Lec 21 More Fun with Arrays: For Loops

Agenda Some backfill for Lab 20: – Using an array in an applet or class – array instance variables – using Math.random() just like you did in Lab5Lab5 index = Math.round(Math.random()*adjectives.length ); For Loops vs While loops – not that big a deal For Loops and Arrays – more ways to process Frogs and Toads – Pad class

For loops Demo While loop that prints the numbers 1 to 10 – For loop that does the same thing While loop that adds the numbers from 50 to 60 – For loop that does the same thing Declaring the loop counter in the for header – scope limited to loop "body" Changing the loop increment Ascending or descending loops Nested for loops – used to draw a grid of squares

4 Arrays – Using for loops For loops are a more compact way of processing arrays Instead of this: System.out.print(a[0]); System.out.print(a[1]); System.out.print(a[2]); System.out.print(a[3]); System.out.print(a[4]); System.out.print(a[5]); You can do this: for (k=0; k<6; k++) System.out.print(a[k]);

For Loop and Array Demo Assigning a sequence of values to an array – suppose we want data to have 10,20,30,40,50 Putting a set of random values into an array Defining array cell k using previous cell k-1 – new value = old value / 2 Counting how many 3's are in an array

Searching for a value in an array Suppose you want to know if 13 is in the data array. Naive approach: – for (int k=0; k<11; k++){ if (data[k]==13) output "found 13 at cell" + k else output "13 not found" } the problem ? if data[0] doesn't match, you can't tell after looking at one cell that it's not there

A solution? assume value 13 is not in the array – boolean found = false; visit each cell in the array, if there's a match – display "found 13 at cell " + k, – set found to true otherwise, do nothing (no else) but continue to check next cell After loop, if found is still false, say "not found"

Finding the max in the array: set max = 0 visit every cell using counter k – if data cell k is bigger than max set max to data cell value at position k set maxindex equal to k (to remember the k value) when the loop ends, max will have largest value in array, maxindex will tell you which cell it was in

9 Quiz Draw the following arrays int [] data = new int[8]; data[0] = 5; for (k=1; k<8; k++) data[k]= data[k-1]*2; int [] data2 = new int[8]; for (k=0; k<8; k++) data2[k]=k+100;

Final Project

Each Week We'll spend Thursdays going over another aspect of the final project You'll get more out of these sessions if you foray ahead on your own before hand Extra credit: 1 point for each day you hand in early!! up to 20 max DO NOT hand your solutions to another student— you'll both get zero for whole project (one grade lower for class) It's supposed to be a challenge—do your best – I expect you to refer to previous labs/solutions for help