How to get the first element of the List or Set? [duplicate]
See the javadoc of List list.get(0); or Set set.iterator().next(); and check the size before using the above methods by invoking isEmpty() !list_or_set.isEmpty()
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Convert NumPy array to Python list
Use tolist(): >>> import numpy as np >>> np.array([[1,2,3],[4,5,6]]).tolist() [[1, 2, 3], [4, 5, 6]] Note that this converts the values from whatever numpy type they may have (e.g. np.int32 or np.float32) to the “nearest compatible Python type” (in a list). If you want to preserve the numpy data types, you could call list() on … Read more
How to get last items of a list in Python?
You can use negative integers with the slicing operator for that. Here’s an example using the python CLI interpreter: >>> a = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12] >>> a [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12] >>> a[-9:] [4, 5, 6, 7, 8, … Read more
How to take the first N items from a generator or list? [duplicate]
Slicing a list top5 = array[:5] To slice a list, there’s a simple syntax: array[start:stop:step] You can omit any parameter. These are all valid: array[start:], array[:stop], array[::step] Slicing a generator import itertools top5 = itertools.islice(my_list, 5) # grab the first five elements You can’t slice a generator directly in Python. itertools.islice() will wrap an object … Read more
Sum a list of numbers in Python
Question 1: To sum a list of numbers, use sum: xs = [1, 2, 3, 4, 5] print(sum(xs)) This outputs: 15 Question 2: So you want (element 0 + element 1) / 2, (element 1 + element 2) / 2, … etc. We make two lists: one of every element except the first, and one … Read more
Check if something is (not) in a list in Python
The bug is probably somewhere else in your code, because it should work fine: >>> 3 not in [2, 3, 4] False >>> 3 not in [4, 5, 6] True Or with tuples: >>> (2, 3) not in [(2, 3), (5, 6), (9, 1)] False >>> (2, 3) not in [(2, 7), (7, 3), “hi”] … Read more
Getting a list item by index
list1[0]; Assuming list’s type has an indexer defined.
Python list vs. array – when to use?
Basically, Python lists are very flexible and can hold completely heterogeneous, arbitrary data, and they can be appended to very efficiently, in amortized constant time. If you need to shrink and grow your list time-efficiently and without hassle, they are the way to go. But they use a lot more space than C arrays, in … Read more
How to sort a List/ArrayList?
Collections.sort(testList); Collections.reverse(testList); That will do what you want. Remember to import Collections though! Here is the documentation for Collections.
How to avoid “RuntimeError: dictionary changed size during iteration” error?
In Python 3.x and 2.x you can use use list to force a copy of the keys to be made: for i in list(d): In Python 2.x calling keys made a copy of the keys that you could iterate over while modifying the dict: for i in d.keys(): But note that in Python 3.x this … Read more