Write the pseudocode for linear search, which scans through the sequence, looking for ν. Using a loop invariant, prove that your algorithm is correct. Make sure that your loop invariant fulfills the three necessary properties.

Question

Computers and Technology

Nicholas Lawson

20 December, 13:38

Write the pseudocode for linear search, which scans through the sequence, looking for ν. Using a loop invariant, prove that your algorithm is correct. Make sure that your loop invariant fulfills the three necessary properties.

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Answers (1)

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Matthew Glover · Answer 1

1 for i = 1 to A. length

2 if A[i] = nu

3 return i

4 return NIL

Explanation:

Loop invariant:

At the start of each iteration of the for loop of lines 1-3, there is no j
Initialization:

At the beginning of the first iteration, we have i=1, so there is no j
Maintenance:

We fix i and assume there is no j
If A[i]=ν, then we return a value, so then there are no more iterations, so the property is preserved.

If A[i]≠ν, then there is no j
Termination:

The loop terminates either for i=A. length+1, or if ever we encounter A[i]=ν.

In the first case, then there is no 1≤j≤A. length such that A[j]=ν, and we are correctly returning NIL

In the second case, if we encounter some i such that A[i]=ν, we are correctly returning i.