(2);
  vectorInts.push_back(3);
  vectorInts.push_back(4);
  vectorInts.push_back(5);
  vectorInts.push_back(6);
  vectorInts.push_back(7);
  
  std::cout<<"vectorInts has "<<vectorInts.size()<<" elements\n";
  
  //printing the contents of vectorInts
  std::cout<<"VectorInts has these elements:\n";
  for (it = vectorInts.begin(); it != vectorInts.end(); ++it)
    std::cout<<*it<<" ";
 
  //clear the vector
  it = vectorInts.begin();
  vectorInts.erase(it + 4);
  std::cout<<"\nAfter clear, the vector has these elements:\n";
  for (it = vectorInts.begin(); it != vectorInts.end(); ++it)
    std::cout<<*it<<" ";

  return 0;
}

输出(删除了第四个之后的一个元素，即第五个元素)：

vectorInts has 8 elements
VectorInts has these elements:
0 1 2 3 4 5 6 7 
After clear, the vector has these elements:
0 1 2 3 5 6 7

15. pop_back

We can remove the last element of a vector using ::pop_back.

//pop the last element off the vector
vectorInts.pop_back();

pop_back DOES NOT return a value!，没有返回值。

// constructing vectors
// constructing vectors
#include <iostream>
#include <vector>


int main ()
{
  //creating a vector of integers
  std::vector<int> vectorInts;  
  //creating an iterator for the vector
  std::vector<int>::iterator it;

  vectorInts.push_back(0);
  vectorInts.push_back(1);
  vectorInts.push_back(2);
  vectorInts.push_back(3);
  vectorInts.push_back(4);
  vectorInts.push_back(5);
  vectorInts.push_back(6);
  vectorInts.push_back(7);
  
  std::cout<<"vectorInts has "<<vectorInts.size()<<" elements\n";
  
  //printing the contents of vectorInts
  std::cout<<"VectorInts has these elements:\n";
  for (it = vectorInts.begin(); it != vectorInts.end(); ++it)
    std::cout<<*it<<" ";
 
  //pop the last element off the vector
  vectorInts.pop_back();
  std::cout<<"\n\nAfter pop_back(), the vector has these elements:\n";
  for (it = vectorInts.begin(); it != vectorInts.end(); ++it)
    std::cout<<*it<<" ";

  //pop_back does not return the element that was removed. 
  //For example, the line of code below will return an error. 
  int a; 
  //a = vectorInts.pop_back(); //this line does not compile
  return 0;
}

vectorInts has 8 elements
VectorInts has these elements:
0 1 2 3 4 5 6 7 

After pop_back(), the vector has these elements:
0 1 2 3 4 5 6