1. Control Structures - Repetition
Chapter 5

2. Chapter Topics Why Is Repetition Needed The Repetition Structure
Counter Controlled Loops
Sentinel Controlled Loops
Flag Controlled Loops
EOF Controlled Loops
for Loops
The do … while Looping Structure

3. Why Is Repetition Needed?
Consider a task that the computer might automate
Calculate and print paychecks
Read invoices from a file and print monthly statements
Read inputs from a sensor and adjust the air-fuel mixture of an engine
In general
The "Input – process – output" cycle

4. Why Is Repetition Needed?
The algorithm requires a set of actions to be done over and over again
Same set of actions
Number of times to be executed is unknown at design time
We need a structure which allows
Statement(s) to be repeated
A method of testing to see whether the statements should be repeated

5. The Repetition Structure
The while loop provides the repetition structure
Statement(s) to be repeated
A means of checking whether the statement(s) will be repeated
Syntax: while ( logicalExpression ) statement;

6. Phases of Loop Execution
Loop entry => flow of control reaches first statement inside loop
Iteration => each pass thru the loop
Loop test => condition tested before each iteration
Loop exit => when termination condition occurs
in while statement, loop is NOT executed another time
while (condition)
statement 1
statement 2
true
false

7. While Loop Illustration
Loop Iteration
Loop Test
Loop Exit
Loop Entry

8. Counter Controlled Loops
For when you know exactly how many times the loop should run
There are 10 employees for which to print checks
There are 60 sensor inputs per second
The program needs a Loop Control Variable

9. Count Controlled Loop Uses a Loop Control Variable (LCV)
x = 0; while (x <= 10) { cout << “count = “ << x << endl; x = x + 1; }
What gets printed?

10. Count Controlled Loops
The LCV must be ...
I nitialized
I ncremented
I nspected
Amen

11. Count Controlled Loops
Initialized
x = 0; while (x <= 10) { cout << “count = “ << x << endl; x = x + 1; }
Inspected
Incremented

12. Sentinel Controlled Loops
Suppose you want to read some positive integers and average them
You do not have a preset number of data items in mind.
Suppose the number –999 marks the end of data.
Then the -999 is your "sentinel"
View sample program
cin>>variable;
while(variable != sentinel) {
cin>> variable; }

13. The flag is set according to some condition
Flag Controlled Loops
A flag controlled loop uses a Boolean variable to control the loop
Example Let found be the Boolean variable found = false;
while(!found)
{ if(expression)
found = true; }
The flag is set according to some condition

14. Do you need to know how many items are in the file? Why? Why not?
EOF Controlled Loops
End of File Controlled
Do you need to know how many items are in the file? Why? Why not?

15. Testing the State of an I/O Stream
The name of the input stream (used by itself) returns a value
returns a 0 if it is NOT successful
it returns a NON zero value if it IS successful

16. Testing the State of an I/O Stream
When reading a file (a named input stream) we wish to know when it reaches the end
Since the name returns a 0 or non-0, this can be used as a Boolean value
Used to control program sequencing, control a file reading loop

17. EOF Controlled Loops
Can also be used in keyboard entry situations cin>>variable;
while(cin) {
cin>>variable; // Ctrl-Z to signify eof }
Although a sentinel controlled loop might be more user friendly

18. EOF Controlled Loops
In addition to checking the value of an input stream variable …
The function eof with an input stream variable can also be used to determine the end of file status.
Syntax: istreamVar.eof()
Example
infile.get(ch);
while(!infile.eof()) {
cout<<ch; }

19. A Special Count Controlled Loop
The for ( … ) statement
Initialization : initializes the LCV
Condition : usually a comparison, acts like a while ( …)
Incrementing statement : LCV is incremented (or decremented)
for ( initialization ; test expression ; update ) {
0 or more statements to repeat }

20. The for ( … ) Loop Example -- what gets printed? How did it Happen?
x gets initialized
value of x inspected
x gets incremented

21. The for ( … ) Loop
All three of the portions inside the parentheses can be multiple statements separated by commas
Any or all of the three portions inside the parenthesis may be missing
accomplish those tasks some other way
the two semicolons MUST be there

22. for Loops Note the sequence of events in a for loop
Is it possible that the initialization and condition could be set so that that the loop statement would never execute?

23. A Different Looping Statement
Recall that the while ( … ) statement always checked the condition BEFORE the loop
In certain situations we wish to check the condition at …
The END of the loop
After the statement

24. The Do-While Statement
Condition tested at end/bottom of loop
Guarantees loop body executes at least once

25. The do-while Illustrated
Note: always at least one iteration
Loop Iteration
Loop Entry
Loop Exit
Loop Trest

26. The Do-While Statement
Can be used for a counting loop
What gets printed??
How do you change it to go 10 times?

27. What makes this easier than the while ( … ) loop for this task?
The Do While Statement
This logic sometimes more suitable for some algorithms
Example : trap for valid input
do { cout << “Enter value (1 - 5) -> “; cin >> value; if (value < 1 || value > 5) cout << “Invalid input\a” << endl; while (value < 1 || value > 5);
What makes this easier than the while ( … ) loop for this task?

28. do-while Loop vs. while Loop
PRE-TEST loop (entry-condition)
The looping condition is tested before executing the loop body.
Loop body may not be executed at all.
POST-TEST loop (exit-condition)
The looping condition is tested after executing the loop body.
Loop body is always executed at least once.

29. The break; Statement We saw it in the switch statement
Causes immediate exit from innermost block
switch, while, do-while, for
Can be used to break out of purposely designed infinite loop
not a good idea … a lazy shortcut
Use only as last resort to avoid baffling combinations of multiple Boolean flags and nested ifs

30. The continue; Statement
Valid only in loops
Terminates current loop iteration
NOT entire loop
Causes branch to bottom of loop
skips rest of loop statements
Loop then prepares for next iteration
for (…) would increment lcv
all loops would check condition

31. Guidelines for Choosing a Looping Statement
Simple count-controlled
use for (…) loop
Event controlled, body always executed at least once
use do-while
Event controlled and nothing known about first execution
use while, possibly for
When in doubt => use while

32. Testing and Debugging
For do-while loops, make sure to try data sets to make loop go exactly one time
For data-dependant loop where expressions based on values other than constants
make sure to test for proper number of iterations
Make sure switch statements have every branch tested
including default

33. Testing and Debugging
Remember to use break at end of case alternatives in switch statements
otherwise next case is also executed!
Case labels in switch statement must be values or named constants -- no variables
Both switch expression & case constant cannot be floating point
Provide default for switch when possibility of case values not being matched

34. Testing and Debugging Make sure all needed switch cases are present
Choose looping structure carefully
for( ; ; ) loop heading must have two semicolons -- even if portions omitted
Break statement can exit only one level of nesting
innermost switch or loop where break is located