Visual Basic.NET How to Program (PDF)

136 Control Structures: Part 1 Chapter 4
Syntax errors are caught by the compiler. Logic errors affect the program only at execution time.
Fatal logic errors cause a program to fail and terminate prematurely. Nonfatal logic errors do not
terminate a program’s execution but cause the program to produce incorrect results.
Nested If/Then/Else structures test for multiple conditions by placing If/Then/Else structures
inside other If/Then/Else structures.
The While and Do While/Loop repetition structures allow the programmer to specify that an
action is to be repeated while a specific condition remains true.
Eventually, the condition in a While or Do While/Loop structure becomes false. At this point,
the repetition terminates, and the first statement after the repetition structure executes.
Failure to provide in the body of a While or Do While/Loop structure an action that eventually
causes the condition to become false is a logic error. Normally, such a repetition structure never
terminates, resulting in an error called an “infinite loop.”
Statements in the body of a Do Until/Loop are executed repeatedly as long as the loop-continuation
test evaluates to false.
Failure to provide the body of a Do Until/Loop structure with an action that eventually causes
the condition in the Do Until/Loop to become true creates an infinite loop.
Visual Basic provides the assignment operators +=, -=, *=, /=, \=, ^= and &= for abbreviating
assignment statements.
In counter-controlled repetition, a counter is used to repeat a set of statements a certain number of
times. Counter-controlled repetition is also called definite repetition because the number of repetitions
is known before the loop begins executing.
A total is a variable used to calculate the sum of a series of values.
It is important that variables used as totals and counters have appropriate initial values before they
are used. Counters usually are initialized to one. Totals generally are initialized to zero.
Data types Double and Single store floating-point numbers. Data type Double requires more
memory to store a floating-point value, but is more accurate and generally more efficient than type
Single.
In sentinel-controlled repetition, the number of repetitions is not known before the loop begins its
execution. Sentinel-controlled repetition uses a sentinel value (also called a signal value, dummy
value or flag value) to terminate repetition.
We approach programming problems with top-down, stepwise refinement, a technique essential
to the development of well-structured algorithms.
The top is a single statement that conveys the overall function of the program. As such, the top is
a complete representation of a program.
Through the process of refinement, we divide the top into a series of smaller tasks that are listed
in the order in which they must be performed. Each refinement, including the top, is a complete
specification of the algorithm; only the level of detail in each refinement varies.
Many algorithms can be divided logically into three phases: An initialization phase that initializes
the program variables, a processing phase that inputs data values and adjusts program variables
accordingly, and a termination phase that calculates and prints the results.
The programmer terminates the top-down, stepwise refinement process when the pseudocode algorithm
is specified in sufficient detail for the pseudocode to be converted to a Visual Basic program.
The implementation of the Visual Basic program then occurs in a normal, straightforward
manner.
The constants vbCrLf and vbTab represent the carriage return/linefeed character and the tab
character, respectively.