Itertools¶
Why I make notes about it¶
I found many of the challenges websites can be solved using these iterators.
Description¶
Itertools is a module which is implementing of the iterator building blocks of other well known programing languages, this is the implementation on python.
These are memory-efficient tools that are useful either using them alone or in combination.
They are divided in three big main categories:
- Infinite iterators
- Iterators on the shortest input sequence
- combinatorics iterators
1. Infinite Iterators¶
These are apparently 3 different iterators to get N repetitions of the same number, Count from an N number a define number of steps in a direction, and cycle over a string.
| Name | Arg | Example |
|---|---|---|
| count() | "start, [step]" | count(10) ⇒ 10 11 12 13 14 |
| cycle() | p | cycle('ABCD') ⇒ A B C D A B C D ... |
| repeat() | "elem [,n]" | "repeat(10,3) ⇒ 10 10 10" |
2. Iterators terminating on the shortest input sequence¶
These are more difficult to understand just by reading the title, but the example will help to make it clear
| Name | arg | Results | Example |
|---|---|---|---|
| accumulate() | p [, func] | p0, p0+p1. p0+p1+p2 | accumulete([1,2,3,4,5]) ⇒ 1 3 6 10 15 |
| chain() | p, q, … | p0, p1, … plast, q0, q1, … | chain('ABC', 'DEF') ⇒ A B C D E F |
| chain.from_iterable() | iterable | p0, p1, … plast, q0, q1, … | chain.from_iterable(['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(lambda x: x<5, [1,4,6,4,1]) ⇒ 6 4 1 |
| filterfalse() | pred, seq | elements of seq where pred(elem) is false | filterfalse(lambda x: x%2, range(10)) ⇒ 0 2 4 6 8 |
| groupby() | iterable [, key] | sub-iterators grouped by value of key(v) | |
| islice() | seq, [start,] stop [, step] | elements from seq[start:stop:step] | islice('ABCDEFG', 2, None) ⇒ C D E F G |
| starmap() | func, seq | func(*seq[0]) func(*seq[1]), … | starmap(pow, [(2,5), (3,2), (10,3)]) → 32 9 1000 |
| takewhile() | pred, seq | seq[0], seq[1] until pred fails | takewhile(lambda x: x<5, [1,4,6,4,1]) → 1 4" |
| tee() | it, n | it1, it2, … itn | splits one iterator into n |
| zip_longest() | p, q, … | (p[0], q[0]), (p[1], q[1]), … | zip_longest('ABCD', 'xy', fillvalue='-') → Ax By C- D- |
3. Combinatoric iterator¶
| Name | arg | Result | Results |
|---|---|---|---|
| product() | p, q, … [repeat=1] | AA AB AC AD BA BB BC BD CA CB CC CD DA DB DC DD | cartesian product, equivalent to a nested for-loop |
| permutations() | p[, r] | AB AC AD BA BC BD CA CB CD DA DB DC | r-length tuples, all possible orderings, no repeated elements |
| combinations() | p, r | AB AC AD BC BD CD | r-length tuples, in sorted order, no repeated elements |
| combinations_with_replacement() | p, r | AA AB AC AD BB BC BD CC CD DD | r-length tuples, in sorted order, with repeated elements |
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