Computer Science Stack Exchange Community Digest

Top new questions this week:

Is there a faster way to solve the following problem?

$A$ is an array of length $n$ $B$ is an $n\times n$ matrix \ I want to return an array C of size n such that: $$C_{i} = \sum_{j=1}^{n} \max(0, a_i - b_{ij}) $$ In pseudocode it could be like ...

algorithms time-complexity  
user avatar asked by Macosso Score of 3
user avatar answered by D.W. Score of 28

Optimal algorithmic complexity of stack varient?

I'm wondering about the optimal complexity - or at the very least, some way of achieving non-terrible complexity - of a particular stack variant, that I'm calling a 'nonrepetitive stack'. A ...

time-complexity data-structures asymptotics  
user avatar asked by TLW Score of 2

Dijkstra as a greedy algorithm

I'm preparing some material for students about greedy algorithms, and there is one point that confuses me: how Dijkstra's algorithm fits into the greedy framework. I would like to say that we have ...

optimization greedy-algorithms dijkstras-algorithm  
user avatar asked by Dmitry Score of 2
user avatar answered by John L. Score of 2

How can we prove that Extract almost minimum operation in a priority queue cannot be done in o(logn)

Given a data structure with 2 operations: insert and extract almost min. Extract almost min operation outputs either the first minimum or a second minimum item from the current structure randomly. How ...

algorithms complexity-theory algorithm-analysis runtime-analysis priority-queues  
user avatar asked by JsonResponse Score of 1
user avatar answered by D.W. Score of 0

Having a 2D matrix with three typed elements, how to efficiently cover one of the types and NOT cover the other one?

I have a matrix with three possible elements: A, B and C. The size of the matrix could be a maximum of 15x16. $$ \begin{bmatrix} A & A & C & A\\ A & C & B & C\\ A & C & ...

algorithms matrices python  
user avatar asked by Predicate Score of 1
user avatar answered by D.W. Score of 0

Efficiently find the distance from a point to the decision boundary for assigning points to a particular $k$-means cluster

I have run $k$-means on a large set of high-dimensional data, and now I want to find the distance from a point $x$ to the Voronoi cell associated with one of the $k$ centroids. (In a previous version ...

algorithms computational-geometry clustering  
user avatar asked by gmr Score of 1
user avatar answered by V.V. Score of 1

Automatic differentiation - Upper bound time

Is there any proof or reference or intuition for the following theorem related to AAD: Any function f of n variables f(x1,...,xn) can be differentiated with respect to every variable xi at a ...

algorithms time-complexity  
user avatar asked by Glork Score of 1
user avatar answered by Glork Score of -1

Greatest hits from previous weeks:

The convoy effect in process scheduling

As I understand the convoy effect, in the context of vehicular traffic in a road system. A slow moving group of vehicles passes through the system, slowing traffic even in areas which were not ...

operating-systems process-scheduling  
user avatar asked by jsj Score of 7
user avatar answered by Ugnes Score of 9

What is most efficient for GCD?

I know that Euclid’s algorithm is the best algorithm for getting the GCD (great common divisor) of a list of positive integers. But in practice you can code this algorithm in various ways. (In my case,...

algorithms recursion arithmetic  
user avatar asked by Jonathan Prieto-Cubides Score of 33
user avatar answered by Gilles 'SO- stop being evil' Score of 28

Why is it best to use a prime number as a mod in a hashing function?

If I have a list of key values from 1 to 100 and I want to organize them in an array of 11 buckets, I've been taught to form a mod function $$ H = k \bmod \ 11$$ Now all the values will be placed ...

data-structures hash hash-tables primes  
user avatar asked by CodyBugstein Score of 99
user avatar answered by Mario Cervera Score of 116

What is the exact difference between a latch & a flipflop?

From what I have understood : A Flip Flop is a clocked latch i.e. flip flop = latch + clock Latch continuously checks for inputs & changes the output whenever there is a change in input Flip Flop ...

computer-architecture sequential-circuit digital-circuits  
user avatar asked by avi Score of 8
user avatar answered by Ran G. Score of 7

How do computers keep track of time?

How are computers able to tell the correct time and date every time? Whenever I close the computer (shut it down) all connections and processes inside stop. How is it that when I open the computer ...

computer-architecture clocks  
user avatar asked by Soham Score of 79
user avatar answered by D.W. Score of 113

Why is quicksort better than other sorting algorithms in practice?

In a standard algorithms course we are taught that quicksort is $O(n \log n)$ on average and $O(n^2)$ in the worst case. At the same time, other sorting algorithms are studied which are $O(n \log n)$ ...

algorithms sorting  
user avatar asked by Janoma Score of 369
user avatar answered by Sebastian Score of 254

What is the average turnaround time?

For the following jobs: The average wait time would be using a FCFS algorithm: (6-6)+(7-2)+(11-5)+(17-5)+(14-1) -> 0+5+6+10+13 -> 34/5 = 7 (6.8) What would the average turnaround time be?

algorithms operating-systems process-scheduling scheduling  
user avatar asked by Sheldon Score of 7
user avatar answered by Gilles 'SO- stop being evil' Score of 10

Can you answer these questions?

Sort array of 3D points to maximize number of tetrahedra in the array

Let $P$ be a indexable set(array) of points in $\mathbb{R}^3$ s.t. $P = \{p_0,p_1,p_2,...,p_n\}, p_i \in\mathbb{R}^3$. I want to sort $P$ so that every 4 consecutive points forms a non-coplanar ...

algorithms computational-geometry simplex  
user avatar asked by yosmo78 Score of 1
user avatar answered by Yves Daoust Score of 0

acceptance of non-recursively enumerable language by the Turing machine

I'd like to know if there's a non-recursively computable language that can be accepted by the Turing machine. From the following definition of the recursively enumerable language: and from the fact ...

turing-machines recursively-enumerable  
user avatar asked by obolenskaya00 Score of 1

Is this problem NP-hard ? (about arithmetic progressions)

Suppose there are n Arithmetic Progressions : Example : $$ AP1: a_1 = 1, d = 14 \\ AP2: a_1 = 8, d = 21 \\ :\\ :\\ APn: a_1 = X, d = X \\ $$ a1 = first term d = common difference and for each AP'...

np-complete np-hard np  
user avatar asked by uccu1573 Score of 1
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