Control Structures - Repetition

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 loopprovides therepetition structure
• Statement(s) tobe repeated
• A means of checking whether the statement(s) will
  be repeated
• Syntax while ( logicalExpression )
  statement

6
Phases of Loop Execution

• Loop entry gt flow of control reaches first
  statement inside loop
• Iteration gt each pass thru the loop
• Loop test gt condition tested before each
  iteration
• Loop exit gt when termination condition occurs
• in while statement, loop is NOT executed another
  time

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 0while (x lt 10) cout ltlt count ltlt
x ltlt endl x x 1
What gets printed?
10
Count Controlled Loops
The LCV must be ...

• I nitialized
• I ncremented
• I nspected

11
Count Controlled Loops
x 0while (x lt 10) cout ltlt count ltlt
x ltlt endl x x 1
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
cingtgtvariable while(variable ! sentinel) . .
. cingtgt variable
13
Flag Controlled Loops

• A flag controlled loop uses a Boolean variable to
  control the loop
• ExampleLet found be the Boolean variablefound
  false
• while(!found)
• . . . if(expression)
• found true

14
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
  situationscingtgtvariable
• while(cin)
• . . .
• cingtgtvariable // 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())
coutltltch infile.get(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 wewish to check the
  condition at
• The END ofthe 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 alwaysat least oneiteration
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
The Do While Statement

• This logic sometimes more suitable for some
  algorithms
• Example trap for valid input

do cout ltlt Enter value (1 - 5) -gt cin
gtgt value if (value lt 1 value gt 5)
cout ltlt Invalid input\a ltlt endl while
(value lt 1 value gt 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 gt 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