Structured Programming

Background

Since the invention by Von Neumann of the stored program computer, computer scientists have known that a tremendous power of computing equipment was the ability to alter its behavior, depending on the input data. Calculating machines had, for some time, been able to perform fixed arithmetic operations on data, but the potential of machines capable of making decisions opened up many new possibilities. Machines that could make decisions were capable of sorting records, tabulating and summarizing data, searching for information, and many more advanced operations that could not even be imagined at the time.

In early programming languages, like Fortran (first invented in 1954) and various low level machine languages, the goto statement allowed the computer to deviate from the sequential execution of the program instructions. The goto statement was recognized to be a very powerful construction, and soon, programs of increasing complexity and power were developed.

However, the increasingly complex code that resulted from goto statements became harder and harder to maintain. Dijkstra, in 1966, was one of the first persons to recognize that this run away complexity of programs was due to the overuse of the goto statement (Dijkstra, E. W., "Go To Considered Harmful," Communications of the ACM, March 1966). In fact, it was determined shortly thereafter, that the goto statement is not needed at all. Dijkstra showed that any program construction that could be created with goto statements could be created more simply with the sequence, repetition and decision constructions that are discussed in the following sections. This was the birth of the discipline of Structured Programming.

Structured Programming in Everyday Life

1. Sequence Execute a list of statements in order.

Example: Baking Bread

Add flour.
Add salt.
Add yeast.
Mix.
Add water.
Knead.
Let rise.
Bake.

2. Repetition Repeat a block of statements while a condition is true.

Example: Washing Dishes

Stack dishes by sink.
Fill sink with hot soapy water.
While moreDishes
    Get dish from counter,
    Wash dish,
    Put dish in drain rack.
End While
Wipe off counter.
Rinse out sink.

3. Selection Choose at most one action from several alternative conditions.

Example: Sorting Mail

Get mail from mailbox.
Put mail on table.
While moreMailToSort
    Get piece of mail from table.
    If pieceIsPersonal Then
       Read it.
    ElseIf pieceIsMagazine Then
       Put in magazine rack.
    ElseIf pieceIsBill Then
       Pay it,
    ElseIf pieceIsJunkMail Then
       Throw in wastebasket.
    End If
End While

Structured Programming in Visual Basic

Structured programming is a program written with only the structured programming constructions: (1) sequence, (2) repetition, and (3) selection.

Sequence. Lines or blocks of code are written and executed in sequential order.
Example:
x = 5
y = 11
z = x + y
WriteLine(z)
Repetition. Repeat a block of code (Action) while a condition is true. There is no limit to the number of times that the block can be executed.
While condition
    action
End While
Example:
x = 2
While x < 100
    WriteLine(x)
    x = x * x
End
Selection. Execute a block of code (Action) if a condition is true. The block of code is executed at most once.
If condition Then
action
End If
Example:
x = ReadLine()
If x Mod 2 = 0
WriteLine("The number is even.")
End If

Extensions to Structured Programming

To make programs easier to read, some additional constructs were added to the basic three original structured programming constructs:

Definite Repetition (For Loop) Combine initialization, checking a condition, and incrementing a counter in a single statement called a for statement. Here is the general form:
For i = 1 To n Step k ' Step k is optional
     action
Next
Example:
For i = 1 To 20
    WriteLine(i)
Next i
Example:
For i = 20 To 1 Step -1
    WriteLine(i)
Next
If-Then-Else Statements Execute the first action whose corresponding condition is true. Here is the general form:
If condition1 Then
    action1
ElseIf condition2 Then
    action2
ElseIf condition3 Then
    action3
Else
    defaultAction
End If
Example:
If n = 1 Then
    WriteLine("One")
ElseIf n = 2 Then
    WriteLine("Two")
ElseIf n = 3 Then
    WriteLine("Three")
Else
    WriteLine("Many")
End If
Select Statement Execute the action corresponding to the value of the expression.
Select Case value1
    Case value1
       action1
    Case value2
       action2
    Case value3
       action3
    Case Else
       defaultAction
End Select
Example:
Select Case n
    Case 1
       WriteLine("One")
    Case 2
       WriteLine("Two")
    Case 3
       WriteLine("Three")
    Case Else
       WriteLine("Many")
End Select