## Wednesday, June 29, 2011

### Blogging my way through CLRS [part 4]

Part 3 here This set is a bit easier than last time.
Question 2.2-1:Express the function $$\frac{n^3}{1000} - 100n^2 - 100n + 3$$ in terms of $\Theta$ notation
A function g(x) is said to be in the set of all functions $\Theta(x)$ if and only if g(x) is also in the set of all functions $\Omega(x)$ and in the set of all functions $O(x)$.
Symbolically: $$g(x) \in \Theta(x) \iff g(x) \in O(x) \And g(x) \in \Omega(x)$$

A function g(x) is in the set of all functions $\Theta(x)$ if and only if after some constant $c$ it is always true that for some constant C, $g(x) \lt Cf(x)$

A function g(x) is in the set of all functions O(x) if and only if after some constant $c$ it is always true that for some constant C, $g(x) \gt Cf(x)$

With our function we could choose practically any function to satisfy either one of these conditions. However we need to satisfy both of them. One thing that makes this easier is that it only has to be true after some constant number. This allows us to throw away the "trivial" parts that are eventually overwhelmed by the faster growing terms. We therefore are only left with $n^3$, which is the answer.

Question 2.2-2: Consider sorting n numbers stored in an array A by first finding the smallest element and exchanging it with the element in A[1], then find the second smallest element and exchange it with A[2], and continue this for the first n-1 elements of A. Write the pseudocode for this algorithm, which is known as Selection Sort. What loop invariant does this algorithm maintain? Why does it need to run only for the first n-1 elements and not for all n? Give the best case and worst case running times in $\Theta$ notation
This question is asking us to analyze selection sort in a variety of ways. I will start with writing out the pseudocode: for $j \leftarrow 1$ to $n-1$
min $\leftarrow$ j
for $i \leftarrow j+1$ to $n$
$\rhd$ if A[i] < A[min] then min $\leftarrow$ i
$\rhd$ if min $\neq$ j then swap A[min] and A[j]
A loop invariant that this algorithm maintains is that every elements prior to A[j] is sorted among the subarray A[1] to A[j] and is less than or equal to every element in the subarray A[j+1] to A[n]. I do not believe a stronger loop invariant is provable. The algorithm only needs to run until n-1 because of the second part of the loop invariant. When $j = n-1$ we know that every element after A[j], which is A[n] is not less than all previous elements. Therefore no check has to be done. In the best case (an already sorted array) and in the worst case (a reverse sorted array) the running time is the same: $\Theta(n^2)$
Question 2.2-3: Consider linear search again. How many elements of the input sequence need to be checked on average, assuming that the element being searched for is equally likely to be any element in the array? How about in the worst case? What are the average-case and worst-case running times of linear search in $\Theta$ notation?
The best case for a linear search algorithm is when the searched-for element is in the first location. In the worst case all n locations must be searched. In the average case $\frac{n}{2}$ locations have to be searched.
Question 2.2-4: How can we modify almost any algorithm to have a good best-case running time?
I have no idea what this question is asking for. I guess checking for the optimal case (as in a pre-sorted array for a sorting algorithm) and then skipping the rest of the procedure might work.

#### 1 comment:

1. http://arxiv.org/abs/0909.3346

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