adamant's blog

By adamant, history, 31 hour(s) ago, In English

Hi everyone!

As Codeforces now supports C++23, it seems to be the right time to discuss some of the particularly interesting features.

Some noteworthy ones that were already mentioned elsewhere include:

  • views::zip that maps two ranges into pairs (A[i], B[i]).
  • views::enumerate that maps range into pairs (i, a[i]).
  • views::adjacent<k> that maps range into tuples (a[i], ..., a[i+k-1]).
  • views::cartesian_product that maps two ranges into pairs (A[i], B[j]) with all possible i and j.
  • Some more specialized views.
  • ranges::to<Container> that creates a container out of a view, e.g. to<vector>(views::iota(0, n)).
  • ranges::fold_left and ranges::fold_right, range versions of std::accumulate/std::reduce.
  • insert_range/append_range/prepend_range/assign_range for containers (not in GCC yet).
  • print/println for formatted printing (it seems that standard formatters for ranges are not in GCC yet).

But there are also two features that weren't covered in as much detail as they deserve, deducing this and generators.

Deducing this and recursive lambdas

Assume that you want to write a recursive lambda. What are your options? Naturally, you'd try something like this:

    auto fact = [&](int x) {
        return x ? x * fact(x - 1) : 1;
    };

You will not be allowed to do it, because inside the lambda, you use fact before its type is deduced. One way to circumvent it is to write function<int(int)> fact = ... instead. While it is attractive, it makes the calls to fact more expensive, and in certain cases might even lead you to TLE, e.g. if you try to use this for depth-first traversal of a big graph.

Until C++23, the best practice was to do something like this:

    auto fact = [&](auto &&self, int x) -> int {
        return x ? x * self(self, x - 1) : 1;
    };
    cout << fact(fact, n) << endl;

While much more efficient on practice, it looks very ugly. And C++23 helps us, allowing to do this instead:

    auto fact = [&](this auto fact, int x) -> int {
        return x ? x * fact(x - 1) : 1;
    };
    cout << fact(n) << endl;

Just as if it was a proper recursive function!

std::generator

Another interesting feature that I haven't seen mentioned in competitive programming discussions at all are coroutines. Assume that you need to factorize a number. If you depend on Pollard's rho algorithm, your flow probably looks as follows:

    vector<int> factors;
    void factorize(uint64_t m) {
        if(is_prime(m)) {
            factors.push_back(m);
        } else if(m > 1) {
            auto g = proper_divisor(m);
            factorize(g);
            factorize(m / g);
        }
    }

And it's always annoying that to store the result, you have to either keep a global vector, or take output vector as an argument by reference (and pass it around each time). Ideally you'd want to return a vector, but then you might be afraid of accidentally getting a quadratic runtime, and even besides that you'd need to write extra code to merge the results. Coroutines allow us to rewrite it as follows:

    std::generator<uint64_t> factorize(uint64_t m) {
        if(is_prime(m)) {
            co_yield m;
        } else if(m > 1) {
            auto g = proper_divisor(m);
            co_yield std::ranges::elements_of(factorize(g));
            co_yield std::ranges::elements_of(factorize(m / g));
        }
    }
    
    for(int p: factorize(m)) {
        ...
    }

In this manner, factorize will return a generator, which is basically a view wrapper to the function above which generates consecutive elements on the range "on the fly", while also suspending execution of the function between accesses to the resulting range. This way, we avoid the need to store the results of the recursive function somewhere, as well as the need to incorporate external logic or callbacks into the function if we want to do something as soon as you get the next element.

From performance perspective, of course, coroutines may be slightly inferior to having a global vector to store the results. For example, this submission to finding strongly connected components takes 278ms and 108 MB of memory, while its global vector version only needs 229ms and 65 MB. Still I think coroutines are pretty nice concept to keep in mind and might simplify code or make it better structured in a lot of cases.

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28 hours ago, # |
Rev. 2   Vote: I like it +15 Vote: I do not like it

It smells like Python.

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    26 hours ago, # ^ |
      Vote: I like it 0 Vote: I do not like it

    Absolutely, and I think Python's generator is even easier to use : /

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27 hours ago, # |
  Vote: I like it +10 Vote: I do not like it

is there a way to make std::print not flush?

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27 hours ago, # |
  Vote: I like it +3 Vote: I do not like it

If you do

using namespace std;
using namespace std::ranges;
using namespace std::ranges::views;

then some stuff will become available from multiple namespaces, and c++ gives an ambigiuity error. Such as with sort that's available as std::sort or std::ranges::sort. Same with iota and a lot of other things.

If you don't want to type ranges:: and views:: everytime then use this trick:

#include <bits/stdc++.h>

using namespace std;

namespace std::ranges::views {
    auto solve() {
        return iota(1, 5) | transform([](int a) { return a + 10; });
    }
};

int main() {
    for (int i : std::ranges::views::solve()) cout << i << " ";
    cout << endl;
}

This way, in case of ambiguities, the compiler will resolve to views and ranges library.

I learned this trick from someone's submission, idr.

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    27 hours ago, # ^ |
      Vote: I like it +3 Vote: I do not like it

    hey arent you that guy on the monkeytype discord

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    23 hours ago, # ^ |
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    Modifying the namespace std is strictly undefined behavior in C++. This is a bad idea.

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      17 hours ago, # ^ |
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      I mean the compiler isn't going to sue us for that.

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        17 hours ago, # ^ |
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        The compiler is legally allowed to do anything in undefined behavior, so this includes suing us...

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20 hours ago, # |
  Vote: I like it +1 Vote: I do not like it

Can someone explain why using function<int(int)> fact = ... is more expensive?

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    17 hours ago, # ^ |
    Rev. 3   Vote: I like it +6 Vote: I do not like it

    std::function is a complex object that can store lambdas, function pointers and functors. if you store a lambda, it allocates on heap (significant performance loss). also, the compiler (and cpu (?)) can't reason which function you are going to call (similar performance loss as to function pointers).

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      17 hours ago, # ^ |
        Vote: I like it 0 Vote: I do not like it

      Thanks for explaining it. I just tested and found that std::function can be 3x slower than auto&& for just a fibonacci lambda (even slower for more complex ones).

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14 hours ago, # |
  Vote: I like it +6 Vote: I do not like it

And it's always annoying that to store the result, you have to either keep a global vector, or take output vector as an argument by reference

Pardon?

vector<int> factorize(uint64_t m) {
    vector<int> factors;
    auto inner = [&](this auto inner, uint64_t m) -> void {
        if(is_prime(m)) {
            factors.push_back(m);
        } else if(m > 1) {
            auto g = proper_divisor(m);
            inner(g);
            inner(m / g);
        }
    } 
    inner(m);
    return factors;
} 
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    14 hours ago, # ^ |
    Rev. 3   Vote: I like it 0 Vote: I do not like it

    Well, "global" in a sense that it should be defined outside of main function's scope and managed externally (or outside the lambda like in your example). I suppose one other way to circumvent it is for function to take a callback as an argument and invoke it with returned values each time this is needed.

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13 hours ago, # |
Rev. 3   Vote: I like it 0 Vote: I do not like it

Ok, fine... I replaced all the recursive function<>s with auto &&self monstrosity in my prewritten library. It was a sad decision to make.

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    12 hours ago, # ^ |
    Rev. 2   Vote: I like it 0 Vote: I do not like it

    If you're fine using C++23, deducing this should be a much nicer alternative :)

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      12 hours ago, # ^ |
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      But c++23 does not exist everywhere yet, so I am not ready to put it in my templates