As per Wiki, a combination is a selection of items from a collection, such that the order of selection does not matter. For example, given three fruits, say an apple, an orange and a pear, there are three combinations of two that can be drawn from this set: an apple and a pear; an apple and an orange; or a pear and an orange.
As per the Documentation, it makes 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.
Unlike regular slicing, islice does not support negative values for start , stop , or step. Can be used to extract related fields from data where the internal structure has been flattened.
We have seen how useful and easy it is to use itertools module and it can do lot of work under the hood in a more memory efficient way.
As a Data Scientist where you have to crunch million of records and iterate thru it to accomplish your task these handy solutions come as a boon. Save Article. Improve Article. Like Article. Previous How to use Iterator in Java?
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While the for each loop iterates over the iterator obtained from the linked list and calls its next method. The iterator maintains the states of the last access and thus does not start all the way from head everytime.
You should also be programming to the list or collection interface so that if you later decided that another data structure would be more efficient you'd be able to swap it out without massive surgery. In that case the case of coding to an interface you won't necessarily know the implementation details and it's probably wiser to defer that to the data structure itself. All the i's, j's, and k's that you may end up manipulating can get confusing very quickly.
The advantage of the new iterator form is that it looks cleaner in your codebase. Edit : I see from the other answers that you actually meant the difference between using get i versus an iterator. I took the original question to mean the difference between the old and new ways of using the iterator. Using get i and maintaining your own counter, especially for the List classes is not a good idea, for the reasons mentioned in the accepted answer.
FYI, whether you use an explicit iterator or an implicit one i. Stack Overflow for Teams — Collaborate and share knowledge with a private group. Create a free Team What is Teams? Collectives on Stack Overflow. Learn more. Asked 11 years, 10 months ago. Active 6 years, 8 months ago. Viewed 59k times. Or simply why should I use Iterator over for loop or vice versa?
Improve this question. Harish Harish 1, 4 4 gold badges 20 20 silver badges 20 20 bronze badges. Note that the Reason a for loop is slower with a linked list, is that each call to get i iterates from the head of the list i times. I'm sure that is intuitively obvious to everyone else here, but it took me a minute to figure out the why of it. Add a comment.
Active Oldest Votes. Hence I've run a small test : iterate over a LinkedList and an ArrayList respecively with , "random" strings summing up their length just something to avoid that compiler optimizes away the whole loop using all 3 loop styles iterator, for each, for with counter Results are similar for all but "for with counter" with LinkedList.
Improve this answer. All the other five took less than 20 milliseconds to iterate over the whole list looks like the JVM dead-code optimization kicked in I've generated , random strings actually UUIDs and summed their lengths which was printed to stdout after the loop. Sure, UUIDs have the same length which makes the output predictable, but the compiler isn't that smart. Believe it or not, but a modern CPU can do that in 20 ms.
So we're talking about billions of instructions per s or millions per ms. Thus, iterating over , strings with several millions of instructions is feasible.
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