Module 5

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Module 5

• Loops

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What is a loop?

• A loop is a repetition control structure
• It lets us do one (or more) things many times
• Two types of loops
• Count controlled loops do x number of times
• Event controlled loops do until some condition
  is true
• For loops, many rules similar to those you
  learned for the if-else statement

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The while loop
while (expression) // single statement
form statement1 while(expression) // many
statement block statement1 statement2 stat
ement3
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Some Simple while Rules

• The expression is evaluated to zero or nonzero
  (true or false if you use Boolean)
• The contents are done while expression evaluates
  to true
• Otherwise we go on and execute the next thing
  following the while
• (Sound familiar?)

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Count-Controlled Loop
int count count 4 while (count gt 0) cout
ltlt count-- ltlt endl cout ltlt Done with count
value ltlt count
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Note On Variables and Blocks
int count count 4 while (count gt
0) cout ltlt count-- ltlt endl cout ltlt
Done with count value ltlt count
// This last line now produces an error!
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Variables and Scope

• Variables only hang around as long as we need
  them
• We demark where they are needed by the blocks we
  createvariables dont persist once the block is
  ended (why?)
• Where the variable is useable (readable and
  writeable) is called the variables scope

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Count-Controlled Loop Example
int thisNum // current number
input by user int total 0 // running total of
input values int count 0 while (count lt 100)
// How many times will this execute? cout ltlt
Please input a number ltlt endl cin gtgt
thisNum total total thisNum count cou
t ltlt The total is ltlt total ltlt endl
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Event-Controlled Loop Example
int thisNum 1 // current number input
by user int total 0 // running total of input
values while (thisNum) // How many times will
this execute? cout ltlt Please input a number
ltlt endl cin gtgt thisNum total total
thisNum cout ltlt The total is ltlt total ltlt
endl
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Important Note About Loops

• Here I deliberately used a while loop for the
  count-controlled loopit required extra
  statements
• There is another loop, called a for loop, that is
  generally better for count-controlled loops
• Thus, each type of loop has a particular purpose
  for which it is intended!

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Important Note, Continued

• Note that the while loop does its test at the
  start of the loop
• There is another loop called a do-while loop
  which does its test at the end of the loop
• Thus, again, each type of loop has a particular
  purpose for which it is intended!
• If you choose wisely, your code is more readable,
  debugable, and maintainable

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Loops Compared
Loop Type Loop Test Placement
Loop Condition for loop at beginning
fixed of times while loop at beginning
test a value do-while loop at end test a
value
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Which Type of Loop is Appropriate?

• If I want to check successive inputs from a user
  to see if the value is zero?
• If I want to compare a name I have to every name
  in a list of 20 other names?
• If I want to divide by the variable A but I need
  to make sure A is not zero the division changes
  the value of A?

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The for loop
for (initialize test increment) statementA f
or (initialize test increment)
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A for-loop example
int count, countFactorial 1 //
Acquire value from user cout ltlt Please enter a
positive integer cin gtgt count // Calculate
factorial of input value for ( count gt 1
count count - 1) countFactorial count //
Output users value and its factorial cout ltlt
count ltlt ! ltlt countFactorial ltlt endl
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for loop execution path

• When entering the loop, the initialization is
  executed first, then the test
• if test is false here, we skip the loop!
• After that, we execute the body, then increment,
  then test until the test is false
• It is possible that some of these items will be
  null statements
• Take time to learn the order of execution
  described hereit will eliminate common errors
  with for loops!

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What gets printed?
for (int j 1 j lt 100 j) cout ltlt j ltlt
// this is a typical loop structure
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What gets printed?
int k for (k 34 k 0 ) // note the
third item here! if ((k 5) 0) cout ltlt
k-- cout ltlt k ltlt ltlt endl
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The do-while loop
do statementA while(expression) do statemen
tA statementB while(expression)
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Example do-while
do cin gtgt response if ((response ! y)
(response ! n)) cout ltlt Please type y or n
while((response ! y) (response !
n))
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Equivalence of loops

• In some sense, these three loop structures are
  equivalent that is, I can simulate any one of
  them with one of the others
• We have three different ones because they make it
  possible for us to do things in ways that are
  more natural to read

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Example of Equivalence
for (j 0 j lt 10 j) statementA //
equivalent while j 0 while(j lt
10) statementA j // can you create
the equivalent do-while?