1. Lec 21 More Fun with Arrays: For Loops

2. Agenda Some backfill for Lab 20: For Loops vs While loops
Using an array in an applet or class
array instance variables
For Loops vs While loops
not that big a deal
Nested for Loops
For Loops and Arrays
more ways to process
Plotting data on the screen

3. 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
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]);

5. 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

6. 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

7. 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"

8. 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 Determine the values generated and stored in the following array
int [] data = new int[8];
data[0] = 5;
for (k=1; k<8; k++) data[k]= data[k-1]*2;

10. Quiz
2. Determine the values generated and stored in the following array
int [] data2 = new int[8];
for (k=0; k<8; k++) data2[k]=k+100;

11. Final Project