cse 373: data structures and algorithms lecture 6: searching 1

CSE 373: Data Structures and Algorithms

Lecture 6: Searching

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Searching and recursion• Problem: Given a sorted array a of integers and

an integer i, find the index of any occurrence of i if it appears in the array. If not, return -1.– We could solve this problem using a standard

iterative search; starting at the beginning, and looking at each element until we find i

– What is the runtime of an iterative search?

• However, in this case, the array is sorted, so does that help us solve this problem more intelligently? Can recursion also help us?

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Binary search algorithm

• Algorithm idea: Start in the middle, and only search the portions of the array that might contain the element i. Eliminate half of the array from consideration at each step.– can be written iteratively, but is harder to get right

• called binary search because it chops the area to examine in half each time– implemented in Java as method Arrays.binarySearch in java.util package

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Binary search example

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Binary search pseudocodebinary search array a for value i: if all elements have been searched, result is -1. examine middle element a[mid]. if a[mid] equals i, result is mid. if a[mid] is greater than i, binary search left half of a for i. if a[mid] is less than i, binary search right half of a for i.

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Runtime of binary search• How do we analyze the runtime of binary search and

recursive functions in general?

• binary search either exits immediately, when input size <= 1 or value found (base case),or executes itself on 1/2 as large an input (rec. case)– T(1) = c– T(2) = T(1) + c– T(4) = T(2) + c– T(8) = T(4) + c– ...– T(n) = T(n/2) + c

• How many times does this division in half take place?

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Divide-and-conquer• divide-and-conquer algorithm: a means for

solving a problem that first separates the main problem into 2 or more smaller problems, then solves each of the smaller problems, then uses those sub-solutions to solve the original problem– 1: "divide" the problem up into pieces– 2: "conquer" each smaller piece– 3: (if necessary) combine the pieces at the end to

produce the overall solution

– binary search is one such algorithm

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Recurrences, in brief• How can we prove the runtime of binary search?

• Let's call the runtime for a given input size n, T(n).At each step of the binary search, we do a constant number c of operations, and then we run the same algorithm on 1/2 the original amount of input. Therefore:

– T(n) = T(n/2) + c– T(1) = c

• Since T is used to define itself, this is called a recurrence relation.

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Solving recurrences• Master Theorem:

A recurrence written in the formT(n) = a * T(n / b) + f(n)

(where f(n) is a function that is O(nk) for some power k)has a solution such that

• This form of recurrence is very common for divide-and-conquer algorithms

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Runtime of binary search• Binary search is of the correct format:

T(n) = a * T(n / b) + f(n)

– T(n) = T(n/2) + c– T(1) = c

– f(n) = c = O(1) = O(n 0) ... therefore k = 0– a = 1, b = 2

• 1 = 20, therefore:T(n) = O(n0 log n) = O(log n)

• (recurrences not needed for our exams)

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Why Sorting?

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• Practical application– People by last name– Countries by population– Search engine results by relevance

• Fundamental to other algorithms

• Different algorithms have different asymptotic and constant-factor trade-offs– No single ‘best’ sort for all scenarios– Knowing one way to sort just isn’t enough

• Many to approaches to sorting which can be used for other problems

Problem statement

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There are n comparable elements in an array and we want to rearrange them to be in increasing order

Pre:– An array A of data records– A value in each data record– A comparison function

• <, =, >, compareTo

Post:– For each distinct position i and j of A, if i < j then A[i] A[j]

– A has all the same data it started with

Sorting Classification

In memory sorting External sorting

Comparison sorting(N log N)

Specialized Sorting

O(N2) O(N log N) O(N)# of tape accesses

• Bubble Sort• Selection Sort• Insertion Sort• Shellsort Sort

• Merge Sort• Quick Sort• Heap Sort

• Bucket Sort• Radix Sort

• Simple External Merge Sort• Variations

in place? stable?

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Comparison Sorting

comparison-based sorting: determine order through comparison operations on the input data: <, >, compareTo, …

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Bogo sort

• bogo sort: orders a list of values by repetitively shuffling them and checking if they are sorted

• more specifically:– scan the list, seeing if it is sorted– if not, shuffle the values in the list and repeat

• This sorting algorithm has terrible performance!– Can we deduce its runtime?

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Bogo sort codepublic static void bogoSort(int[] a) { while (!isSorted(a)) { shuffle(a); }}

// Returns true if array a's elements// are in sorted order.public static boolean isSorted(int[] a) { for (int i = 0; i < a.length - 1; i++) { if (a[i] > a[i+1]) { return false; } } return true;}

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Bogo sort code, helpers// Shuffles an array of ints by randomly swapping each// element with an element ahead of it in the array.public static void shuffle(int[] a) { for (int i = 0; i < a.length - 1; i++) { // pick random number in [i+1, a.length-1] inclusive int range = a.length-1 - (i + 1) + 1; int j = (int)(Math.random() * range + (i + 1)); swap(a, i, j); }}

// Swaps a[i] with a[j].private static void swap(int[] a, int i, int j) { if (i == j) return;

int temp = a[i]; a[i] = a[j]; a[j] = temp;}

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Bogo sort runtime

• How long should we expect bogo sort to take?– related to probability of shuffling into sorted order– assuming shuffling code is fair, probability equals

1 / (number of permutations of n elements)

– bogo sort takes roughly factorial time to run• note that if array is initially sorted, bogo finishes

quickly!– it should be clear that this is not satisfactory...

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O(n2) Comparison Sorting

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Bubble sort

• bubble sort: orders a list of values by repetitively comparing neighboring elements and swapping their positions if necessary

• more specifically:– scan the list, exchanging adjacent elements if they are not

in relative order; this bubbles the highest value to the top– scan the list again, bubbling up the second highest value– repeat until all elements have been placed in their proper

order

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• Traverse a collection of elements– Move from the front to the end– "Bubble" the largest value to the end using pair-

wise comparisons and swapping

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Swap42 77

"Bubbling" largest element

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512357742 101

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No need to swap

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Largest value correctly placed

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Bubble sort codepublic static void bubbleSort(int[] a) { for (int i = 0; i < a.length; i++) { for (int j = 1; j < a.length - i; j++) { // swap adjacent out-of-order elements if (a[j-1] > a[j]) { swap(a, j-1, j); } } }}

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Bubble sort runtime• Running time (# comparisons) for input size n:

– number of actual swaps performed depends on the data; out-of-order data performs many swaps

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Selection sort• selection sort: orders a list of values by

repetitively putting a particular value into its final position

• more specifically:– find the smallest value in the list– switch it with the value in the first position– find the next smallest value in the list– switch it with the value in the second position– repeat until all values are in their proper places

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Selection sort example

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Index0 1 2 3 4 5 6 7

Value27 63 1 72 64 58 14 9

1st pass1 63 27 72 64 58 14 9

2nd pass1 9 27 72 64 58 14 63

3rd pass1 9 14 72 64 58 27 63

…

Selection sort example 2

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Selection sort codepublic static void selectionSort(int[] a) { for (int i = 0; i < a.length; i++) { // find index of smallest element int min = i; for (int j = i + 1; j < a.length; j++) { if (a[j] < a[min]) { min = j; } }

// swap smallest element with a[i] swap(a, i, min); }}

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Selection sort runtime• Running time for input size n:– in practice, a bit faster than bubble sort. Why?

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Insertion sort

• insertion sort: orders a list of values by repetitively inserting a particular value into a sorted subset of the list

• more specifically:– consider the first item to be a sorted sublist of length 1– insert the second item into the sorted sublist, shifting the

first item if needed– insert the third item into the sorted sublist, shifting the

other items as needed– repeat until all values have been inserted into their proper

positions

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Insertion sort• Simple sorting algorithm.– n-1 passes over the array– At the end of pass i, the elements that occupied

A[0]…A[i] originally are still in those spots and in sorted order.

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Insertion sort example

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Insertion sort codepublic static void insertionSort(int[] a) { for (int i = 1; i < a.length; i++) { int temp = a[i];

// slide elements down to make room for a[i] int j = i; while (j > 0 && a[j - 1] > temp) { a[j] = a[j - 1]; j--; }

a[j] = temp; }}

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Insertion sort runtime• worst case: reverse-ordered elements in

array.

• best case: array is in sorted ascending order.

• average case: each element is about halfway in order.

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Comparing sorts

• We've seen "simple" sorting algos. so far, such as:– selection sort– insertion sort

• They all use nested loops and perform approximately n2 comparisons

• They are relatively inefficient

comparisons swaps

selection n2/2 n

insertionworst: n2/2best: n

worst: n2/2best: n

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Average case analysis• Given an array A of elements, an inversion is an

ordered pair (i, j) such that i < j, but A[i] > A[j]. (out of order elements)

• Assume no duplicate elements.

• Theorem: The average number of inversions in an array of n distinct elements is n (n - 1) / 4.

• Corollary: Any algorithm that sorts by exchanging adjacent elements requires O(n2) time on average.

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Shell sort description• shell sort: orders a list of values by comparing

elements that are separated by a gap of >1 indexes– a generalization of insertion sort– invented by computer scientist Donald Shell in 1959

• based on some observations about insertion sort:– insertion sort runs fast if the input is almost sorted– insertion sort's weakness is that it swaps each

element just one step at a time, taking many swaps to get the element into its correct position

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Shell sort example

• Idea: Sort all elements that are 5 indexes apart, then sort all elements that are 3 indexes apart, ...

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Shell sort codepublic static void shellSort(int[] a) { for (int gap = a.length / 2; gap > 0; gap /= 2) { for (int i = gap; i < a.length; i++) { // slide element i back by gap indexes // until it's "in order" int temp = a[i]; int j = i; while (j >= gap && temp < a[j - gap]) { a[j] = a[j – gap]; j -= gap; } a[j] = temp; } }}

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Sorting practice problem• Consider the following array of int values.

[22, 11, 34, -5, 3, 40, 9, 16, 6]

(a) Write the contents of the array after 3 passes of the outermost loop of bubble sort.

(b) Write the contents of the array after 5 passes of the outermost loop of insertion sort.

(c) Write the contents of the array after 4 passes of the outermost loop of selection sort.

(d) Write the contents of the array after a pass of bogo sort. (Just kidding.)

cse 373: data structures and algorithms lecture 6: searching 1

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