Chapter 5' Looping

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Chapter 5. Looping
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5.1 The Increment and Decrement Operators

• and -- are operators that add and subtract one
  from their operands.
• Num Num 1
• Num 1
• Num

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The Difference between Postfix and Prefix
The cout statement 1) displays Num and 2) causes
Num to be incremented after it is displayed.
This is postfix. Num 4 cout ltlt Num // 4
is displayed The cout statement 1) displays Num
and 2) causes Num to be incremented before it is
displayed. This is prefix. Num 4 cout ltlt
Num // 5 is displayed
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Using and -- in Mathematical Expressions

• X 2
• Y 5
• Z X Y
• cout ltlt X ltlt ltlt Y ltlt ltlt Z
• Results 2 6 10

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Using and -- in Relational Expressions

• X 10
• if ( X gt 10)
• cout ltlt X is greater than 10.\n
• Two operations are happening
• the value in X is tested to determine if it is
  greater than 10
• then X is incremented

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The while Loop

• A loop is part of a program that repeats.
• A while loop is a pre test loop - the
  expression is tested before the loop is executed
• while (expression)
• statement

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WHILE is a PRE-TEST LOOP int number
10 while( number ! 99 ) cin gtgt number
number ! 99
cin gtgt number
YES
while( number ! 99 ) cin gtgt number
NO
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while LOOPS
General format while( expr ) statement(s)
// end while Next statement
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Terminating a Loop

• A loop that does not have a way of stopping is
  called an infinite loop
• int Test 0
• while (Test lt 10)
• cout ltlt Hello\n
• A null statement is also an infinite loop, but it
  does nothing forever
• while (Test lt 10)

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// This is ok int test 0 while (Test lt 10
) cout ltlt Hello\n Test // BUT - Dont
do this! int test 0 while ( Test lt 10
) cout ltlt Hello\n Test
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Dont do this either! while( Remainder 1 ) //
assignment operator cout ltlt Enter a number
cin gtgt Num Remainder Num 2
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INFINITE LOOPS
// This is an infinite loop while( 1 ) // always
true cout ltlt Howdy doo to you! ltlt endl
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Programming Style and the while Loop

• If there is only one statement repeated by the
  loop, it should appear on the line after the
  while statement and be indented one additional
  level
• If the loop repeats a block, the block should
  begin on the line after the while statement and
  each line inside the braces should be indented

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Counters

• A counter is a variable that is incremented or
  decremented each time a loop iterates.

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User Controlled Loops

• Loops can be designed to repeat until the user
  enters a particular value.

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Keep a Running Total

• A running total is a sum of numbers that
  accumulates with each iteration of a loop. The
  variable used to keep the running total is called
  an accumulator.

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Sentinels

• A sentinel is a special value that marks the end
  of a list of values.

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DO-WHILE is a POST-TEST LOOP int number 0 do
// loop until zero cout ltlt Enter a
non-zero number cin gtgt number while(
number ! 0 )
cout ltlt cin gtgt number
number ! 0
YES
NO
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The for Loop

• Ideal for situations that require a counter
  because it has built-in expressions that
  initialize and update variables.
• for (initialization test update)
• statement(s)

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for (Num 1 Num lt 10 Num) cout ltlt Num ltlt
"\t\t" ltlt (Num Num) ltlt endl
num 1
YES
cout statement
num
NO
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Omitting the for Loops Expressions

• int Num 1
• for ( Num lt 10 Num)
• cout ltlt Num ltlt \t\t ltlt (Num Num) ltlt
  endl

int Num 1 for ( Num lt 10 ) cout
ltlt Num ltlt \t\t ltlt (Num Num) ltlt endl
Num
// Infinite loop for ( ) cout ltlt
Hello World!\n
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LOOK OUT BELOW!!!
THIS LOOP WONT EXECUTE! for( x 11 x lt 10
x ) cout ltlt x ltlt endl WATCH THE DOUBLE
INCREMENT HERE!! for( x 1 x lt 10 x
) cout ltlt x ltlt ( x x ) ltlt endl x DONT
DO THIS!!!! NOBODY!!! NO BODY!!! for( x 11
x lt 10 x ) cout ltlt x ltlt endl
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Focus on Software Engineering Deciding Which
Loop to Use

• The while Loop
• A pre-test loop.
• Use when you do not want the loop to iterate if
  the condition is false from the beginning.
• Ideal if you want to use a sentinel.
• The do-while Loop
• A post-test loop.
• Use if you always want the loop to iterate at
  least once.
• The for Loop
• A pre-test loop.
• Automatically executes an update expression at
  the end of each iteration.
• Ideal for situations where a counter variable is
  needed.
• Used when the exact number of required iterations
  is known.

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OTHER FORMS OF THE UPDATE EXPRESSION
for( num 2 num lt 100 num 2) for( num
10 num gt 0 num-- ) Scarey? for( num 1
num lt 10 cout ltlt Num ltlt endl) for( num 1
num lt 10 num, cout ltlt pow(num, 2) ) for( int
num 1 num lt 10 cout ltlt Num ltlt endl) int
num 1 for( num lt 100) Cout ltlt Num ltlt
\t\t ltlt (num num ) ltlt endl Num for(
) cout ltlt Hello World\n // Infinite loop
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The comma Operator
expr1, expr2 // expr1 is evaluated first int a
0, b 1 // left to right associativity for(
int sum 0, i 1 i lt n i) sum i //
Another way to do the same thing for( int sum
0, i 1 i lt n sum i, i) // NOT the
same thing for( int sum 0, i 1 i lt n
i, sum i)
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THE for STATEMENT
// This is valid, but dont do it! for() for()
for() statement // This is better use
those braces!! for() for() for()
statement
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Nested Loops

• A loop that is inside another loop is called a
  nested loop.
• while(expr1)
• while(expr2)
• statements
• // end while doing expr2
• // end while doing expr1

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Breaking Out of a Loop

• The break statement causes a loop to terminate
  early.

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The continue Statement

• The continue statement causes a loop to stop its
  current iteration and begin the next one.

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Using Loops for Input Validation

• Loops can be used to create input routines that
  repeat until acceptable data is entered.

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THE break AND continue STATEMENTS
for( expr1 expr2 expr3 ) expr1 while(
expr2 ) statements continue goto
next more statements more statements n
ext expr3