1. While and Do While Syntax, semantics and examples
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2. The two simple loops
Either the while or do while is simpler than the for
However, all loops need all the pieces:
Initialization
Test
Body of the loop
Advancement
From a programming perspective very similar
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3. Syntax of while The form is: while(condition) one statement
While is a reserved word
Condition is a boolean of the same type seen elsewhere
The one statement is any statement
Most often a compound statement
Unlike the for the while almost never has an empty statement here
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4. Syntax of do while The form is: do one statement while(condition);
Do is also a reserved word
Condition is a boolean of the same type seen elsewhere
Evaluated at end of loop
One statement is most often a compound statement
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5. Flowchart Representation
Leading decision loop
such as a while
Trailing decision loop such as a do while
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6. Why Both?
The while is used instead of a for when the advancement issue is fuzzy
The do is used when the initialization becomes part of the loop body
The do is the least used loop
Never the less, the experienced programmer uses all three in their code
Let’s consider some examples
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7. While Revisited Suppose a cash register machine program
We take in the purchase price and the amount the customer has given us
Compute the change
Make the change in terms of numbers of bills and coins the teller has to give
Try first part of this with a while
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8. Cash Register Program double charge, tender; … // get charge, tender
double change = tender-charge;
int twenties = 0;
while(change >= 20.00){
change -= 20;
twenties++; }
String s;
if(twenties>0){
s = “Give them “ +twenties+ “ twenties.”;
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9. Example Revisited The loop would be duplicated for tens, fives, etc.
Both the while and for are leading decision loops
They may execute the body of the loop zero times, if the condition is false
This is most common form of loop
This is desirable here, we may not give them one of some bill denomination
Leading decision loops must initialize before the first test
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10. Another Situation
Sometimes we end up in duplication in initialization and body of loop
Consider the case of reading in the age of a hospital patient, that we attempt to verify int age; age = scan.nextInt(); while(age > 120){ System.out.println( “Bad age entered”); age = scan.nextInt(); }
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11. Another Loop
This type of situation may be best handled with a trailing decision loop
This checks the condition at the end of the loop rather than the beginning
Particularly handy for when the initialization is part of the loop processing
Consider this with a do while
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12. Consider Example Again
Get a Hospital patient’s age: int age; do { System.out.println( “Enter age”);
age = scan.nextInt(); }
while(age > 120);
The loop reads and verifies
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13. One last thought Some loops are predictable and others not
For example: for(int i = 0;i<100;i++)… for(Pixel ap: p.getPixels()) …
Prior to the first execution of these two we can say precisely how many times they will execute
These two are deterministic
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14. In contrast
The do while: do { System.out.println( “Enter age”); age = scan.nextInt(); } while(age > 120);
Is unpredictable
The cash register while is also non-deterministic
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15. Determinism Counting loops usually deterministic, use a for
Most often use whiles or do whiles for non-deterministic loops
There are several other things we think about with loop choice as well
However the while and for are equivalent in that any for can be converted to a while and the reverse
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16. Conclusion There is more to learn about the three loops
There are also more presentations:
Logical loop types
Using break and continue
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