C++20: Functional Patterns with the Ranges Library

17 February 2020

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My last post C++20: The Ranges Library, gave you the first impression of the ranges library. Today's post is about functional patterns: function composition and lazy evaluation. They become first-class citizens in C++20.

TimelineCpp20

The algorithms of the Standard Template Library (STL) are sometimes a little inconvenient. They need a begin and an end iterator. This is often more than you want to write.

#include <numeric>
#include <iostream>
#include <vector>

int main() {
    
    std::vector<int> myVec{1, 2, 3, 4, 5, 6, 7, 8, 9};
    auto res = std::accumulate(std::begin(myVec), std::end(myVec), 0); 
    // std::accumulate(myVec, 0);                                      // (1)
    std::cout << res << std::endl;      //  45 
    
}

Would it be nice if std::accumulate could be executed directly on the container such as in line (2)?

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Direct on the Container

The following program creates direct views on the keys (line 1) and the values (line 3) of the std::unordered_map.

// rangesEntireContainer.cpp

#include <iostream>
#include <ranges>
#include <string>
#include <unordered_map>


int main() {
    
    std::unordered_map<std::string, int> freqWord{ {"witch", 25}, {"wizard", 33}, {"tale", 45},
                                                   {"dog", 4}, {"cat", 34}, {"fish", 23} };
    
    std::cout << "Keys" << std::endl;
    auto names = std::views::keys(freqWord);                                            // (1)
    for (const auto& name : names){ std::cout << name << " "; };
    std::cout << std::endl;
    for (const auto& name : std::views::keys(freqWord)){ std::cout << name << " "; };   // (2)
    
    std::cout << "\n\n";
    
    std::cout << "Values: " << std::endl;
    auto values = std::views::values(freqWord);                                          // (3)
    for (const auto& value : values){ std::cout << value << " "; };
    std::cout << std::endl;
    for (const auto& value : std::views::values(freqWord)){ std::cout << value << " "; } // (4)
                           
}

Of course, the keys and values can be displayed directly (line 2 and line 4). The output is identical.

rangesEntireContainer

Working directly on the container is not revolutionary, but function composition and lazy evaluation are.

Function Composition

In my next example, I use std::map, because the ordering of the keys is crucial.

// rangesComposition.cpp

#include <iostream>
#include <ranges>
#include <string>
#include <map>


int main() {
    
    std::map<std::string, int> freqWord{ {"witch", 25}, {"wizard", 33}, {"tale", 45},
                                                   {"dog", 4}, {"cat", 34}, {"fish", 23} };
                                                
    std::cout << "All words: ";                     // (1)
    for (const auto& name : std::views::keys(freqWord)) { std::cout << name << " "; };                                               
                                                   
    std::cout << std::endl;
    
    std::cout << "All words reverse: ";             // (2)
    for (const auto& name : std::views::keys(freqWord) | std::views::reverse) { std::cout << name << " "; };  
     
    std::cout << std::endl;
    
    std::cout << "The first 4 words: ";             // (3)
    for (const auto& name : std::views::keys(freqWord) | std::views::take(4)) { std::cout << name << " "; }; 
    
    std::cout << std::endl;
                                
    std::cout << "All words starting with w: ";     // (4)
    auto firstw = [](const std::string& name){ return name[0] == 'w'; };
    for (const auto& name : std::views::keys(freqWord) | std::views::filter(firstw)) { std::cout << name << " "; };
    
    std::cout << std::endl;
                                                     // (5)
    auto lengthOf = [](const std::string& name){ return name.size(); };
    auto res = ranges::accumulate(std::views::keys(freqWord) | std::views::transform(lengthOf), 0);
    std::cout << "Sum of words: " << res << std::endl;
   
}

In this case, I'm only interested in the keys. I display all of them (line 1), all of them reversed (line 2), the first four (line 3), and the keys starting with the letter 'w' (line 4). The line (5) sums up all the lengths of all words.

The pipe symbol | is syntactic sugar for function composition. Instead of C(R) you can write R | C. Consequentially, the next two lines are equivalent.

auto rev2 = std::views::reverse(ranges::views::keys(freqWord));
auto rev = std::views::keys(freqWord) | ranges::views::reverse;

Finally, here is the output of the program. rangesComposition

Lazy Evaluation

I use in my example std::views::iota. This function is a range factory for creating a sequence of elements by successively incrementing an initial value. This sequence can be finite or infinite. The following example fills a std::vector with 10 int's, starting with 0.

// rangesIota.cpp

#include <iostream>
#include <numeric>
#include <ranges>
#include <vector>

int main() {
    
    std::cout << std::boolalpha;
    
    std::vector<int> vec;
    std::vector<int> vec2;
    
    for (int i: std::views::iota(0, 10)) vec.push_back(i);                      // (1)         
         
    for (int i: std::views::iota(0) | std::views::take(10)) vec2.push_back(i);  // (2)
    
    std::cout << "vec == vec2: " << (vec == vec2) << '\n'; // true
    
    for (int i: vec) std::cout << i << " ";                // 0 1 2 3 4 5 6 7 8 9
                           
}

The first iota call (line 1) creates all numbers from 0 to 9, incremented by 1. The second iota call (line 2) creates an infinite data stream, starting with 0 and incremented by 1. std::views::iota(0) is lazy. I only get a new value if I ask for it. I ask for it ten times. Consequentially, both vectors are identical.

Now, I want to solve a small challenge. I want to find the first 20 prime numbers, starting with 1000000.

// rangesLazy.cpp

#include <iostream>
#include <ranges>


bool isPrime(int i) {
    for (int j=2; j*j <= i; ++j){
        if (i % j == 0) return false;
    }
    return true;
}

int main() {
                                        // (1)
    std::cout << "Numbers from 1000000 to 1001000 (dispayed each 100th): " << std::endl;
    for (int i: std::views::iota(1000000, 1001000)) {
        if (i % 100 == 0) std::cout << i << " ";  
    }
    
    std::cout << "\n\n";
                                        // (2)
    auto odd = [](int i){ return i % 2 == 1; };
    std::cout << "Odd numbers from 1000000 to 1001000 (displayed each 100th): " << std::endl;
    for (int i: std::views::iota(1000000, 1001000) | std::views::filter(odd)) {
         if (i % 100 == 1) std::cout << i << " ";  
    }
    
    std::cout << "\n\n";
                                         // (3)
    std::cout << "Prime numbers from 1000000 to 1001000: " << std::endl;
    for (int i: std::views::iota(1000000, 1001000) | std::views::filter(odd) 
                                                   | std::views::filter(isPrime)) {
        std::cout << i << " ";  
    }
    
    std::cout << "\n\n";
                                         // (4)
    std::cout << "20 prime numbers starting with 1000000: " << std::endl;
    for (int i: std::views::iota(1000000) | std::views::filter(odd) 
                                          | std::views::filter(isPrime) 
                                          | std::views::take(20)) {
        std::cout << i << " ";  
    }

}

Here is my iterative strategy:

Of course, I don't know when I have 20 primes greater than 1000000. To be on the safe side, I create 1000 numbers. For obvious reasons, I displayed only each 100th.
I'm only interested in the odd numbers; therefore, I remove the even numbers.
Now, it's time to apply the next filter. The predicate isPrime returns if a number is a prime. As you can see in the following screenshot, I was too eager. I got 75 primes.
Laziness is a virtue. I use std::iota as an infinite number factory, starting with 1000000 and ask precisely for 20 primes.

rangesLazy

What's next?

Python has many very nice functions such as range, map, filter, reduce, and zip. And of course, there is also the slice operator and list comprehension. All of these functions are eager in Python 2, but map, filter, and reduce are lazy in Python 3. I try an experiment in my next post. Which of these functions can I implement with similar functionality in C++ by using the ranges library?

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