Questions tagged [hoare-logic]
Questions about Hoare's logical framework for program correctness proofs and variants.
66
questions
0
votes
0answers
12 views
Proving a Hoare Triple does not hold
I have this program specification annotated with a Hoare Triple. Trying to prove that is the case.
...
0
votes
1answer
24 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 ...
1
vote
0answers
95 views
How to prove program solves the problem?
I am preparing for the exam on Theory of Programming class. Now I am trying to solve the task from the sample paper:
Task description starts here
Given the following problem:
A problem is given by ...
0
votes
0answers
32 views
Using Hoare logic to show an invariant holds or using induction?
I want to know if given a while loop:
x = 0
while(x < 5){
x = x + 1
}
I want to show that x (at the a ith iteration of the loop), the value of i is ...
0
votes
1answer
72 views
Confusion about assignment axiom in Hoare logic
I wanted to know if we are given the f.f.g. Hoare triple:
{x = 43}x := x + 1{x = 44}
How do we show that this is a valid Hoare triple?
My attempt was:
Using the assignment axiom:
{x + 1 = 43} x := ...
0
votes
0answers
20 views
How to solve Hoare's problem when precondition contains meta symbols?
This is the proram for which I have to prove correctness using Hoare's Axioms:
{X = |x|}
if(x<=0)
x:=-x;
else
skip;
{X=x}
This is my solution so ...
2
votes
0answers
22 views
Formal semantics of a mutable/imperative stack
When introducing formal semantics for data structures, immutable stacks are a nice simple example :
$\mathit{is\_empty}(\mathit{create()})=\mathrm{True}$
$\mathit{is\_empty}(\mathit{push}(e, s))=\...
0
votes
1answer
94 views
Hoare triple: Loop invariant and correctness
The following Hoare triple in which variable a is an array of integers, and len, max, i, n, j and m are integer-valued variables. Provide a loop invariant (using predicate logic) suitable for proving ...
2
votes
0answers
85 views
Hoare triple: Loop invariant and partial correctness
Below there is Hoare triple in which variable $a$ is an array of integers, $len$, $x, i$ are integer-valued variables, and $r$ is a Boolean-valued variable. I have to provide a loop invariant (using ...
1
vote
0answers
46 views
Can we ignore the postcondition in the Hoare conditional rule when there is a return statement?
I'm proving the correctness of naive string matching using Hoare logic. I have the following pseudocode:
...
2
votes
1answer
114 views
Hoare logic, proving conjunction rule from basic rules, possible or not?
(This is HW.) Suppose I have these following proof rules given.
I am currently considering if I can prove the conjunction rule (given below) from the above given Hoare logic rules.
My answer would ...
3
votes
1answer
140 views
The Law of Excluded Miracle in the language of guarded commands
The definition of weakest precondition is familiar (let me use Isabelle's syntax here):
definition "wp c Q s ≡ ∃t. (c,s) ⇒ t ∧ Q t"
the weakest precondition ...
2
votes
1answer
37 views
Bottleneck in Hoare Logic unable to arrive at my {P} from {Q}
{Q} = {n>0}
C1 = i := 1;
C2 = c := 1;
C3 = p := 0;
{P} = {i<=n, p = fib(i-1), c = fib(i)}
My lack of understanding towards the rule of consequence in hoare logic is blocking me from find the ...
0
votes
1answer
50 views
How to solve for the precondition give a postcondtion that must satisfy two conditions
I tried solving past exam question but there is this one that I haven't been able to solve.
The question states that a suitable precondition should be found for the statement.
$$
a= i +2; i++\{(a = ...
1
vote
2answers
53 views
How to prove a side effect in a function
I asked a question earlier about Saving to the Database, which was very general and about the requirements for a proof when you go through many layers of non-verified systems such as the network and ...
1
vote
1answer
88 views
How to prove $c = a + b$ using Program Verification Techniques
I am trying to prove an elementary thing, but it seems at some point you get down to atoms where you can't prove anything else. This is why I am wondering about proving $c = a + b$, it seems like an ...
4
votes
2answers
108 views
The difference between a Hoare Triple/Assertion and a Typed Function
I have been trying to wrap my head around applying Hoare Logic and am running into the question of how Hoare triples are any different from (simply) a typed function.
That is, say you have a typed ...
1
vote
1answer
168 views
How to use Hoare Logic to Prove this Assertion
Given this assertion in Hoare Logic:
\begin{align}
&\mathbf{\{p >= 0\}}\\
&s = 0 ; n = 1 ;\\
&\mathtt{while}\ (n <= p)\ \{\\
&\quad s = s + n ;\\
&\quad n = n + 1\\
&\}\\...
2
votes
1answer
161 views
Understanding Hoare Logic Axioms
Given these 5 axioms of Hoare Logic:
\begin{array}{cl}
\frac{}{\{\phi([x \leftarrow E])\}\ x := E\ \{\phi(x)\}} & \mathtt{Assignment}\\\\
\frac{\{\phi\}\ P_1\ \{\eta\} \quad \{\eta\}\ P_2\ \{\psi\...
1
vote
0answers
73 views
How to apply Hoare Logic to this function
I would like to apply Hoare Logic to realistic, complex imperative programs that use while loops, if statements, functions, modules, etc. and have side effects (e.g. access the network). In order to ...
1
vote
1answer
21 views
Hoare-Logic: Requirements for imperfect data types
Theoretically, Hoare-Logic let's one prove the correctness of an algorithm, given pre- and post-condition.
However, as far as I've seen it so far, one idealizes his data-types to a mathematical set ...
1
vote
1answer
99 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
145 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
61 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):
...
4
votes
1answer
91 views
Developing invariants for comparing two strings
The following algorithm is supposed to compare two strings $S_1$ and $S_2$ ("/\" for empty string):
...
1
vote
1answer
62 views
what is the 'x :=' part mean in a hoare triple?
In Hoare logic, there's a thing called a Hoare triple, e.g.$$
\begin{array}{ccccc}
\{x = 2\} & & x := x+1 & & \{x = 3\}
\end{array}.
$$
What does '$x :=$' mean?
1
vote
2answers
900 views
how to solve Hoare logic problems
I'm having trouble proving Hoare logic questions as I'm not sure of the process that is taken to prove them. I understand that they're rules such as assignment axiom, pre-condition strengthening, post-...
0
votes
0answers
139 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
167 views
True Postcondition, with true Precondition
In my exam preparation I stumbled across the follwoing exercise regarding pre and postconditions:
As far as I understood the question, we need to express some condition for p, such that if that ...
0
votes
1answer
3k views
Finding weakest precondition
I have the following program
x := y + 1;
if (y > 0) then
x := x + y
else x := y + 100;
x := x + y;
I want to compute the weakest precondition for getting <...
1
vote
1answer
440 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 ...
4
votes
1answer
122 views
Continuation-passing style: what is meant by “CPS'ing”?
I'm reading Dijkstra Monads for Free for a presentation I'll be doing and it's pretty meaty. One of the things that I keep running into is the term "CPS'ing". I've read a little bit on continuation-...
3
votes
0answers
284 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
72 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 ...
0
votes
2answers
62 views
Using the Consequence Rule
I have the following example that I have to prove
{a>7 ^ b>=0} n:=a-b {n<a ^ a+b>=0}
Using the Consequence Rule I assumed that the P is true
...
1
vote
1answer
110 views
How to prove the equivalence between Hoare and Floyd assignment axioms?
How to show that these two axioms are equivalent:
1: $\{G[v/e]\} v:=e \{G\}$
2: $\{F\} v:=e \{\exists v' (F[v/v'] \land v=e[v/v'])\}$
I've tried with $G = \exists v' (F[v/v'] \land v=e[v/v']) $and ...
5
votes
1answer
123 views
Relation between Hoare Type Theory and pointers
My understanding is that in Hoare Type Theory every imperative statement has a type of the form {Pre}res:T{Post} where T is the ...
0
votes
1answer
136 views
Why does (y < 10) imply everywhere (x > 0 ^ y < 10)?
My lecturer recently released solutions to an assignment. One of the questions was to determine the weakest precondition of:
...
7
votes
1answer
298 views
Hoare logic - total correctness of loops
Consider a while loop of the form :
$\texttt{while (C) {S}}$
with $\texttt{C}$ the condition and $\texttt{S}$ the body of the loop.
Let $\texttt{I}$ and $\texttt{V}$ respectively be an invariant ...
7
votes
1answer
217 views
What was the major breakthrough between Hoare-Floyd logic and Scott–Strachey semantics?
I'm reading through a commentary on Milner's "The use of machines to assist in rigorous proof" by Mike Gordon. In this paper, he explains how LCF was born from the ideas of denotational semantics by ...
2
votes
1answer
843 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:
...
18
votes
5answers
5k views
Example of an algorithm that lacks a proof of correctness
We have Hoare logic. Why is it still possible that an algorithm is right but there is no proof that it's correct? Suppose the algorithm is expressed in C. Then we can argue step by step that it's ...
5
votes
3answers
604 views
Intuitive explanation of Hoare assignment axiom
$\small\textit{''The obvious things are the most difficult to understand''}$
May be the question does not make sense, but let me ask it anyway.
The Hoare assignment axiom is
$$
\dfrac{}{\{Q[v \...
3
votes
1answer
46 views
What's the meaning of the $\top$ symbol in a Hoare triple?
I'm studying program verification and came across the following triple:
$$ \{\top\} \;P \; \{y=(x+1)\} $$
What's the meaning of the $\top$ symbol on the precondition? Does it mean $P$ can take any ...
4
votes
2answers
2k views
What does it mean to “strengthen the precondition and weaken the postcondition” in Hoare logic?
Having learned a rough summary of Hoare logic (i.e. learning just the basic concept of Hoare triples and a few of the rules) I kept seeing a statement along these lines:
The rule of consquence ...
-1
votes
1answer
42 views
Does this program satisfy the postcondition?
Does the following program $P$:
a = 2
b = a + 3
c = a * b
Satisfy the following formula?
$$\{ \top \} \; P \; \{ a < (b - 2) + c \}$$
I want to use integers ...
0
votes
0answers
358 views
How can I find the strongest postcondition for those two Hoare triples?
I'm trying to solve an exercise in which I have to find the strongest post-condition of the two Hoare triples.
\begin{gather*}
(| a=9 |)\ a=2; b=a+1; a=b*b;\ (| ?? |)\\
(| i=-j |)\ i=i+1; j=j-...
1
vote
0answers
23 views
How to define the dimension function for searching in a 3D-array?
Problem:
Propose a plan (pre- and post-condition, invariant, dimension function) for searching a value in a 3D-array.
Solution:
This is the first solution I've foreseen:
$[\text{Ctx}\ C: m\...
2
votes
2answers
70 views
How to change the algorithm implication by changing the invariant?
I have this specification in GCL:
$[Ctx C: n\geqslant 0\ \wedge\ b:[0..n-1]\ \text{of int}$
$\{Q:\text{True}\}$
$sum,i:=0,0;$
$\{\text{Invariant}\ P: 0\leqslant i \leqslant n\ \wedge sum=\sum_{j=0}...
3
votes
1answer
863 views
What is a predicate transformer?
I'm reading Programming - The derivation of algorithms, and I want to understand the purpose of a predicate transformer. This is the excerpt (p. 14-15):
A more precise way in which constructs may ...
