The repetition structure, or loop, processes one or more instructions repeatedly. The repeatable instructions in a loop are called the loop body. When a loop’s condition evaluates to true, the instructions in the loop body are processed. Otherwise, the instructions are skipped and processing continues with the first instruction after the loop. After each processing of the loop body (iteration), the loop’s condition is reevaluated to determine if the instructions should be processed again.

It contains four elements:

• Repetition statement – a body of the loop
• Condition – a boolean condition that controls whether the instructions are repeated.
• Initial value – starting value for the condition
• Loop termination - there must be a statement within the repeating section of code that allows the condition to become false. This is necessary to ensure that, at some point, the repetition stops.

A repetition structure can be pretest or posttest:

• A pretest loop is an entrance-controlled loop where the condition is evaluated before the instructions in the loop are processed. If the condition is false, the statements in the loop body will never be executed. Examples of pretest loops are while and for loop.
• A posttest loop is an exit-controlled loop where the condition is evaluated after the instructions in the loop are processed. In this loop, the instructions will always be processed at least once. An example of a posttest loop is a do-while loop.

Some loops require the user to enter a special sentinel value to end the loop. Sentinel values should be easily distinguishable from the valid data recognized by the program.

A loop whose instructions are processed indefinitely is called an infinite loop or endless loop .

while loop

while statement is used to create a while loop.

Syntax:

1. while (condition)
2. statement;

• condition can contain constants, variables, functions, arithmetic operators, comparison operators, and logical operators.
• statement(s) are processed repeatedly as long as the condition is true.
• If there is more than one statement in the loop body then it must be entered as a statement block (enclosed in braces). There can be braces even if there is only one statement in the statement block however it is not mandatory.

for loop

A for loop is a counter-controlled loop which uses the for statement to code any pretest loop.

Syntax:

1. for ([initialization]; condition; [update])
2. statement;

• initialization and update arguments are optional.
• initialization argument usually creates and initializes a counter variable.
• condition argument specifies the condition that must be true for the loop body to be processed. It may contain variables, constants, functions, arithmetic operators, comparison operators, and logical operators.
• update argument usually contains an expression that updates the counter variable Loop ends when the condition evaluates to false
• If there is more than one statement in the loop body then it must be entered as a statement block (enclosed in braces). There can be braces even if there is only one statement in the statement block however it is not mandatory.

do-while loop

In this loop, the condition is tested at the end of the loop therefore all the statements in the body of this loop are executed at least once. A do-while statement is used to code posttest loops.

Syntax:

1. do {
2. //one or more statements to be processed one time, and thereafter as long as the condition is true
3. } while (condition);

• condition argument specifies condition that must be true for the loop body to be processed. It may contain variables, constants, functions, arithmetic operators, comparison operators, and logical operators.
• Braces are required around statements if there are more than one.

Counters and Accumulators

A counter can be used to control the loop instead of using a sentinel value. A counter is a numeric variable used for counting something. An accumulator is a numeric variable used for accumulating (adding together) multiple values. It is used to perform calculations such as total or average.

Two tasks are associated with counters and accumulators:

1. Initializing means assigning a beginning value to a counter or accumulator (usually 0) – happens once, before the loop is processed
2. Updating (or incrementing) means adding a number to the value of a counter or accumulator. A counter is updated by a constant value (usually 1). An accumulator is updated by a value that varies. Update statements are placed in the body of a loop since they must be performed at each iteration.

break statement

A break statement forces an immediate break or exit from switch, while, for, and do-while statements. It is useful for breaking out of loops when an unusual condition is detected

continue statement

A continue statement applies to while, do-while, and for statements; causes the next iteration of the loop to begin immediately. It is useful for skipping over data that should not be processed in this iteration while staying within the loop.

Nested loops

Like selection structures, repetition structures can be nested. You can place one loop (the inner, or nested loop) inside another loop (the outer loop). Both loops can be pretest loops or posttest loops, or the two loops may be different types. The programmer decides whether a problem requires a nested loop by analyzing the problem specification.