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Table of Contents
Learning C++:
An Index of Entry Points
2. The A reference document on the basic elements of C++.
3. The Patterns
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A. Algorithms - One Component of the Design
Phase Programming problems always involve two components:
2. A set of actions involving the objects, for example: The complexity of whatever part of the world (real or imaginary ) we
are attempting to model in a program is reflected in the interactions
among the objects modeled in the program. As a programmer you must have a
good handle on that complexity before you proceed to coding. Humans almost
always handle complexity using a divide and conquer strategy. We
break down or decompose complex problems into easier, more manageable
components. Later, we will discover that there are two basic approaches in
the programming world to this decomposition process. One can focus on the
objects in the problem environment and examine each object separate from
others. Or, one can focus on the actions/tasks involved in a problem and
focus on each task separate from all others. At present, however, we are
not ready to handle any real complexity. Instead, we will focus on
problems that are so simple as to have only one object and only one basic
task. A caution is in order - in reality even these problems can be
further decomposed but let's pretend otherwise for the moment. The amazing thing is that even with programming problems this simple it
is unwise to simply start coding. You still have to make sure that you
know how to accomplish the task - after all, how can you tell a computer
how to solve a problem if you do not know how to solve the problem
yourself. Your first goal then is to come up with some general plan or
description for solving the problem and then express that plan in a
systematic way. In other words, your goal is to state clearly and in order
the exact steps necessary to accomplish the task/problem . In Computer
Science terms, your goal is to write an algorithm.
Good recipes, especially those written for beginners, can be considered
algorithms. They are step by step descriptions for solving the problem of
preparing some kind of food. Whenever you write instructions for someone else, you need to ask
yourself, "What can I leave out?". In other words, how much does the
person already know and how much detail must I provide. For example, in a
cookie recipe, do you need to tell the person to grease the cookie sheet?
The answer is an emphatic 'YES' if you are writing for someone like at
least one of the authors of this text! You should provide all possible
details and you should do so in as simple a language as possible. . The same is true for computer based algorithms - algorithms that will
be turned into code in some programming language. Computers assume
nothing. They do not perform any action they are not explicitly told to
perform. In addition, the instructions they perform are simple and
programming languages such as C++ tend to reflect that simplicity. For
that reason algorithms should be written in simple, concise English. - no
compound sentences, no skipping of details. Think of yourself as writing
instructions for a ten year old who can read, knows the basics of
arithmetic and can follow instructions. In addition, to keep your
algorithms as general as possible, try to avoid writing your algorithms
for a specific language such as C++. B. Your First Algorithm Compare this with the actual code and you will
discover a number of points. The algorithm goes through the same steps as
the code but it is not written as C++. There are no variable declarations,
the rules for variables are not followed, it uses words like 'get' and
'output' that are clear English but not part of C++ and it does not
include some of the peculiarities of C++ syntax. (Some might object to the
use of '{' and '}' in the algorithms here, arguing that they should be
replaced by English terms such as 'Begin' and "End'. They are used here to
support learning C++.) Also, the algorithm does not worry about special points such as
constants. Actually, we could have written this algorithm with names such
as REGULAR HOURS and OVERTIME ADJUSTMENT for the values 40 and 1.5
respectively and it would have been more general. That would improve the
algorithm. On the other hand, programming languages include constants to
allow programs to be easily modified. At the algorithm development stage,
such considerations are not important. In this case, it was decided to not
include constants simply to make the algorithm easier to read. If we had not yet written the code but had carefully worked out this
algorithm, it would be easy to now translate this algorithm into code.
Each variable and constant would be appropriately declared; the Get
instructions would be turned into the appropriate pair of cout and cin
instructions; the 'if' statement would have been written with the
arithmetic instructions inside it; and the output instruction would have
been translated as cout..... The moral of the story is,
C. Purpose and Goal Statements Second, one must have a sense of what happens when instructions are
executed on a computer. As you write each instruction in an algorithm you
need to ask yourself:
Computers have what is
called a state which includes:
Each instruction modifies one or more elements of the state.
Instructions that include the assignment statement (=), modify or change
memory; 'if' statements may cause the computer to jump to a completely
different set of instructions; output (cout) instructions modify what is
on the monitor. The purpose of an instruction can be described as the
modification of the state of the computer in some way. The goal can be
described as the state of the computer AFTER the instruction is executed.
The goal can be described as the state of the computer AFTER the
instruction is executed, that is, the goal represents the state we want
the computer to be in after the line or section of code is executed.
Let's add 'Purpose and Goal' statements to our salary algorithm. Purpose: To get the rate of pay for an employee from the user
Purpose: To get the hours worked for an employee from the user
Purpose: To decide if the employee has worked overtime
Purpose: To calculate the overtime rate of pay Purpose: To calculate the overtime salary Purpose: To calculate the full salary of the employee (overtime
and regular) } } Purpose: To output the employee's salary to the screen
This sure is full of redundancy isn't it! In essence we have a series
of checks and balances here. (Remember your high school civics class?) The
goal captures the purpose and the instruction captures both the purpose
and the goal. If the goal and the instruction you write are redundant to
the purpose and the purpose is the correct purpose for this point in the
algorithm, then the instruction is correct. Because writing out algorithms
in this redundant format can be so tedious, we will not do it often.
However, to get you used to thinking carefully about the meaning and
purpose of instructions, you may be asked to follow this "Purpose, Goal,
Instruction" format in some exercises.
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