Computer Science Stack Exchange Community Digest

Top new questions this week:

Are there associative cryptographic / collision resistant hashes?

Git uses SHA hashing as a content address key. This relies on their being a vanishingly small probability that two different pieces of content will ever hash to the same key. For content ...

algorithms hash  
user avatar asked by Benjohn Score of 10
user avatar answered by D.W. Score of 9

Find Minimum Transformation Between Multisets of Lists of Cards

Brief: I have two configurations, a and b, of the same set of Rummikub tiles (a may not be a ...

algorithms data-structures optimization combinatorics board-games  
user avatar asked by xdaimon Score of 3
user avatar answered by D.W. Score of 5

Why Is Inverse Quantization and Inverse Transform Taken Before Motion Estimation in H.264?

Here is the block diagram of the pipeline for the H.264 encoder (Fundamentals of Multimedia by Ze-Nian Li, Mark S. Drew and Jiangchuan Liu). The feedback loop is used for motion estimation, but why ...

user avatar asked by Xosrov Score of 2
user avatar answered by Topquark Score of 0

Asymptotics of the sum of a geometric series

I have a parameter $q$ which is the probability of selecting a vertex (among $n$ vertices...) to be in a certain set. I am constructing the sets in an iterative way, having the vertex $v_i$ be in the ...

user avatar asked by V. Prasad Score of 2
user avatar answered by Steven Score of 8

number of edges that appeared at least in one shortest path

In a simple weighted graph, with n vertices and m edges , for each pair of vertices we want to find the number of edges that appeared at least in one of the shortest paths between these two vertices. ...

graphs graph-traversal weighted-graphs  
user avatar asked by mostamele Score of 1
user avatar answered by Inuyasha Yagami Score of 0

Polynomial Hierarchy - the difference between $\Pi$ and $\Sigma$

The way I understand is that complexity classes which are denoted with $\Sigma_i$ are representing a combination of alternating variable quantifiers which must start with an existential quantifier $\...

user avatar asked by Meki21 Score of 1
user avatar answered by Arno Score of 2

Greedy Algorithm and Proof of Correctness for Minimum Denominations of US Coinage System Problem

I've come up with a greedy algorithm proof for the minimum denominations problem, and I'm curious if someone can verify the correctness of the proof for me. I have simplified the problem by ...

algorithms proof-techniques greedy-algorithms correctness-proof  
user avatar asked by Gary Drocella Score of 1
user avatar answered by Gary Drocella Score of 1

Greatest hits from previous weeks:

Time complexity of min() and max() on a list of constant size?

If you use min() or max() on a constant sized list, even in a loop, is the time complexity O(1)?

time-complexity python  
user avatar asked by anshur Score of 8
user avatar answered by gnasher729 Score of 28

How/when is calculus used in Computer Science?

Many computer science programs require two or three calculus classes. I'm wondering, how and when is calculus used in computer science? The CS content of a degree in computer science tends to focus ...

education mathematical-analysis  
user avatar asked by Victor Score of 112
user avatar answered by Jay Score of 124

How to solve T(n)=2T(√n)+log n with the master theorem?

I'm trying to solve the recurrence $$T(n)=2T(\sqrt{n})+\log n$$ using the master theorem. Which case applies here?

recurrence-relation master-theorem  
user avatar asked by Nimbus9000 Score of 3
user avatar answered by John L. Score of 9

Why does Randomized Quicksort have O(n log n) worst-case runtime cost

Randomized Quick Sort is an extension of Quick Sort in which the pivot element is chosen randomly. What can be the worst case time complexity of this algorithm. According to me, it should be $O(n^2)$, ...

algorithm-analysis runtime-analysis sorting quicksort  
user avatar asked by Atinesh Score of 22
user avatar answered by James Evans Score of 23

Why does a processor have 32 registers?

I've always wondered why processors stopped at 32 registers. It's by far the fastest piece of the machine, why not just make bigger processors with more registers? Wouldn't that mean less going to the ...

user avatar asked by Matt Capone Score of 59
user avatar answered by Wandering Logic Score of 91

How to prove greedy algorithm is correct

I have a greedy algorithm that I suspect might be correct, but I'm not sure. How do I check whether it is correct? What are the techniques to use for proving a greedy algorithm correct? Are there ...

algorithm-analysis proof-techniques correctness-proof greedy-algorithms  
user avatar asked by D.W. Score of 48
user avatar answered by D.W. Score of 41

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 107
user avatar answered by Mario Cervera Score of 126

Can you answer these questions?

How do I find time complexity of while loops?

I am designing a simple algorithm to be used at hardware level. And I want to find its time complexity. i=1 while(i<=n) { i=i*2; } How I'd go about finding ...

algorithms time-complexity  
user avatar asked by barnyard9 Score of 1
user avatar answered by Gary Drocella Score of 0

Confusion on the use of the chain-rule for the total derivative of the NLL Loss function

So my question is about when we want to find the total derivative of the NLL Loss function $L$ w.r.t. $w_i$. So the "pipeline" is often expressed as: $$\frac{\partial L}{\partial w_i} = \...

algorithms algorithm-analysis derivation  
user avatar asked by ZenPyro Score of 1

Assumption on SuccessiveShortestPaths

I read that one assumption for Successive Shortest Paths algorithm for computing the minimum cost flow problem is that every cost is non-negative. I also read that this assumption can be removed with ...

graphs network-flow  
user avatar asked by Wuac Score of 1
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