lecture 7 - loop structures

8/14/2019 Lecture 7 - Loop Structures

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Lecture 7: Loop Structures


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Outline

In this lecture, we will discuss loop structures Our second class of VB structures for controlling program

flow.

We will discuss two broad classes of loops:

1. ForNext Loops The ForNext Loop

The For Each

Next Loop

2. DoLoop Loops The Do WhileLoop Loop

The Do Until

Loop Loop The DoLoop While Loop

The DoLoop Until Loop

Combining Loops: Nested Loops


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Loop Structures

So far, our programs have been acyclic:

We have discussed both linear and branched programs In which process paths are executed once or not at all.

With no repeated processes (cycles).

However, real algorithms often use of loops:

i.e., the programmed (controlled) repetition of a basic process.

Example: Make a telephone bill for allof your customers (not just one).

The ability to repetitively cycle a process is called iteration.

Blocks of code for looping are called Loop Structures.

As noted in the outline, VB .NET provides several flavors of loops:

ForNext loops DoWhile loops

Each of these provides a means to control program flow

Via controlled cycling.

Lets take a look at the simplest: The ForNext loop.


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The ForNext Loop

The simplest and most popular loop structure is the For

Next loop. This allows the repeated execution of a set of statements

With loop termination based on a loop index.

The VB syntax for the ForNext loop is:

For index=startTo EndStep stepstatements

Next

index: is a loop variable which: Indexes (counts) the number of loop iterations; Is Initialized to the value, start

End specifies a loop condition , evaluated at each iteration: Determines whether statementsare executed.

If index < start, conditions are executed; otherwise, the loop ends. Step sets the increment value of the loop (default = 1):

After each execution of statements, indexis incremented by step; In other words: index += step

Note: if step is negative, you can loop backwards.

Next index: tells the running program to loop back to For index And test the loop condition, based on index,

to see if another cycle is to be executed.


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Flow Chart for the ForNext Loop

Example: Calculate the sum of all integer values from 1 to 10.

We use a ForNext loop

With loop variable, i to both index the 10 integers and control looping.

And variable Total to store the sum. VB code:

Dim Total, i As Integer

Total = 0

For i=1 To 10 Step 1

Total = Total + i

Next

The ForNext loops flow of control isshown clearly using a Flow Chart:

Our loop is executed 10 times.

Starting cycle 1: i = 1; Total = 0; After cycle 10: i= 11; Total = 55.

The i=11 value fails the test, which stops the loop.


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Program: Simple ForNext Loop


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DoLoop Loops

A different type of loop structure is a Do

Loop loop: Allow repeated execution of a set of statements

With loop termination based on a conditional test (True/False).

Can be used to execute a loop an indefinite number of times.

Compare with a For

Next loop (set number of cycles).

There are 4 flavors of DoLoop loops: Made using two keywords: While and Until

Loop Condition tested at the start of each loop cycle:

Do WhileLoop loop Loops while a condition is true.

Do UntilLoop loop

Loops until a condition becomes true.

Note: loop body statements might be executed 0 times. Loop Condition tested at the end of each cycle:

DoLoop While loop

Loops while a condition is true

DoLoop Until loop Loops until a condition becomes true

Note: loop body statements are executed at least 1 time.


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The Do WhileLoop Loop

The Do While

Loop loop: Keeps looping while a loop conditionis True; Tests the condition at thestartofeach cycle.

Halts when the conditionbecomes False.

The VB syntax for the Do While

Loop loop is:

Do While conditionstatements

Loop

Keyword Do says, start the cycle

Keyword While says test ourcondition:

Tested at the start of each loop

If condition True, statementsare executed once;

Example: (sum < 1000)

Keyword Loop then tells control to loop (return to Do) And another cycle begins


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The Do UntilLoop Loop

The Do Until

Loop loop: Keeps looping while a loop conditionis False; Tests the condition at thestartofeach cycle.

Halts after the cycle in which the conditionbecomes True.

The VB syntax for the Do Until

Loop loop is:

Do Until conditionstatements

Loop

Keyword Do says, start the cycle

Keyword Until says test ourcondition:

Tested at the start of each loop

If condition False, statementsare executed once;

Example: (sum >= 1000)

Keyword Loop then tells control to loop (return to Do). And anther cycle begins.


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The DoLoop While Loop

The Do

Loop While loop: Keeps looping while a loop conditionis True; Tests the condition at the endofeach cycle.

Halts after the cycle in which the conditionbecomes False.

The VB syntax for the Do

Loop While loop is:Do

statementsLoop While condition

Keyword Do says: Execute our statements once.

Keywords Loop While say test our condition:

Tested at the bottom of each loop If condition True, control returns to

Do And another cycle begins.

Otherwise, the loop ends.

Example: (sum < 1000)


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The Do Loop Until Loop

The Do

Loop Until loop: Keeps looping while a loop conditionis False; Tests the condition at the endofeach cycle.

Halts after the cycle in which the conditionbecomes True.

The VB syntax for the Do

Loop Until loop is:Do

statementsLoop Until condition

Keyword Do says: Execute our statements once.

Keywords Loop Until say test our condition:

Tested at the bottom of each loop If condition False, control returns to

Do And another cycle begins.

Otherwise, the loop ends.

Example: (sum >= 1000)


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Nested Loops

Sometimes it is desirable to place loops inside of loops. These are referred to as Nested Loops

Multiple loops of all types may be nested in the obvious way

Example: A simple nested ForNext loop Purpose: Display all pairs of positive integers, each


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Program to Demonstrate Loops


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Program (cont.)


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Program (cont.)


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Program (cont.)

P ( t )


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Program (cont.)


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lecture 7 - loop structures

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