range-based for loop add-ons inspired by the Python builtins and itertools library. Like itertools and the Python3 builtins, this library uses lazy evaluation wherever possible.

Note: Everthing is inside the iter namespace.

This library is header-only and relies only on the C++ standard library. The only exception is zip_longest which uses boost::optional. #include <cppitertools/itertools.hpp> will include all of the provided tools except for zip_longest which must be included separately. You may also include individual pieces with the relevant header (#include <cppitertools/enumerate.hpp> for example).

range
enumerate
zip
zip_longest
imap
filter
filterfalse
unique_everseen
unique_justseen
takewhile
dropwhile
cycle
repeat
count
groupby
accumulate
compress
sorted
chain
chain.from_iterable
reversed
slice
sliding_window
grouper

product
combinations
combinations_with_replacement
permutations
powerset

Uses an underlying iterator to acheive the same effect of the python range function. range can be used in three different ways:

Only the stopping point is provided. Prints 0 1 2 3 4 5 6 7 8 9

for (auto i : range(10)) {
    cout << i << '\n';
}

The start and stop are both provided. Prints 10 11 12 13 14

for (auto i : range(10, 15)) {
    cout << i << '\n';
}

The start, stop, and step are all provided. Prints 20 22 24 26 28

for (auto i : range(20, 30, 2)) {
    cout << i << '\n';
}

Negative values are allowed as well. Prints 2 1 0 -1 -2

for (auto i : range(2, -3, -1)) {
    cout << i << '\n';
}

In addition to normal integer range operations, doubles and other numeric types are supported through the template

Prints: 5.0 5.5 6.0 ... 9.5

for(auto i : range(5.0, 10.0, 0.5)) {
    cout << i << '\n';
}

Can be used with any class with an iterator. Continually "yields" containers similar to pairs. They are basic structs with a .index and a .element. Usage appears as:

vector<int> vec{2, 4, 6, 8};
for (auto&& e : enumerate(vec)) {
    cout << e.index
         << ": "
         << e.element
         << '\n';
}

Called as filter(predicate, iterable). The predicate can be any callable. filter will only yield values that are true under the predicate.

Prints values greater than 4: 5 6 7 8

vector<int> vec{1, 5, 4, 0, 6, 7, 3, 0, 2, 8, 3, 2, 1};
for (auto&& i : filter([] (int i) { return i > 4; }, vec)) {
    cout << i <<'\n';
}

If no predicate is passed, the elements themselves are tested for truth

Prints only non-zero values.

for(auto&& i : filter(vec)) {
    cout << i << '\n';
}

Similar to filter, but only prints values that are false under the predicate.

Prints values not greater than 4: 1 4 3 2 3 2 1

vector<int> vec{1, 5, 4, 0, 6, 7, 3, 0, 2, 8, 3, 2, 1};
for (auto&& i : filterfalse([] (int i) { return i > 4; }, vec)) {
    cout << i <<'\n';
}

If no predicate is passed, the elements themselves are tested for truth.

Prints only zero values.

for(auto&& i : filterfalse(vec)) {
    cout << i << '\n';
}

This is a filter adaptor that only generates values that have never been seen before. For this to work your object must be specialized for std::hash.

Prints 1 2 3 4 5 6 7 8 9

vector<int> v {1,2,3,4,3,2,1,5,6,7,7,8,9,8,9,6};
for (auto&& i : unique_everseen(v)) {
    cout << i << ' ';
}

Another filter adaptor that only omits consecutive duplicates.

Prints 1 2 3 4 3 2 1 Example Usage:

vector<int> v {1,1,1,2,2,3,3,3,4,3,2,1,1,1};
for (auto&& i : unique_justseen(v)) {
    cout << i << ' ';
}

Yields elements from an iterable until the first element that is false under the predicate is encountered.

Prints 1 2 3 4. (5 is false under the predicate)

vector<int> ivec{1, 2, 3, 4, 5, 6, 7, 6, 5, 4, 3, 2, 1};
for (auto&& i : takewhile([] (int i) {return i < 5;}, ivec)) {
    cout << i << '\n';
}

Yields all elements after and including the first element that is true under the predicate.

Prints 5 6 7 1 2

vector<int> ivec{1, 2, 3, 4, 5, 6, 7, 1, 2};
for (auto&& i : dropwhile([] (int i) {return i < 5;}, ivec)) {
    cout << i << '\n';
}

Repeatedly produces all values of an iterable. The loop will be infinite, so a break or other control flow structure is necessary to exit.

Prints 1 2 3 repeatedly until some_condition is true

vector<int> vec{1, 2, 3};
for (auto&& i : cycle(vec)) {
    cout << i << '\n';
    if (some_condition) {
        break;
    }
}

Repeatedly produces a single argument forever, or a given number of times. repeat will bind a reference when passed an lvalue and move when given an rvalue. It will then yield a reference to the same item until completion.

The below prints 1 five times.

for (auto&& e : repeat(1, 5)) {
    cout << e << '\n';
}

The below prints 2 forever

for (auto&& e : repeat(2)) {
    cout << e << '\n';
}

Effectively a range without a stopping point.
count() with no arguments will start counting from 0 with a positive step of 1.
count(i) will start counting from i with a positive step of 1.
count(i, st) will start counting from i with a step of st.

Technical limitations: Unlike Python which can use its long integer types when needed, count() will eventually exceed the maximum possible value for its type (or minimum with a negative step). When using a signed type it is up to the API user to ensure this does not happen. If the limit is exceeded for signed types, the result is undefined (as per the C++ standard).

The below will print 0 1 2 ... etc

for (auto&& i : count()) {
    cout << i << '\n';
}

Separate an iterable into groups sharing a common key. The following example creates a new group whenever a string of a different length is encountered.

vector<string> vec = {
    "hi", "ab", "ho",
    "abc", "def",
    "abcde", "efghi"
};

for (auto&& gb : groupby(vec, [] (const string &s) {return s.length(); })) {
    cout << "key: " << gb.first << '\n';
    cout << "content: ";
    for (auto&& s : gb.second) {
        cout << s << "  ";
    }
    cout << '\n';
}

Note: Just like Python's itertools.groupby, this doesn't do any sorting. It just iterates through, making a new group each time there is a key change. Thus, if the group is unsorted, the same key may appear multiple times.

Differs from std::accumulate (which in my humble opinion should be named std::reduce or std::foldl). It is similar to a functional reduce where one can see all of the intermediate results. By default, it keeps a running sum. Prints: 1 3 6 10 15

for (auto&& i : accumulate(range(1, 6))) {
    cout << i << '\n';
}

A second, optional argument may provide an alternative binary function to compute results. The following example multiplies the numbers, rather than adding them. Prints: 1 2 6 24 120

for (auto&& i : accumulate(range(1, 6), std::multiplies<int>{})) {
    cout << i << '\n';
}

Note: The intermediate result type must support default construction and assignment.

Takes an arbitrary number of ranges of different types and efficiently iterates over them in parallel (so an iterator to each container is incremented simultaneously). When you dereference an iterator to "zipped" range you get a tuple of the elements the iterators were holding.

Example usage:

array<int,4> i{{1,2,3,4}};
vector<float> f{1.2,1.4,12.3,4.5,9.9};
vector<string> s{"i","like","apples","alot","dude"};
array<double,5> d{{1.2,1.2,1.2,1.2,1.2}};

for (auto&& e : zip(i,f,s,d)) {
    cout << std::get<0>(e) << ' '
         << std::get<1>(e) << ' '
         << std::get<2>(e) << ' '
         << std::get<3>(e) << '\n';
    std::get<1>(e)=2.2f; // modifies the underlying 'f' array
}

Terminates on the longest sequence instead of the shortest. Repeatedly yields a tuple of boost::optional<T>s where T is the type yielded by the sequences' respective iterators. Because of its boost dependency, zip_longest is not in itertools.hpp and must be included separately. The following loop prints either "Just " or "Nothing" for each element in each tuple yielded.

vector<int> v1 = {0, 1, 2, 3};
vector<int> v2 = {10, 11};
for (auto&& t : zip_longest(v1, v2)) {
    cout << '{';
    if (std::get<0>(t)) {
        cout << "Just " << *std::get<0>(t);
    } else {
        cout << "Nothing";
    }
    cout << ", ";
    if (std::get<1>(t)) {
        cout << "Just " << *std::get<1>(t);
    } else {
        cout << "Nothing";
    }
    cout << "}\n";
}

The output is:

{Just 0, Just 10}
{Just 1, Just 11}
{Just 2, Nothing}
{Just 3, Nothing}

Takes a function and one or more iterables. The number of iterables must match the number of arguments to the function. Applies the function to each element (or elements) in the iterable(s). Terminates on the shortest sequence.

Prints the squares of the numbers in vec: 1 4 9 16 25

vector<int> vec{1, 2, 3, 4, 5};
for (auto&& i : imap([] (int x) {return x * x;}, vec)) {
    cout << i << '\n';
}

With more than one sequence, the below adds corresponding elements from each vector together, printing 11 23 35 47 59 71

vector<int> vec1{1, 3, 5, 7, 9, 11};
vector<int> vec2{10, 20, 30, 40, 50, 60};
for (auto&& i : imap([] (int x, int y) { return x + y; }, vec1, vec2)) {
    cout << i << '\n';
}

Note: The name imap is chosen to prevent confusion/collision with std::map, and because it is more related to itertools.imap than the python builtin map.

Yields only the values corresponding to true in the selectors iterable. Terminates on the shortest sequence.

Prints 2 6

vector<int> ivec{1, 2, 3, 4, 5, 6};
vector<bool> bvec{false, true, false, false, false, true};
for (auto&& i : compress(ivec, bvec) {
    cout << i << '\n';
}

Allows iteration over a sequence in sorted order. sorted does not produce a new sequence, copy elements, or modify the original sequence. It only provides a way to iterate over existing elements. sorted also takes an optional second comparator argument. If not provided, defaults to std::less.
Iterables passed to sorted are required to have an iterator with an operator*() const member.

The below outputs 0 1 2 3 4.

unordered_set<int> nums{4, 0, 2, 1, 3};
for (auto&& i : sorted(nums)) {
    cout << i << '\n';
}

This can chain any set of ranges together as long as their iterators dereference to the same type.

vector<int> empty{};
vector<int> vec1{1,2,3,4,5,6};
array<int,4> arr1{{7,8,9,10}};

for (auto&& i : chain(empty,vec1,arr1)) {
    cout << i << '\n';
}

Similar to chain, but rather than taking a variadic number of iterables, it takes an iterable of iterables and chains the contained iterables together. A simple example is shown below using a vector of vectors to represent a 2d ragged array, and prints it in row-major order.

vector<vector<int>> matrix = {
    {1, 2, 3},
    {4, 5},
    {6, 8, 9, 10, 11, 12}
};

for (auto&& i : chain.from_iterable(matrix)) {
    cout << i << '\n';
}

Iterates over elements of a sequence in reverse order.

for (auto&& i : reversed(a)) {
    cout << i << '\n';
}

Returns selected elements from a range, parameters are start, stop and step. the range returned is [start,stop) where you only take every step element

This outputs 0 3 6 9 12

vector<int> a{0,1,2,3,4,5,6,7,8,9,10,11,12,13};
for (auto&& i : slice(a,0,15,3)) {
    cout << i << '\n';
}

Takes a section from a range and increments the whole section.

Example: [1, 2, 3, 4, 5, 6, 7, 8, 9]

take a section of size 4, output is:

1 2 3 4
2 3 4 5
3 4 5 6
4 5 6 7
5 6 7 8
6 7 8 9

Example Usage:

vector<int> v = {1,2,3,4,5,6,7,8,9};
for (auto&& sec : sliding_window(v,4)) {
    for (auto&& i : sec) {
        cout << i << ' ';
        i.get() = 90;
    }
    cout << '\n';
}

grouper is very similar to sliding window, except instead of the section sliding by only 1 it goes the length of the full section.

Example usage:

vector<int> v {1,2,3,4,5,6,7,8,9};
for (auto&& sec : grouper(v,4))
//each section will have 4 elements
//except the last one may be cut short
{
    for (auto&& i : sec) {
        cout << i << " ";
        i.get() *= 2;
    }
    cout << '\n';
}

Generates the cartesian project of the given ranges put together

Example usage:

vector<int> v1{1,2,3};
vector<int> v2{7,8};
vector<string> v3{"the","cat"};
vector<string> v4{"hi","what","up","dude"};
for (auto&& t : product(v1,v2,v3,v4)) {
    cout << std::get<0>(t) << ", "
        << std::get<1>(t) << ", "
        << std::get<2>(t) << ", "
        << std::get<3>(t) << '\n';
}

Generates n length unique sequences of the input range.

Example usage:

vector<int> v = {1,2,3,4,5};
for (auto&& i : combinations(v,3)) {
    for (auto&& j : i ) cout << j << " ";
    cout << '\n';
}

Like combinations, but with replacement of each element. The below is printed by the loop that follows:

{A, A}
{A, B}
{A, C}
{B, B}
{B, C}
{C, C}

for (auto&& v : combinations_with_replacement(s, 2)) {
    cout << '{' << v[0] << ", " << v[1] << "}\n";
}

Generates all the permutations of a range using std::next_permutation. The iterators of the sequence passed must have an operator*() const

Example usage:

vector<int> v = {1,2,3,4,5};
for (auto&& vec : permutations(v)) {
    for (auto&& i : vec) {
        cout << i << ' ';
    }
    cout << '\n';
}

Generates every possible subset of a set, runs in O(2^n).

Example usage:

vector<int> vec {1,2,3,4,5,6,7,8,9};
for (auto&& v : powerset(vec)) {
    for (auto&& i : v) {
        cout << i << " ";
    }
    cout << '\n';
}