Module: Dynamic arrays: vector


Problem

2/8

Iterator in vector

Theory Click to read/hide

Iterators
One way to iterate over elements in containers (data structures) is the iterator (iterator), which come in several types.
 
Iterator - data structure that « indicates» on some container element, and (for some containers) can navigate to the previous/next element.

The vector uses the most powerful - random-access iterator(random-access Iterator).  A random access iterator can access an arbitrary element of the vector in addition to sequential movement.

The Benefits of Iterators
1)  When deleting elements and iterating over elements using indices ([]), we need to keep track of the number of remaining elements all the time so as not to go beyond the vector, and using an iterator, you can use end()< /code> indicating the end of the vector.
2) Using an iterator, you can easily dynamically remove and insert elements in a vector. 
 
Iterator declaration
1) Declaring an iterator for an integer vector and pointing it to the first element in the vector. vector <int> myvector = { 1, 2, 3, 4, 5 }; vector <int>::iterator it = myvector.begin();
2) Declaring an iterator for an integer vector and pointing it to the element after the last in the vector. vector <int> myvector = { 1, 2, 3, 4, 5 }; vector <int>::iterator it = myvector.end(); // points to the element after the last one, vector <int>::iterator it1 = myvector.end() - 1 ; // pointing to the last element.  
Getting and displaying a value
Getting and displaying the element pointed to by the iterator. cout << *it;  
Move iterator position
Move the iterator position 3 positions forward. 
advance(it, 3);
 
Creating a new iterator from an existing one
Create a new iterator based on an existing one, advancing 3 positions. auto it1 = next(it, 3);  
Displaying vector values ​​using an iterator
vector<int>::iterator it; for (it = myvector.begin(); it != myvector.end(); ++it) {   cout<<*it<<" ";  }
Vector traversal
To traverse the vector from the last element to the first, a reverse iterator reverse_iterator is used, it is obtained by:
1) rbegin() -  returns a reverse iterator pointing to the last element of the vector, the application of the ++ operation leads to the transition to the previous element;
2) rend() -  returns a reverse iterator pointing to the previous element of the vector, the application of the ++ operation leads to the transition  to the next. vector<int>::reverse_iterator it = myvector.rbegin();  // points to the last element vector<int>::reverse_iterator it = myvector.rend();    // points to an element,   // which comes before the first one,

Problem

You are given a sequence of integers.  Write a program that reverses a vector using reverse_iterator.

Input
First given number N - the number of elements in the sequence (1<= N <= 100). Then N numbers are written separated by a space.
 
Output
Output all the elements of the resulting vector in one line, separating them with spaces.
 
Example
# Input Output
1 5
1 2 3 4 5 		5 4 3 2 1

time 1000 ms
memory 256 Mb
Rules for program design and list of errors in automatic problem checking

Teacher commentary

Insert the missing code snippets
C++
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#include <iostream>
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#include <vector>
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using namespace std;
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int main()
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{
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	int n;
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	vector<int> myvector;
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       cin>>n;
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       for(int i=0;i<n;i++)
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         {
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          int a;
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          cin>>a;
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         myvector.push_back(a);
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         }
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    vector<int>::reverse_iterator it;
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	 {
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         cout<<*it<<" ";
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     }
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 }

     

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