Rotate the complete array using rotate function

Question

A array contains element 10,20,30,40,50 What i wanna rotate the complete array so as it will cout will stream elements 50,40,30,20,10

I want to solve this problem using rotate function

i tried to write rotate(arr,arr+4,arr+1);

  #include<iostream>
  #include<algorithm>
  using namespace std;
  int main()
  {
      int arr[]={10,20,30,40,50};

      rotate(arr,arr+4,arr+1);
      int i;

      for(i=0; i<5; ++i)
      {
       cout<<arr[i]<<"  ";
      }
   }

by running above program i getting output 50 10 20 30 40 which is wrong the actual output is 50 40 30 20 10

You do not want to rotate (this is done correctly, but not what you wanted), you want to mirror/reverse the array. — MrSmith42, Aug 02 '19 at 08:29

score 3 · Accepted Answer · answered Aug 02 '19 at 08:42

3

The issue is that you picked the wrong algorithm (quotes from https://en.cppreference.com/w/, emphasis mine):

Specifically, std::rotate swaps the elements in the range [first, last) in such a way that the element n_first becomes the first element of the new range and n_first - 1 becomes the last element.

What you need is std::reverse(first, last), which

Reverses the order of the elements in the range [first, last) Behaves as if applying std::iter_swap to every pair of iterators first+i, (last-i) - 1 for each non-negative i < (last-first)/2

answered Aug 02 '19 at 08:42

Bob__

12,361
3
28
42

thank u so much Bob_ for replying i already referred https://en.cppreference.com/w/cpp/header in which i found using vector.h file Can i write function without vector type – aman_49 Aug 02 '19 at 09:16
@AlexStacy Not sure to understand your comment, but you can use either `std::reverse(std::begin(arr), std::end(arr));` or `std::reverse(arr, arr + 5);` with your array. – Bob__ Aug 02 '19 at 09:35

Amit G. · Answer 2 · 2019-12-24T03:39:56.903

1

std::rotate: "Rotates the order of the elements in the range (first,last), in such a way that the element pointed by middle becomes the new first element."

Your std::rotate middle points to "arr+4" i.e. the 5th element: 50.

You expect "50 40 30 20 10" i.e. to reverse the array; not to rotate it. So, you should use std::reverse:

#include <array>
#include <algorithm>
#include <iostream>

int main()
{
    std::array<int, 5> arr { 10,20,30,40,50 };

    std::reverse(arr.begin(), arr.end());

    for (auto i : arr)
        std::cout << i << "  ";

    return 0;
}

edited Dec 24 '19 at 03:39

answered Aug 02 '19 at 08:44

Amit G.

2,546
2
22
30

can u please tell me why did u take std::array arr{10,20,30,40,50,}; instead of vector in more details – aman_49 Aug 02 '19 at 09:30
@AlexStacy From [std::array](https://en.cppreference.com/w/cpp/container/array) reference: "std::array is a container that encapsulates **fixed size arrays**. ... The struct combines the performance and accessibility of a C-style array with the benefits of a standard container, such as knowing its own size, supporting assignment, random access iterators, etc." – Amit G. Aug 02 '19 at 09:45
can u please tell me difference between std::array and std::vector – aman_49 Aug 02 '19 at 09:55
@AlexStacy , you can read [here](https://stackoverflow.com/questions/4424579/stdvector-versus-stdarray-in-c). – Amit G. Aug 02 '19 at 10:02

score 0 · Answer 3 · answered Aug 02 '19 at 08:56

This is not a good idea, but I tried to solve this with using rotate()...

#include <iostream>
#include <algorithm>
#include <vector>

using namespace std;
int main()
{
    vector<int> v{10,20,30,40,50};
    int i;

    for(i=0; i<5; ++i)
    {
       cout<<v[i]<<"  ";
    }
    cout << endl ;

     for(i=0; i<5; ++i)
     {
        rotate(v.rbegin(),v.rbegin()+1,v.rend());
        cout<<v[0]<<"  ";
     }
     cout << endl ;
}

g++ rotatev.cpp --std=c++11

./a.out
10  20  30  40  50  
50  40  30  20  10

Rotate the complete array using rotate function

3 Answers3