Itertools for java

A library for efficiently generating and iterating over various combinatorial constructs in java.

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The aim of this library is to provide an interface similar to itertools in python.This module implements a number of iterator building blocks inspired by constructs from APL, Haskell, SML and Python. Each has been recast in a form suitable for Java.

For instance, SML provides a tabulation tool: tabulate(f) which produces a sequence f(0), f(1), .... The same effect can be achieved using this library by combining Itertools.imap() and Itertools.count() to form Itertools.imap(f, Itertools.count()).

Features

This library uses generator like constructs to provide iterators and hence has no memory or speed limitations.
A particular element is generated only when it is needed.
As a result permutations of large arrays can be easily evaluated.

The following methods have been implemented in the library and are ready for use.

Documentation

Infinite Iterators

Iterator	Arguments	Results	Example
`count()`	start,[step]	start, start+step , start+2*step,...	count(10) --> 10 11 12 13 14 ...
`cycle()`	p	p0, p1, … plast, p0, p1, …	cycle('ABCD') --> A B C D A B C D ...
`repeat()`	elem [,n]	elem, elem, elem, … endlessly or up to n times	repeat(10, 3) --> 10 10 10

Iterators terminating on the shortest input sequence:

Iterator	Arguments	Results	Example
`chain()`	p, q, …	p0, p1, … plast, q0, q1, …	chain('ABC', 'DEF') --> A B C D E F
`compress()`	data, selectors	(d[0] if s[0]), (d[1] if s[1]), …	compress('ABCDEF', [1,0,1,0,1,1]) --> A C E F
`dropwhile()`	pred, seq	seq[n], seq[n+1], starting when pred fails	dropwhile( x-> x<5, [1,4,6,4,1]) --> 6 4 1
`ifilter()`	pred, seq	elements of seq where pred(elem) is true	ifilter( x-> x%2, Arrays.asList(1,2,3...10) --> 1 3 5 7 9
`ifilterfalse`	pred, seq	elements of seq where pred(elem) is false	ifilterfalse( x-> x%2, Arrays.asList(1,2,3...10) --> 2 4 6 8 10
`islice()`	seq, [start,] stop [, step]	elements from seq[start:stop:step]	islice('ABCDEFG', 2, None) --> C D E F G
`imap()`	func, p, q, …	func(p0, q0), func(p1, q1), …	imap(t -> Math.pow(t[0],t[1]), (2,3,10), (5,2,3)) --> 32 9 1000
`takewhile()`	pred, seq	seq[0], seq[1], until pred fails	takewhile(x-> x<5, [1,4,6,4,1]) --> 1 4
`izip()`	p, q, …	(p[0], q[0]), (p[1], q[1]), …	izip('ABCD', 'xy') --> Ax By
`izipLongest()`	p, q, …	(p[0], q[0]), (p[1], q[1]), …	izip_longest('ABCD', 'xy', fillvalue='-') --> Ax By C- D-

Combinatoric generators

Iterator	Arguments	Results
`product()`	p, q, r …	cartesian product, equivalent to a nested for-loop
`permutations()`	p,r	r-length tuples, all possible orderings
combinations()	p,r	r-length tuples, in sorted order
product("ABCD", "EFGH")		AE AF AG AH BE BF BG BH CE CF CG CH DE DF DG DH
permutations('ABCD', 2)		AB AC AD BA BC BD CA CB CD DA DB DC
combinations('ABCD', 2)		AB AC AD BC BD CD

How to use ?

Include jar file from releases.
Use them as shown below.

Itertool functions and their usage examples

1. `Itertools.chain(List<T>... iterables)`

Make an iterator that returns elements from the first iterable until it is exhausted, then proceeds to the next iterable, until all of the iterables are exhausted. Used for treating consecutive sequences as a single sequence.

public void chainExample() {
        String[] strings = {"Hello", "World", "Samagra", "Sammy", "P", "NP", "PSPACE"};
        int i = 0;
        for (String s :
                Itertools.chain(Arrays.asList(strings[0], strings[1],
                        strings[2]), Arrays.asList(), Arrays.asList(
                        strings[3], strings[4], strings[5], strings[6]
                ))) {
            assertEquals(strings[i], s);
            i++;
        }

2. `Itertools.compress(List<T> data, List<Boolean> selectors)`

Make an iterator that filters elements from data returning only those that have a corresponding element in selectors that evaluates to True. Stops when either the data or selectors iterables has been exhausted.

public void compressExample() {
        String[] data = {"Hello", "World", "Samagra", "Sammy", "P", "NP", "PSPACE"};
        Boolean[] selector = {false, true, true, false, false, false, true};
        int i = 0;
        for (String s :
                Itertools.compress(Arrays.asList(data), Arrays.asList(selector))) {
            if (i == 0)
                assertEquals("World", s);
            if (i == 1)
                assertEquals("Samagra", s);
            if (i == 2)
                assertEquals("PSPACE", s);
            i++;
        }
    }

3. `Itertools.dropWhile(Predicate<T> pred,List<T> seq)`

Make an iterator that drops elements from the iterable as long as the predicate is true; afterwards, returns every element. Note, the iterator does not produce any output until the predicate first becomes false, so it may have a lengthy start-up time.

public void dropwhileExample() {
        int i = 0;
        for (int a :
                Itertools.dropWhile(integer -> integer < 5, new ArrayList<>(Arrays.asList(1, 4, 6, 4, 1)))) {
            if (i == 0)
                assertEquals(6, a);
            if (i == 1)
                assertEquals(4, a);
            if (i == 2)
                assertEquals(1, a);
            i++;
        }
    }

4. `Itertools.ifilter(Predicate<T> predicate, List<T> list)`

Make an iterator that filters elements from iterable returning only those for which the predicate is True.

public void ifilterTest() {
        int i = 0;
        for (int a :
                Itertools.ifilter(integer -> (integer % 2) != 0, Arrays.asList(1, 2, 3, 4, 5, 6, 7, 8, 9))) {
            if (i == 0) {
                assertEquals(1, a);
            }
            if (i == 1) {
                assertEquals(3, a);
            }
            if (i == 2) {
                assertEquals(5, a);
            }
            if (i == 3) {
                assertEquals(7, a);
            }
            if (i == 4) {
                assertEquals(9, a);
            }
            i++;
        }
    }

5. `Itertools.ifilterfalse(Predicate<T> predicate, List<T> list)`

Make an iterator that filters elements from iterable returning only those for which the predicate is False.

 public void ifilterfalseExample() {
        int i = 0;
        for (int a :
                Itertools.ifilterfalse(integer -> (integer % 2) != 0, Arrays.asList(1, 2, 3, 4, 5, 6, 7, 8, 9))) {
            if (i == 0) {
                assertEquals(2, a);
            }
            if (i == 1) {
                assertEquals(4, a);
            }
            if (i == 2) {
                assertEquals(6, a);
            }
            if (i == 3) {
                assertEquals(8, a);
            }
            if (i == 4) {
                assertEquals(9, a);
            }
            i++;
        }
    }

6. `Itertools.islice(List<T> seq, int start, int stop,int step)`

Make an iterator that returns selected elements from the iterable. If start is non-zero, then elements from the iterable are skipped until start is reached. Afterward, elements are returned consecutively unless step is set higher than one which results in items being skipped. If stop is None, then iteration continues until the iterator is exhausted, if at all; otherwise, it stops at the specified position.

 public void isliceExample(){
        int i = 0;
        for (int a :
                Itertools.islice(Arrays.asList(1, 2, 3, 4, 5, 6, 7, 8, 9), 5, 2)) {
            if (i == 0)
                assertEquals(1, a);
            if (i == 1)
                assertEquals(3, a);
            if (i == 2)
                assertEquals(5, a);
            assertTrue(i < 3);
            i++;
        }
    }

7. `Itertools.imap(Function<T,U> function,List<T>... lists)`

Make an iterator that computes the function using arguments from each of the iterables.

public void imapExample() {
        int i = 0;
        for (double a :
                       Itertools.imap((t) -> Math.pow(t.get(0), t.get(1)), Arrays.asList(2, 3, 10), Arrays.asList(5, 2, 3))) {
            if (i == 0)
                assertEquals(32, (int) a);
            if (i == 1)
                assertEquals(9, (int) a);
            if (i == 2) {
                assertEquals(1000, (int) a);
            }
            i++;
        }
    }

8. `Itertools.takeWhile(Predicate<T> pred,List<T> seq)`

Make an iterator that returns elements from the iterable as long as the predicate is true.

public void takewhileTest() {
        int i = 0;
        for (int a :
                Itertools.takeWhile(integer -> integer < 5, new ArrayList<>(Arrays.asList(1, 4, 6, 4, 1)))) {
            if (i == 0)
                assertEquals(1, a);
            if (i == 1)
                assertEquals(4, a);
            i++;
        }
    }

9. `Itertools.izip(List<T> ... lists)`

Make an iterator that aggregates elements from each of the iterables.

public void izipTest(){
        String[] listOne = {"A","B","C","D"};
        String[] listTwo = {"x","y"};
        int i = 0;
        for (List<String> list :
                Itertools.izip(Arrays.asList(listOne),Arrays.asList(listTwo))) {
            assertTrue(i<2);
            assertEquals(listOne[i],list.get(0));
            assertEquals(listTwo[i],list.get(1));
            i++;
        }
    }

10. `Itertools.izipLongest(T fillValue,List<T> ... lists)`

Make an iterator that aggregates elements from each of the iterables. If the iterables are of uneven length, missing values are filled-in with fillvalue. Iteration continues until the longest iterable is exhausted.

public void izipLongestExample(){
        String[] listOne = {"A","B","C","D"};
        String[] listTwo = {"x","y"};
        int i = 0;
        for (List<String> list :
                Itertools.izipLongest("filler",Arrays.asList(listOne),Arrays.asList(listTwo))){
            assertTrue(i<4);
            if (i<2){
                assertEquals(listOne[i],list.get(0));
                assertEquals(listTwo[i],list.get(1));
            }
            else {
                assertEquals(listOne[i],list.get(0));
                assertEquals("filler",list.get(1));
            }
            i++;
        }

11. `Itertools.product(List<T> ... lists)`

Cartesian product of input iterables.

Roughly equivalent to nested for-loops in a generator expression. The nested loops cycle like an odometer with the rightmost element advancing on every iteration. This pattern creates a lexicographic ordering so that if the input’s iterables are sorted, the product tuples are emitted in sorted order.

    public void exampleProduct(){
        String[] str1 = {"alpha","Beta","gamma"};
        String[] str2 = {"a","b"};
        String[] str3 = {"Samagra","Sammy","Shanu"};
        for (List<String> products :
                Itertools.product(Arrays.asList(str1),Arrays.asList(str2),Arrays.asList(str3))) {
            for (String s :
                    products) {
                System.out.print(s+" ");
            }
            System.out.println();
        }
    }

12. `Itertools.combinations(List<T> list, int r)`

Return r length subsequences of elements from the input iterable.

Combinations are emitted in lexicographic sort order. So, if the input iterable is sorted, the combination tuples will be produced in sorted order.

Elements are treated as unique based on their position, not on their value. So if the input elements are unique, there will be no repeat values in each combination.

public void exampleCombinations(){
        String[] str1 = {"alpha","Beta","gamma","a","b","Samagra","Sammy","Shanu"};
        int i = 0;
        for (List<String> products :
                Itertools.combinations(Arrays.asList(str1),4)) {
            for (String s :
                    products) {
                System.out.print(s+" ");
            }
            i++;
            System.out.println(i);
        }
    }

13. `Itertools.permutations(List<T> list,int r)`

Return successive r length permutations of elements in the iterable. Permutations are emitted in lexicographic sort order. So, if the input iterable is sorted, the permutation tuples will be produced in sorted order.

Elements are treated as unique based on their position, not on their value. So if the input elements are unique, there will be no repeat values in each permutation.

public void testPermutations(){
        String[] str1 = {"alpha","Beta","gamma","a","b","Samagra"};
        int i = 0;
        for (List<String> products :
                Itertools.permutations(Arrays.asList(str1),5)) {
            for (String s :
                    products) {
                System.out.print(s+" ");
            }
            i++;
            System.out.println(i);
        }
    }

Contributing

Feel free to send any PRs right away.

License

Check out LICENSE.md

References

Wikipedia page for Generators
This project is heavily inspired from Python itertools

Name		Name	Last commit message	Last commit date
Latest commit History 34 Commits
.gradle		.gradle
build		build
gradle/wrapper		gradle/wrapper
src		src
test		test
.gitignore		.gitignore
.travis.yml		.travis.yml
LICENSE.md		LICENSE.md
README.md		README.md
build.gradle		build.gradle
combinatorial_lib.iml		combinatorial_lib.iml
gradlew		gradlew
gradlew.bat		gradlew.bat

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Itertools for java

Table of Contents

Features

Documentation

Infinite Iterators

Iterators terminating on the shortest input sequence:

Combinatoric generators

How to use ?

Itertool functions and their usage examples

1. `Itertools.chain(List<T>... iterables)`

2. `Itertools.compress(List<T> data, List<Boolean> selectors)`

3. `Itertools.dropWhile(Predicate<T> pred,List<T> seq)`

4. `Itertools.ifilter(Predicate<T> predicate, List<T> list)`

5. `Itertools.ifilterfalse(Predicate<T> predicate, List<T> list)`

6. `Itertools.islice(List<T> seq, int start, int stop,int step)`

7. `Itertools.imap(Function<T,U> function,List<T>... lists)`

8. `Itertools.takeWhile(Predicate<T> pred,List<T> seq)`

9. `Itertools.izip(List<T> ... lists)`

10. `Itertools.izipLongest(T fillValue,List<T> ... lists)`

11. `Itertools.product(List<T> ... lists)`

12. `Itertools.combinations(List<T> list, int r)`

13. `Itertools.permutations(List<T> list,int r)`

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Repository files navigation

Itertools for java

Table of Contents

Features

Documentation

Infinite Iterators

Iterators terminating on the shortest input sequence:

Combinatoric generators

How to use ?

Itertool functions and their usage examples

1. Itertools.chain(List<T>... iterables)

2. Itertools.compress(List<T> data, List<Boolean> selectors)

3. Itertools.dropWhile(Predicate<T> pred,List<T> seq)

4. Itertools.ifilter(Predicate<T> predicate, List<T> list)

5. Itertools.ifilterfalse(Predicate<T> predicate, List<T> list)

6. Itertools.islice(List<T> seq, int start, int stop,int step)

7. Itertools.imap(Function<T,U> function,List<T>... lists)

8. Itertools.takeWhile(Predicate<T> pred,List<T> seq)

9. Itertools.izip(List<T> ... lists)

10. Itertools.izipLongest(T fillValue,List<T> ... lists)

11. Itertools.product(List<T> ... lists)

12. Itertools.combinations(List<T> list, int r)

13. Itertools.permutations(List<T> list,int r)

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1. `Itertools.chain(List<T>... iterables)`

2. `Itertools.compress(List<T> data, List<Boolean> selectors)`

3. `Itertools.dropWhile(Predicate<T> pred,List<T> seq)`

4. `Itertools.ifilter(Predicate<T> predicate, List<T> list)`

5. `Itertools.ifilterfalse(Predicate<T> predicate, List<T> list)`

6. `Itertools.islice(List<T> seq, int start, int stop,int step)`

7. `Itertools.imap(Function<T,U> function,List<T>... lists)`

8. `Itertools.takeWhile(Predicate<T> pred,List<T> seq)`

9. `Itertools.izip(List<T> ... lists)`

10. `Itertools.izipLongest(T fillValue,List<T> ... lists)`

11. `Itertools.product(List<T> ... lists)`

12. `Itertools.combinations(List<T> list, int r)`

13. `Itertools.permutations(List<T> list,int r)`

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