C++ Loops: Pretest and Posttest

• A repetition structure, also known as a loop, executes one or more instructions repeatedly.
• Loops contain a Boolean condition to determine when to repeat the instructions.
• A looping condition specifies when to continue the loop, while a loop exit condition specifies when to stop.
• Each looping condition has an opposite loop exit condition.
• C++ uses looping conditions in repetition structures.
• Repetition structures can be pretest or posttest.

Pretest Loop

• The condition is evaluated before the instructions are processed.
• If the condition is initially false, the instructions may not be processed.

Posttest Loop

• The condition is evaluated after the instructions are processed.
• The instructions are always processed at least once.
• The condition is evaluated with each repetition in both types of loops.
• The repeatable instructions in a loop are called its body.
• Sentinel values used to end loops should be easily distinguishable from valid program data.
• When a loop's condition is true, the instructions in the loop body are processed.
• Otherwise, the instructions are skipped, and processing continues after the loop.
• After each iteration, the loop's condition is reevaluated.
• A priming read initializes the loop before it starts.
• An update read allows the user to enter a new value in each loop iteration.

Flowcharts of Pretest Loops

• The diamond symbol represents decision-making and repetition structures.
• A diamond in a repetition structure contains a Boolean condition that determines if the loop proceeds.
• Diamonds have one flowline entering and two flowlines exiting which are marked "T" and "F" for true and false.
• The "T" line directs to the loop body.
• The "F" line directs to the instructions processed when the loop condition is false.
• The flowline entering the diamond and the path of the true condition form a loop, distinguishing it from selection structures.

For Statement

• It can code any pretest loop and is often used for counter-controlled loops.
• In the syntax for ([initialization]; condition; [update]), only one statement or a statement block is processed as long as the condition is true.
• Initialization and update arguments are optional.
• The initialization argument typically creates and initializes a counter variable that is local to the for statement.
• The condition argument must be a Boolean expression that needs to be true for the loop body to be processed, and may contain variables, constants, functions, and operators.
• Mark the end of for loops with a comment like //end for.
• Loops end if the condition evaluates to false.
• Update Expression contains the expression that updates the counter variable

For Loop

• A for loop is useful when the number of statement repeats is known when writing code
• The format is for(initialization; test condition; loop update)
• Statements are executed only if the condition is true

While Loop

• It is used when number of times loop is to be executed is unknown
• The condition is checked at only once, and it is possible that the loop doesn't execute at all
• It can also be used to perform loop an exact number of times
• The while loop has the following structure, where code is executed only if the codition is true:

while (condition)
{
//these done if condition is true
}

Counters and Accumulators

• Often used to calculate to totals and averages
• Counter: numeric variable used for counting
• Accumulator: numeric variable used to accumulate
• Initializing: Giving a beginning value, assign 0 before loop
• Updating: (or incrementing) add a number to the value of a counter or accumulator

• Counters are updated by a constant value (usually 1)
• Accumulators are updated by a value that varies
• Update statements are placed in the body of a loop since they must be performed at each iteration

Do While

• This loop is similar to the while loop, but the condition is checked at the end
• Loop statements are always performed exactly once, unlike the for and while loops
• Is coded as follows

do
{
            //loop statements
} while(condition);
     

POSTTEST and PRETEST LOOPS

• PRETEST: condition evaluated before instructions are processed
• POSTTEST: condition evaluated after instructions are processed. This loops instructions are always processed at least once