CPS120 Introduction to Computer Science

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CPS120 Introduction to Computer Science

• Iteration (Looping)

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Looping

• Loops allow your program to repeat groups of
  statements a specified number of times. Loops are
  an example of an iteration structure.

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Iterate

• A program loop is a form of iteration. A computer
  can be instructed to repeat instructions under
  certain conditions.

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Looping with Keyword goto

• Uses concepts covered in decision making
  discussion
• Implemented with a simple if..then structure
• loop counter
• cout ltlt"Processing Counter "ltlt counter ltlt
  endl
• if (counter lt 5)
• goto loop

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Iteration Control Structures

• Iteration control structures are looping
  mechanisms.
• Loops repeat an activity until stopped. The
  location of the stopping mechanism determines how
  the loop will work
• Leading decisions
• Trailing decisions

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Leading Decisions

• If the stop is at the beginning of the iteration,
  then the control is called a leading decision.
• The command DO WHILE performs the iteration and
  places the stop at the beginning.

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For Loops

• A for loop always executes a specific number of
  iterations.
• Use a for loop when you know exactly how many
  times a set of statements must be repeated
• A for loop is called a determinant or definite
  loop because the programmer knows exactly how
  many times it will iterate

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Syntax of a for Loop

• for (initializing expression control expression
  step expression) // one or more
  statements
• The initializing expression sets the counter
  variable for the loop to its initial value.
• The control expression ends the loop at the
  specified moment.
• The step expression changes the counter variable
• Semi-colons, not commas, divide the expressions

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Things to Remember About For Loops

• It is possible to have variable increase by a
  value other than one
• for (num 1 num lt 10 num num 3)
• It is possible to initialize the loop variable
  within the initializing expression. But I
  recommend against it
• Declare loop variables at the top of the function
  where they belong
• If an if statement only contains one body
  statement, the compiler doesn't require the use
  of curly braces

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WHILE Loop
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While Loops

• A while loop does not necessarily iterate a
  specified number of times
• As long as its control expression is true, a
  while loop will continue to iterate
• An indeterminate or indefinite loop because only
  at run-time can it be determined how many times
  it will iterate
• While is considered a top-checking loop
• The control expression is located on the first
  line of code
• If the control expression initially evaluates to
  FALSE, the loop will not execute even once.

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While Loop Syntax

• while (control expression) // one or
  more statements
• The control expression must evaluate to TRUE in
  order for the while loop to iterate even once

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While (true) Loops

• This type of loop will never end unless you put a
  break in it
• while (true)
• counter
• if (counter gt 10)
• break

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Trailing Decisions

• If the stop is at the end of the iteration, the
  control mechanism is called a trailing decision.
• The command DO / WHILE performs the iteration and
  puts the stop at the end of the loop.

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DO WHILE Loop
Loop statement a
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Do While Loops

• As with a while loop, a do while loop does not
  necessarily iterate a specified number of times
• A do while loop will iterate at least one time
  because its control expression is placed at the
  end of the loop
• Considered to be an indeterminate or indefinite
  loop
• Considered to be a bottom-checking loop, since
  the control expression is located after the body
  of the loop

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Do While Syntax

• do // body statements would be placed
  herewhile (control expression)
• Don't forget to include the required semicolon
  after the control expression

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Break and Continue Statements

• A break statement is used to stop the execution
  of a loop immediately and to continue by
  executing the statement that comes directly after
  the loop
• A continue statement is used to stop the
  execution of the statements in the loop's body on
  that particular iteration and to continue by
  starting the next iteration of the loop

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Nested Loops

• A loop of any kind may be placed in another loop
  (of any kind).
• Be sure to entirely encapsulate the inner loop
  inside of the outer loop, otherwise an error is
  sure to occur.
• Two loops are considered to be nested loops if
  one is enclosed within the other

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In Summary

• Loops
• goto loops -- simple ifthen structure
• for -- fixed number of loops
• while -- may never be run
• while (true) -- always true, needs break
• do while -- always run once
• continue causes while, do while, and for loops
  to start over
• break causes while, do while, for and switch
  statements to end