1. CS2011 Introduction to Programming I Loop Statements (I)
Chengyu Sun
California State University, Los Angeles

2. Certain Things Can Be Done by Both Human and Computer
For example, in our labs we have done: convert mile to kilometer, sort three numbers, calculate the decimal value of a 4-digit binary number …

3. Certain Things Are More Suitable for Computers
Display "Welcome to Java" a thousand times
Sort one million numbers
… …
Loop statements allow us to make computers do what they do best: repetitive work at very high speed

4. Example: Welcome20
Display "Welcome to Java" 20 times

5. while statement while ( boolean-expression ) { statement(s) }
If the boolean expression evaluates to true, execute the statement(s) in {}; repeat until the condition is no longer true
{} can be omitted if there's only one statement

6. Flow of while Statement
while(condition)
false
true
statement(s)

7. if vs. while if(condition) false while(condition) false true true
statements
statements

8. Key Things for a Loop Statement
Initial state
Loop condition
Change of state inside the loop that affects the loop condition
Iteration counter
Must-have
Often used

9. Avoid Common Problems When Using Loop Statements
Make sure the loop condition eventually become false to avoid infinite loop
Specify the loop condition carefully to avoid off-by-one error
E.g. should it be count < 20 or count <= 20 in the Welcome20 example?

10. Example: Bin2Dec Calculate the decimal value of a given binary number
For example:
1101 = 1 x x x x 23
= 1 x x x x 8 =
= 13

11. Construct the Loop
Find the repeating pattern and decide the unit of work done in each iteration
Initial state ??
Change of state ??
Loop condition ??

12. Example: Addition Quiz Revisited
The program asks if the user wants to continue after each question
"Y" or "y" : create another question
"N" or "n": exit

13. do … while statement do { statement(s) } while ( boolean-expression );
It's like a while loop except that the statement(s) will be executed at least once
Notice the ; at the end
Flow chart ??
AdditionQuiz is more natural with do…while