Questions tagged [loop-invariants]
Properties that hold before and after every execution of a loop. Used to prove correctness of algorithms.
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questions
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2answers
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quicksort invariant 3 conditions with loop invariant
in studying Quicksort using the book "Introduction to Algorithms" by Cormen, Leiserson, Rivest and Stein, they describe in order to show correctness, an invariant must hold for the 3 stages of the ...
0
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0answers
23 views
How to determinne the loop invariant?
The following code returns the number of trailing zeros of an integer x (number of 0s at the end of the decimal number representation of x). I had to find its loop invariant, which is ...
1
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1answer
23 views
Hoare Logic for Factorial
I came across this hoare logic for factorials but I don't quite understand it. We multiply F and X but we're not adding up all values of F so how do we get the sum/factorial at the end?
Precondition: ...
0
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0answers
14 views
Finding bound function of loop invariant
In Programming in the 1990s, by Edward Cohen, the author gives an example of a bound function.
For example, if we have $B \equiv 10 - n$ where $n = 0$, say, then $B$ will eventually be falsified if ...
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0answers
21 views
How to write the invariants for one version of binary search insertion point (or leftmost entry) algorithm?
If we compare the binary search algorithm (leftmost or insertion point) on Wikipedia:
Algorithm 1:
...
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0answers
75 views
Loop invariant of a search algorithm
I have to come up with a proof of correctness of the following algorithm: GuardedSearch(A; v)
Input: an array A of n numbers and a number v
Output: an index i such that A[i] = v, or NotFound if no ...
4
votes
1answer
70 views
Is there a model of ZF¬C where some program always terminates but has no loop variant?
Wikipedia has a proof that every loop that terminates has a loop variant—a well-founded relation on the state space such that each iteration of the loop results in a state that is less than the ...
0
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1answer
31 views
I cannot find an invariant for the following program
I have the following:
(|$y=0; x=c$|) while(x > 0){y=y+a; x=x-1;} (|$y= a*c$|)
This seems like a fairly simple program and I can intuitively tell that the post ...
2
votes
2answers
213 views
Implementing Tarjan's strongly connected components algorithm in a language without exceptions or undefined behavior
I asked this question on Stack Overflow, but I have not obtained an actual answer to the question.
Tarjan's strongly connected components algorithm is stunningly beautiful, and inexpressible in a ...
0
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1answer
39 views
How do I “apply” a loop invariant?
A homework problem I have asks that I "state" a loop invariant, then "prove" it, then "apply" it. I get state and prove, but my textbook doesn't explicitly state how to "apply" a loop invariant.
1
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1answer
36 views
When are invariants true inside of a loop?
I am new to the idea of invariants and hope to find more information about it.
It can make an algorithm or loop show a high level of certainty / confidence that the code is correct, such as in the ...
0
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1answer
24 views
Loop invariant initialisation confusion
Consider the algorithm LastMatch below, which returns the offset (shift) of the last
occurrence of the pattern P in text T, or -1 if P does not occur in T:
...
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0answers
43 views
Proof that this sorting algorithm sorts the input
I'm given this "sorting" algorithm and now I'm supposed to prove, that if given an array of integers of length $n$, sort(A,0,n-1) will sort it.
...
1
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3answers
152 views
Dealing with test condition '=' for a while loop when determining a bound function/loop variant
The following is the definition of what a bound function for a while loop must satisfy:
The bound function is an integer-valued, total function of some of the inputs, variables and global data that ...
2
votes
1answer
59 views
How to prove a bound function for a sequence of numbers?
Let $G_n$ be defined by
$$G_n = \begin{cases} 1 & n=0 \\
2 & n = 1 \\
3 & n = 2 \\
4 & n = 3 \\
2G_{n-1}-2G_{n-3}+G_{n-4} & n\geq4
\end{cases}$$
How can I prove that $f(n) = n$...
0
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1answer
63 views
Loop Invariant Code Motion - am I missing something?
I've been implementing LICM for a project, and came upon a strange observation.
Let's say we have a loop
int i = 10;
while (i > 0) {
a = 2;
i--;
}
...
0
votes
1answer
218 views
Loop-Invariant for a program fragment
Q) Consider the following program fragment:
var x,y:integer;
x:=1; y:=0;
while y < x do
begin
x:=2āx;
y:=y+1;
end;
For the above fragment , ...
0
votes
1answer
59 views
Finding invariant when detecting a cycle
Let consider a connected graph $G = (V, E)$ which is not oriented. One way to detect a cycle in such a graph is :
Create an array : seen of size $\mid V \mid$ with seen[i] = false for all $i$
...
1
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1answer
164 views
Proving correctness of the Newton's Method for finding the square root of a number
I'm trying to prove the correctness of this simple square root calculation algorithm using SPARK:
...
0
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1answer
136 views
Find an invariant in the minimum algorithm
I have the following simple algorithm to find the smallest element of an array $A$ of numbers:
...
2
votes
2answers
439 views
Iterative Fibonacci algorithm correctness proof, finding loop invariants
The algorithm take in an integer $n$ and outputs the $n$th number in the Fibonacci sequence ($F_n$). The sequence starts with $F_0$. I am trying to prove the correctness assuming valid input:
...
1
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1answer
16 views
How to understand out of bound in the following theoretical context?
Consider the following initialization step of loop invariant for merge procedure
Initialization: Prior to the first iteration of the loop, we have
$k=p$, so that the subarray $A[p .. k - 1]$ is ...
0
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0answers
29 views
Equivalent Algorithm with Sharman Morrison inversion
I am trying to invert a matrix using Woodbury identity. The inversion using Cholesky decomposition has the following pseudo-code:
For $t=1,2,...$
$(1)\;\; \text{Read}\;x_t\in\mathbb{R}^n$ ...
1
vote
0answers
35 views
Does code motion/hoisting happen on if statements/branches? [closed]
Consider the following C code:
for(int i=0; i<100000; i++) {
if (operation == 0) {
a[i] *= 2;
} else {
a[i] += 1;
}
}
Would a real ...
2
votes
1answer
195 views
Loop Invariant summation
So I am stuck:
I have this algorithm from which I need to find a loop invariant but I just can't get my head around it :
...
1
vote
1answer
88 views
Finding a strong loop invariant
Pseudocode:
if n < 2
return false
while n != 1
if n % 2 != 0
return false
n = n/2
return true
The loop will terminate when n is odd. If n =...
1
vote
1answer
515 views
Is Loop Invariant Proof a form of Induction?
As far as I see, what computer scientists refer to as loop invariant proofs are exact replicas of induction proof. Is it true? Can I state that loop invariant proof implies an induction? Is there a ...
3
votes
2answers
227 views
Invariant on “Find K Closest Elements” problem
I run across this problem:
Given a sorted array, two integers k and x, find the k closest ...
1
vote
1answer
123 views
Loop termination - Loop invariant
(x >= 0 && y >= 0)
q = 0;
r = x;
while ( r >=y ) {
r = r - y;
q = q + 1;
}
(x = q*y +r) && (r >= 0) && (r < y)
For ...
2
votes
1answer
171 views
Find the loop invariant of the given while loop
I don't know how to find a loop invariant. I'm not sure where to start. Can anyone find the loop invariant of the given program and explain your method please.
...
0
votes
1answer
68 views
Inference rules for deriving invariants in Hoare logic
The following algorithm is supposed to compare two strings $S_1$ and $S_2$ ("/\" for empty string):
...
0
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0answers
53 views
Loop invariant for
The programme returns the number of digits of an integer $n>0$.
I still have some difficulties to understand the difference between the loop invariant condition and what the loop should actually ...
4
votes
1answer
96 views
Developing invariants for comparing two strings
The following algorithm is supposed to compare two strings $S_1$ and $S_2$ ("/\" for empty string):
...
0
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1answer
309 views
Loop invariant condition IsPrime program
I'm new to the concept of loop invariant and I'm trying to figure out the loop invariant for a program that returns if an integer is prime and, if not, one possible factorization.
My intuition is that ...
2
votes
1answer
589 views
Proving correctness of an iterative Fibonacci algorithm
One of the questions in the problem sets that I'm struggling in is this specific number that asks me to prove an iterative Fibonacci algorithm. The algorithm is written below:
...
0
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0answers
194 views
Counting the number of occurences - loop invariant
I'm trying to come up with loop invariant for the following program.
k = a[0]
m = 1
p = 1
while p < n:
if a[p] == k:
m += 1
p += 1
return m
I ...
1
vote
1answer
557 views
How to get Loop invariants to prove program is correct in Hoare logic
start off stating the definition of a loop invariant. It is a condition that must be true at the start of the loop and must be true after each iteration of the loop. I understand what it is but i have ...
0
votes
2answers
41 views
Deterministic Nonfinite Automaton - Longest Block of Ones
How to Think About Algorithms - Jeff Edmonds
Exercise 2.2.2
The problem asks to find $\sum$, Q, Ī“, s, and F of this program if we see it as an automaton. Edmonds characterized this as deterministic ...
3
votes
0answers
382 views
Expression of the weakest precondition of a while loop
I am interested in computing weakest preconditions (WP) of loops.
If I refer to Wikipedia "Predicate Transformer semantics", the WP for e.g. total correction of a loop annotated with an invariant I ...
1
vote
0answers
80 views
Hoare correctness proof for a recursive definition of multiplication
Given the program:
{y=y0 ^ y>=0}
z=0;
while (y>0){
z=z+x; (1)
y=y-1;
}
{z=x*y0}
I am having trouble finding the ...
1
vote
1answer
83 views
How to create an algorithm when given the invariant
I am given the following invariant:
Invariant The greatest $i$ keys of an Array are always in sorted order on the last $i$ indices of the Array.
I am supposed to ...
1
vote
1answer
132 views
What is the loop invariant of the following function?
x = n; y = 0
while x >= b:
x = x DIV b
y = y + 1
return y
This function takes in $n,b\in\mathbb{N}, n > 0, b > 1$, and returns $k\in\mathbb{N}$ ...
0
votes
2answers
213 views
Loop invariant for adding 2 arrays storing binary value of integer
Question:
Given 2 arrays of size $n$, $A = \langle a_1,a_2,\dots,a_n \rangle$ and $B = \langle b_1,b_2,\dots,b_n\rangle$, where $A$ and $B$ store the binary value of an integer. Find $A + B$ and ...
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votes
2answers
298 views
Loop invariant for a while loop
I need help understanding why the two statements I have underlined with red are correct. I am confused as to how we can get $n + 1$.
And how $i \le n$ is true when it says $i < n$ in the while.
3
votes
1answer
681 views
Loop invariant for a division algorithm
I'm having problems trying to understand the concept of loop invariants.
I have the following code, where M and N are predefined constants.
...
2
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1answer
316 views
0
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1answer
54 views
Verification condition in case of array theory
As far as i can understand the problem of checking safety property, can be reduce to solving for inductive invariants in VC's and there is following parts to it:
Generate VC's from the program.
Solve ...
2
votes
1answer
879 views
Proving the loop invariant for a simple program in Hoare logic
I was studying loop invariant and came across Tomas Petricek's example. Here's the equivalent(I believe) program after I revised it a bit for proof in Hoare logic:
...
2
votes
0answers
711 views
Finding the weakest precondition and invariant of while loop
I am trying to find the weakest precondition and invariant of this while loop. I have read and am trying to understand the concept of calculating preconditions based on postconditions. I was wondering ...
0
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1answer
107 views
