1. Computer Programming Methodology Input and While Loop
COS 130
Computer Programming Methodology
Input and While Loop

2. Demo Input Program

3. New Issues How to read data into the program How many times to loop
In this case from the key board
How many times to loop
Don’t know in advance how many numbers the user will type in
How to go between characters and int

4. How to read data – Scanner one of many possibilities
import java.util.Scanner;
public class Avg {
public static void main(String args[]) {
Scanner kb = new Scanner(System.in);
int num = 0; double sum = 0; int cnt = 0;
// more stuff
while(num != -1) {
System.out.print("enter number: ");
num = kb.nextInt(); }
This finds the library
This creates a
Scanner object
This specifies
the keyboard or
redirected file
Reads and converts
characters to int

5. Open Loop - while (not counted)
while(num != -1) {
// do stuff }

6. Open Loop - while (not counted)
boolean done = false;
while(!done) {
// do stuff }

7. while can count too int cnt = 0; while(cnt < 10) { // do stuff
cnt = cnt + 1; }
for(int cnt = 0; cnt < 10; cnt = cnt + 1) {
// do stuff }

8. When to use which loop
for – use when the number of repeations is known before the loop starts
while – use when the number of repeations is not known before the loop starts

9. Another way to convert string to int
All Java primitives (int, double, etc.) have a corresponding “wrapper” class which contain “helper” methods.
int -> Integer
double -> Double
long -> Long
boolean -> Boolean

10. Another way to convert string to int
int num = Integer.parseInt(“123”);
double num = Double.parseDouble(“1.23”);

11. Example of use String word = kb.next(); if("quit".equals(word)) {
// code to quit
} else {
num = Integer.parseInt(word); }

12. Class vs. Object
Notice the difference between: String name = “John Doe”; int len = name.length(); And int num = Integer.parseInt(“123”);