COMP 14 Introduction to Programming Miguel A. Otaduy June 4, 2004

Today Review arrays Searching arrays for a particular value Sorting arrays

Exercises 1. Find Sum and Average of Array 2. Determine Largest and Smallest Elements in Array 01 23

Example double[] sale={3.5, 4.6, 5.2, 3.8}; double sum = 0; double average; Find Sum and Average of Array 01 23

Example double[] sale={3.5, 4.6, 5.2, 3.8}; double sum = 0; for(int ind = 0; ind < sale.length; ind++) { sum = sum + sale[ind]; } double average; if(sale.length != 0) average = sum / sale.length; else average = 0.0; Find Sum and Average of Array 01 23

Example double[] sale={3.5, 4.6, 5.2, 3.8}; int maxIndex = 0, minIndex = 0; double largestSale; double smallestSale; Determining Largest/Smallest Element in Array 01 23

Example double[] sale={3.5, 4.6, 5.2, 3.8}; int maxIndex = 0, minIndex = 0; for (int ind = 1; ind < sale.length; ind++) { if (sale[ind] > sale[maxIndex]) maxIndex = ind; else if (sale[ind] < sale[minIndex]) minIndex = ind; } double largestSale = sale[maxIndex]; double smallestSale = sale[minIndex]; Determining Largest/Smallest Element in Array 01 23

Searching Arrays Find one/several particular element(s) in an array of many elements Complexity (How Long To Search?) –find a parking space - linear –look up a word in a dictionary - complex 500K+ words in Oxford Dictionary search - very complex over 3 trillion web pages

Time Complexity Important feature in Computer Science research: –How long does an algorithm take to complete? –Given an input (e.g. an array) of size n, how long does it take for the algorithm (e.g. search for a particular value) to complete? Linear algorithm: time = k 1 * n Quadratic algorithm: time = k 2 * n 2 etc.

Linear Searching Algorithm: –Get a test value and a list of values list can be ordered or unordered –loop through the list repeatedly ask: Is this a match? quit when the answer is yes (use break stmt) –if you finish all items, there is no match Inefficient –worst time to search is the length of the list Relatively easy to program

Example int[] list={3, 6, 27, 8, 33, 54, 23}; int foundAt = -1, element = 33; Linear Search 01 23

Example int[] list={3, 6, 27, 8, 33, 54, 23}; int foundAt = -1, element = 33; for(int i=0; i<list.length; i++) { if(list[i] == element) { foundAt = i; break; } } Linear Search 01 23

Array of Objects Declare array of Student objects (ref. variables) Instantiate array of size 10 Instantiate each of the Student objects –Ask for age (int) and name (String) –Instantiate object

Example Student[] students; students = new Student[10]; for(int i = 1; i < students.length; i++) { //get int age //get String name students[i]=new Student(name, age); } Array of Student objects 01 23

Example int foundAt=-1; String name=“Mark”; for(int i = 1; i < students.length; i++) { if(students[i].getName().equals(name)) { fountAt=i; break; } Search for student Mark 01 23

Binary Search Requires ordered (sorted) list Set searchRange to the entire list Repeat: –pick a “test value” in the middle of searchRange –if test value == value searching for Stop! –if test value > value searching for searchRange = lower half of searchRange –if test value < value searching for searchRange = upper half of searchRange

Example Looking for 46 Trial

Sorting Sort students by birthdate Get a group of 5 students Another student will sort them by birth date Analyze the sorting strategy –Can we devise an algorithm following that strategy? Goal: sort following a methodology –We should be able to write it as an algorithm and then program it Demonstrate selection sort Write algorithm

Selection Sort General Algorithm Scan the list to find the smallest value Swap that value with the value in the first position in the list Scan rest of list to find the next smallest value Swap that value with the value in the second position in the list And so on, until you get to the end of the list

Selection Sort Methods Scan the list to find the smallest value Swap that value with the value in the first position in the list Scan rest of list to find the next smallest value Swap that value with the value in the second position in the list And so on, until you get to the end of the list loop

Selection Sort Sorts in ascending order Can be changed to sort in descending order –look for max instead of min

Homework 6 Read/write files Array of ints Array of Participant objects Next: –Sort array of ints –Sort array of participants

Monday Java applets HTML GUIs Reading Assignment: –Chapter 6 (pp ) –skim Chapter 13 (pp )