Questions tagged [counting]
The term Counting in Computer Science is usually used to refer to counting objects in certain arrangements or with certain properties.
171
questions
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1answer
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Counting all subsequences constrained to a condition
I was trying to find all subsequences constrained to the following conditions:
remove element from the end.
remove element from the beginning.
remove element from both sides.
For example, given the ...
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0answers
19 views
Count number of permutation under certain condition [closed]
I am new to this community and don't know well how to ask questions. Please give any comments if I made a mistake
The question is how many permutations of input numbers exist under certain. K is any ...
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Determining the number of reachable vertices from every vertex in a directed acyclic graph
Let $G = (V, E)$ be a directed acyclic graph, which is quite sparse (in the examples I have in mind, $|E| \approx 10|V|$ or so). For each vertex $v \in V$, let $f(v)$ be the number of vertices ...
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1answer
34 views
How many functions require precisely $n^2$ gates?
I'm trying to determine an asymptotic bound on the cardinality of the following set of functions. It is the functions with $n$-bit inputs, $\{0,1\}$ output, and requires precisely $n^2$ NAND gates. I'...
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1answer
45 views
Proving that deterministic approximate counting uses log(n) space
We just saw the Morris algorithm in class and we were asked the following:
In class, we saw a constant factor approximate randomized counting
algorithm with space complexity $O(\log \log n)$, where $...
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34 views
Is there a reduction from 2sat to bpm?
Given a 2SAT instance can we convert into bipartite perfect matching in parsimonious reduction?
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1answer
67 views
Sorting elements into k subarrays
Given are $n$ integer numbers in the range $0$ to $5n$.
A SubSort algorithm organizes the numbers into $k=n/100$ sets,
$s_{1}$, …, $s_{k}$ , each containing $100$ numbers, such that the
following ...
1
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1answer
38 views
Determine if for given some $L$, $S_L={L(M) : <M>\in L}$ then for any $L$, if $S_L=RE$ then $L\in R$ is True or False and explain
Determine if for given some $L$, $S_L=\{\ L(M) | <M>\in L \}$ then for any
$L$, if $S_L=RE$ then $L\in R$. Correct or Incorrect and explain why.
I think the claim is incorrect, and I'm trying ...
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1answer
40 views
Find the total no. of strings ( len n ) possible given a set of sets of letters such that no two letter from a single set should be in that string
This was an algorithm problem but I am having problems in formulating it.
I have a certain approach but I do not know how to fully execute:
Given
26 letters in total
All possible strings of length n
...
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31 views
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568 views
How can we count the number of pairs of coprime integers in an array of integers? (CSES)
For reference, I am trying to solve this CSES Problem.
The problem basically states that given up to $10^5$ positive integers in the range $[1, 10^6]$, find the number of pairs of those positive ...
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32 views
Counting the number of comparisons in red black tree
I have an array with $N$ elements and I run an algorithm that find how many distinct elements are in the array by using a red black tree as follows:
for each element
if element not in tree
insert ...
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0answers
18 views
Counting distinct elements in array using hash table and measuring efficiency
I need to implement an algorithm to solve the Count-distinct problem given an array of $N$ elements using a hash table, and devise metrics with which to measure my algorithm efficiency. I was told I ...
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47 views
Count of different ways to express N as the sum of given numbers
I'm working on a case and I need some help :)
I need to find number of ways and solutions itself to express N as the sum of given numbers.
So, Sum (N) = 600 and the numbers from which I need to get ...
4
votes
1answer
39 views
Model Counting for Sum of Conjunctive Formulas
Problem:
Let $X=\{x_1, ..., x_N \}$ be a set of binary variables. Each variable can be assigned to either $0$ or $1$ so there are $2^N$ possible assignments.
Input: We are given a positive integer $C$ ...
4
votes
2answers
136 views
Why does my code work: bijecting binary trees to Dyck paths
The number of Dyck paths (paths on a 2-d discrete grid where we can go up and down in discrete steps that don't cross the y=0 line) where we take $n$ steps up and $n$ steps down follows the Catalan ...
2
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1answer
79 views
Counting substrings that belong to a regular language
Given a regular language $L$ and a string $x$ give an efficient algorithm to count the occurrences of substrings $x[i,j] \in L$.
More in particular, I am looking for a linear time algorithm in the ...
2
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1answer
169 views
Number of substrings possible with even characters
Consider a string 'ABBAA'
Possible substrings with even number of characters are $4$
'ABBA' : Count of 'A' is even and 'B' is even
'AA' : Count of 'A' is even and 'B' is even - ($0$)
Similarly 'BB' ...
0
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1answer
51 views
Is there a #$P$-complete counting problem such that every (valid) instance of its decision version is a Yes-instance?
I want to know whether there is a decision problem, written EasyProblem, satisfying the follow property:
For every valid instance $x$, $x$ is a Yes-instance for EasyProblem (if we construct ...
2
votes
1answer
31 views
#perfectMatchings is self-reducible
How can one show that the counting problem:
Given a graph, output the number of perfect matchings
Is self reducible?
I found a hint in Moore's Chapter on Counting, Sampling and Statistical Physics:
...
1
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1answer
62 views
What is the solve of F(n,n) = F(n-1,n) + F(n, n-1) + 1 Where F(0,a) = 1 and F(a, 0) = 1 for every a
I'm given the following python function:
...
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0answers
34 views
Counting paths by their type
An edge-labelled directed graph is the data of $G = (V, E, l)$ where $(V, E)$ is a directed graph, and $l \colon E \to \mathbb{P}$ is some function. (For the graph I am considering, labels take values ...
1
vote
1answer
45 views
Iterate unique sets of integers
I'm trying to figure out of if there's a way to generate all unique sets of integers of length K, where each member has an upper bound of N, and a lower bound of M, without tracking which sets have ...
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votes
2answers
360 views
Efficient Algorithm to Find the n-th Odious Number
An odious number is defined as an integer that has odd binary Hamming weight. I need an implementation of algorithm that finds the nth odious number, preferably recursive. Any ideas? A python script ...
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1answer
35 views
Polynomial time counter of solutions of 2SAT expression with pure literals
As per the title, is there any polynomial time algorithm to count the number of satisfying arguments for a 2SAT expression with pure literals? An even shallower case: Is there any such counter when ...
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1answer
92 views
Count number of ways in which atomic operation(s) of n different processes can be interleaved
PROBLEM: Count the number of ways in which atomic operation(s) of n different processes can be interleaved. A process may crash mid way before completion.
Suppose there are a total of n different ...
2
votes
1answer
48 views
Enumerating every "partnering" without repeating partners
I'm taking a class. In this class every week we have a partner. There are an even number of people in the class. We'd like avoid having repeat partners if possible so that everyone gets to work with ...
2
votes
2answers
267 views
Unambiguous context-free grammar for strings with at least as many a's as b's
I have designed this Grammar but it is ambiguous:
$$S\to aSbS \mid bSaS \mid aS \mid\epsilon$$
Would anyone help me make it unambiguous? Assume the alphabet is $\{a,b\}$.
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0answers
86 views
Prove that $\#k-colouring$ graph problem is $\#P-complete$
I need to prove, that the $\#k-colouring$ graph problem is $\#P-complete$. I want to construct the reduction from $\#3SAT$ problem, so $\#3SAT \leq \#k-colouring$. The reduction between the counting ...
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vote
2answers
249 views
Fastest algorithm for finding the number of primes in a range
Is there an algorithm for finding the number of primes in a given range $[N, M)$ that works in time linear to $M-N$? For context, $N$ and $M$ can go up to $10^{10}$, but the distance between N and M ...
1
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1answer
113 views
Can all types of computational problems be modeled as decision problems?
Can all types of computational problems (search, counting, optimization...) be modeled as (sets of) decision problems? Rephrased: For every type of computational problem is there a set of decision ...
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2answers
68 views
To calculate how many times a certain year repeats itself in the calendar within a given year range
Let's say I have a year $Y$. I wanna know how can I calculate the number of times the calendar configuration of the year $Y$ repeats itself in the year range $[A, B]$.
Is there any method to it ...
2
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0answers
80 views
Counting triplets from three arrays satisfying the equation x^2 = yz
Let's say I have three arrays of positive integers X, Y and Z. You can assume that each of ...
0
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1answer
41 views
Number of permutations of set {1, 2, ..., n} for which insertion sort will perform exactly n permutations
I have had the following problem at my last exam:
For how many permutations of set {1, 2, ..., n} where n > 2 will insertion sort (without guard element) perform exactly n comparisons.
My thinking ...
3
votes
1answer
2k views
Count total number of k length paths in a tree
This is a question from a competitive programming competition.
Given a tree with n nodes and a number k, find the total number of paths of length k in that tree.
I know for a fact that a solution can ...
0
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2answers
106 views
How is the set of functions from ${\{a,b\}}$ to $N$ countable?
Assume a set of functions from ${\{a,b\}}$ to $N$
Where $N$ is the set of Natural numbers.
Let us assume that the size of $N$ is $n$.
i.e $|N|=n$
The first element $a$ have $n$ choices for mapping....
4
votes
1answer
67 views
Counting big enough elements
Let $v$ be a vector of positive integers of length $n$. I want to find the highest $k$ such as there are at least $k$ elements of $v$ that are greater or equal to $k$.
What would be the best ...
3
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0answers
34 views
Estimating number of points in 1D space
There are some arbitrary-chosen points in 1D space. What needs to be found is the approximate number of them without counting all of them. It is possible to choose some coordinates (numbers) and for ...
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1answer
154 views
Build a histogram from a very large data sample
I want to calculate a histogram from an array of size N. N is very large.
I know 2 ways to do so:
The naive approach is to ...
3
votes
2answers
484 views
Fastest algorithm to find all the possible paths of length $n$ from a give node in a directed graph?
I am trying to find the fastest algorithm to find all the possible paths of length $N$ from a given node in a directed graph.
My solution is to do a modification of breadth first search from the ...
3
votes
1answer
56 views
Given a set of intervals $(I_n)_n$ contained in $[0, L]$, compute the longest interval in $[0, L]$ which has empty intersection with all $(I_n)_n$
Let be $(I_n)_n$ a set of $p$ intervals each contained in $[0, L]$ for $L \geq 1$.
I define $(J_n = [a_n, b_n])_n$ the set of intervals which have empty intersection with $I_n$ for all $n \in [[1, p]]...
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0answers
19 views
Calculating Time Complexity of Algorithm Using Incrementor Variable [duplicate]
I am trying to calculate the time complexity of an algorithm using n in the code below.
I have a working solution to a coding challenge to sort a stack using only ...
2
votes
1answer
313 views
Number of ways painting graph in two colors, such that two nodes of same color are linked by edge
We are given undirected graph of $N$ nodes and $M$ edges, we want to count the number of possible ways to paint this graph in $2$ colors such that for each two nodes having the same color, there must ...
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2answers
126 views
Are $\mathsf{\#P}$ problems harder than $\mathsf{NP}$ problems
I have a method to solve the $\mathsf{\#P}$ version of 3SAT in a way that seemingly reduces it to an $\mathsf{NP}$ problem. - I don't have a formal understanding of these terms so I will just show an ...
3
votes
1answer
55 views
Estimation of the number of solutions by Counting
This is a question from a quantum computation textbook.
Consider a classical algorithm for counting the number of solutions to a problem. The algorithm samples uniformly and independently $k$ times ...
1
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1answer
474 views
How to find the Big-O for finding combinations of balanced parentheses?
Given n pairs of parentheses, a function which returns the total number of all combinations well-formed parentheses could be:
...
3
votes
2answers
170 views
Prove that this language is NP-Hard
Given
$$\mathrm{\#3SAT} = \{ (w, y) \mid w\text{ is a $\mathrm{3SAT}$ instance with at least $y$ satisfying assignments}\}\,,$$
prove that $\mathrm{\#3SAT}$ is NP-Hard.
I am currently stuck with ...
3
votes
1answer
67 views
Why does not valiant's reduction show NP=RP?
Valiant converts $SAT$ formula to a $0/1$ matrix such that $Permanent$ of the matrix is $4^m\#SAT$.
We know $Permanent$ can be approximated to $1+\epsilon$ factor with probability $1-\frac1\delta$ in ...
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1answer
2k views
How many ways to express N as sum of 2, 3 and 5?
I've learnt about problems about express N as sum of 2, 3, 5. For examples, if N = 7:
N = 5 + 2
N = 2 + 5
N = 2 + 2 + 3
N = 2 + 3 + 2
N = 3 + 2 + 2
But most of I found on the Internet that the ...
3
votes
1answer
81 views
The expectation of the total number of pairs of keys in a hash table that collide using universal hashing
I am reading CLRS relating to perfect hashing. When computing the
$$
\mathbb{E}[\sum_{j=0}^{m-1}{n_j\choose{2}}]
$$
where $m$ is the number of slots in the hash table, and $n_j$ is the number of keys ...
