Kemal's blog

Range-based for loop in C++
By Kemal, history, 9 years ago, In English

Hey there!

I think everyone know what does this piece of code do:

vector <int> v = {1, 2, 3, 4};
for (int x : v)
  cout << x << " ";

prints: 1 2 3 4

Is there any way to do this in reverse order ?!

  • Vote: I like it
  • +8
  • Vote: I do not like it

Comments Comments (20)
| Write comment?
»
9 years ago, # |
  Vote: I like it +9 Vote: I do not like it

http://ideone.com/AS8l6J

→ Reply
»
9 years ago, # |
  Vote: I like it 0 Vote: I do not like it 
template<class T>
struct reversed_iter {
  T & container;
  
  reversed_iter(T & container): container(container) {}
  
  auto begin() -> decltype(container.rbegin()) {
    return container.rbegin();
  }
  
  auto end() -> decltype(container.rend()) {
    return container.rend();
  }
};

template<class T>
reversed_iter<T> make_reversed(T & container) {
  return reversed_iter<T>(container); 
}

int main() {  
  vector <int> v = {1, 2, 3, 4};
  for (auto x : make_reversed(v))
    cout << x << " ";
    
  // or just
  for (auto x = v.rbegin(); x != v.rend(); ++x)
    cout << *x << " ";
}
→ Reply
  • »
    »
    9 years ago, # ^ |
    ← Rev. 2   Vote: I like it +4 Vote: I do not like it

    to Bassel and MrDindows

    The main point of this C++ 11 feature is not to write long code, even the traditional way is better than those options

    for (int i=v.size()-1;i>=0;i--)
  cout << v[i] << " ";
    → Reply
    • »
      »
      »
      9 years ago, # ^ |
        Vote: I like it 0 Vote: I do not like it

      What advantages does the method using a struct and template have over the simple method kingofnumbers provided?

      → Reply
      • »
        »
        »
        »
        9 years ago, # ^ |
          Vote: I like it 0 Vote: I do not like it

        I don't think they have any advantages, they are long code and even slower.

        however, Kemal asked for even shorter method using C++ 11, I don't know it,though. I was just saying that even the way I wrote is better than those struct and template ways

        → Reply
    • »
      »
      »
      9 years ago, # ^ |
        Vote: I like it +9 Vote: I do not like it

      * int(v.size()) - 1 :)

      → Reply
      • »
        »
        »
        »
        5 years ago, # ^ |
          Vote: I like it -8 Vote: I do not like it

        I got TLE once :) thanks for reminding me :)

        → Reply
    • »
      »
      »
      9 years ago, # ^ |
        Vote: I like it +11 Vote: I do not like it

      That's a great way to illustrate the usefulness of reverse adapter. The bug you just made couldn't be possible in the original code. (Not to mention that the original one is shorter, clearer and works with any container.)

      → Reply
      • »
        »
        »
        »
        9 years ago, # ^ |
          Vote: I like it 0 Vote: I do not like it

        can you tell which bug are you talking about? I've just tested my code and it's working even if the vector is empty.

        and I will repeat my point, I think that Kemal actually requested something short and practical to use during the contest. for me I don't prefer to copy/paste that template every time I want to traverse a container in reversed order.

        → Reply
        • »
          »
          »
          »
          »
          9 years ago, # ^ |
            Vote: I like it 0 Vote: I do not like it

          Yeah, you're right, my mistake :) The other points still stand though:

          1. Reverse adapter is cleaner, shorter and less error prone.
          2. I don't see the words "short" or "programming competition" in the OP.
          3. Even if you do use it in a programming competition, there's no need to copy/paste anything ;)
          → Reply
        • »
          »
          »
          »
          »
          9 years ago, # ^ |
          ← Rev. 2   Vote: I like it +8 Vote: I do not like it

          Casting UINT_MAX from unsigned to signed is implementation-defined, so it could possibly be a bug in some machine / compiler. For programming contests this is a moot point because you know the environment you're dealing with.

          → Reply
          • »
            »
            »
            »
            »
            »
            9 years ago, # ^ |
              Vote: I like it 0 Vote: I do not like it

            but in this case it isn't casting -1, it is unsigned zero subtract 1, and it equals to max value of unsigned(4294967295). Then it casts to int and produces undefined (not unspecified) behavior

            → Reply
  • »
    »
    9 years ago, # ^ |
      Vote: I like it +11 Vote: I do not like it

    One problem here that it won't work with temporaries, e.g

    for (auto x: make_reveresed(get_vector()){
  cout << x;
}
    → Reply
  • »
    »
    9 years ago, # ^ |
      Vote: I like it -26 Vote: I do not like it

    Please, don't invent bicycle. It's already in Boost.

    #include <iostream>
#include <vector>
#include <boost/range/adaptor/reversed.hpp>
using namespace std;

int main()
{
    vector<int> v = {1, 2, 3, 4};
    for (int x : boost::adaptors::reverse(v)) {
        cout << x << " ";
    }
}
    → Reply
»
9 years ago, # |
  Vote: I like it +29 Vote: I do not like it

Such simple!

Array [1 2 3 4] will be printed in reverse order.

vector <int> v = {4, 3, 2, 1};
for (int x : v)
  cout << x << " ";
→ Reply
»
9 years ago, # |
  Vote: I like it +15 Vote: I do not like it

There was a post: http://mirror.codeforces.com/blog/entry/10124 where using templates you can write in C++ like this:

for (auto i: range(1000))
    sum += i;
or
for (auto i: range(10, 20))
    cout << i << endl;
→ Reply
»
9 years ago, # |
← Rev. 2   Vote: I like it -10 Vote: I do not like it 

You can use reverse function for vector.


vector <int> v = {1, 2, 3, 4};

reverse(v.begin(), v.end());

for(int i : v) {

      cout << i << " ";

}
→ Reply
»
18 months ago, # |
  Vote: I like it +15 Vote: I do not like it

Using C++20:

#include <ranges>

vector v = {1, 2, 3, 4};
for (int x: v | views::reverse) 
  cout << x << ' ';

// or
for (int x: views::iota(1, 5) | views::reverse) 
  cout << x << ' ';
→ Reply
  • »
    »
    18 months ago, # ^ |
      Vote: I like it 0 Vote: I do not like it

    Any effect on time complete?

    → Reply
    • »
      »
      »
      18 months ago, # ^ |
        Vote: I like it +3 Vote: I do not like it

      I did a few experiments running a loop for $$$10^9$$$ iterations, looks like performance depends on compiler optimizations. If no optimizations used at all, then ranges perform quite slow, however if you turn on some optimizations, then the difference with regular loop becomes really insignificant.

      No optimizations (-O0):

      for loop regular: 2023ms
for loop views::iota: 6070ms
for loop views::iota | views::reverse: 18875ms

      Using -O2 optimizations:

      for loop regular: 449ms
for loop views::iota: 451ms
for loop views::iota | views::reverse: 468ms

      Using -O3 optimizations:

      for loop regular: 247ms
for loop views::iota: 246ms
for loop views::iota | views::reverse: 299ms
      → Reply