chapter01.html

think like a computer scientist

<P>For example, when you hear the sentence, ``The other shoe fell,'' you 
understand that ``the other shoe'' is the subject and ``fell'' is the verb.  
Once you have parsed a sentence, you can figure out what it means, that is, the
semantics of the sentence.  Assuming that you know what a shoe is, and what it 
means to fall, you will understand the general implication of this sentence.</P>

<P>Although formal and natural languages have many features in common---tokens,
structure, syntax and semantics---there are many differences.</P>

<DL>
  <DT>ambiguity:</DT><DD>Natural languages are full of ambiguity, which people 
  deal with by using contextual clues and other information.  Formal languages 
  are designed to be nearly or completely unambiguous, which means that any 
  statement has exactly one meaning, regardless of context.</DD>

  <DT>redundancy:</DT><DD>In order to make up for ambiguity and reduce 
  misunderstandings, natural languages employ lots of redundancy.  As a result,
  they are often verbose.  Formal languages are less redundant and more concise.
  </DD>

  <DT>literalness:</DT><DD>Natural languages are full of idiom and metaphor.  
  If I say, ``The other shoe fell,'' there is probably no shoe and nothing 
  falling.  Formal languages mean exactly what they say.</DD>
</DL>

<P>People who grow up speaking a natural language (everyone) often have a hard 
time adjusting to formal languages.  In some ways the difference between formal
and natural language is like the difference between poetry and prose, but more 
so:</P>

<DL>
  <DT>Poetry:</DT><DD>Words are used for their sounds as well as for their 
  meaning, and the whole poem together creates an effect or emotional response.
  Ambiguity is not only common but often deliberate.</DD>

  <DT>Prose:</DT><DD>The literal meaning of words is more important and the 
  structure contributes more meaning.  Prose is more amenable to analysis than 
  poetry, but still often ambiguous.</DD> 
  
  <DT>Programs:</DT><DD>The meaning of a computer program is unambiguous and 
  literal, and can be understood entirely by analysis of the tokens and 
  structure.</DD>
</DL>

<P>Here are some suggestions for reading programs (and other formal languages).
First, remember that formal languages are much more dense than natural 
languages, so it takes longer to read them.  Also, the structure is very 
important, so it is usually not a good idea to read from top to bottom, left to
right.  Instead, learn to parse the program in your head, identifying the 
tokens and interpreting the structure.  Finally, remember that the details 
matter.  Little things like spelling errors and bad punctuation, which you can 
get away with in natural languages, can make a big difference in a formal 
language.</P>

<BR><BR>
<H3>1.5 The first program</H3>

<P>Traditionally the first program people write in a new language is called 
``Hello, World.'' because all it does is print the words ``Hello, World.''  In 
C++, this program looks like this:</P>

<PRE>
#include &lt;iostream.h&gt;

// main: generate some simple output

void main ()
{
  cout << "Hello, world." << endl;
}
</PRE>

<P>Some people judge the quality of a programming language by the simplicity 
of the ``Hello, World.'' program.  By this standard, C++ does reasonably well.
Even so, this simple program contains several features that are hard to explain
to beginning programmers.  For now, we will ignore some of them, like the first
line.</P>

<P>The second line begins with <TT>//</TT>, which indicates that it is a 
<B>comment</B>.  A comment is a bit of English text that you can put in the 
middle of a program, usually to explain what the program does.  When the 
compiler sees a <TT>//</TT>, it ignores everything from there until the end of 
the line.</P>

<P>In the third line, you can ignore the word <TT>void</TT> for now, but 
notice the word <TT>main</TT>.  <TT>main</TT> is a special name that indicates 
the place in the program where execution begins.  When the program runs, it 
starts by executing the first statement in <TT>main</TT> and it continues, in 
order, until it gets to the last statement, and then it quits.</P>

<P>There is no limit to the number of statements that can be in <TT>main</TT>, 
but the example contains only one.  It is a basic <B>output</B> statement, 
meaning that it outputs or displays a message on the screen.</P>

<P><TT>cout</TT> is a special object provided by the system to allow you to 
send output to the screen.  The symbol <TT>&lt;&lt;</TT> is an <B>operator</B> 
that you apply to <TT>cout</TT> and a string, and that causes the string to be 
displayed.</P>

<P><TT>endl</TT> is a special symbol that represents the end of a line.  When 
you send an <TT>endl</TT> to <TT>cout</TT>, it causes the cursor to move to the 
next line of the display. The next time you output something, the new text 
appears on the next line.</P>

<P>Like all statements, the output statement ends with a semi-colon 
(<TT>;</TT>).</P>

<P>There are a few other things you should notice about the syntax of this 
program.  First, C++ uses squiggly-braces ({ and }) to group things together.
In this case, the output statement is enclosed in squiggly-braces, indicating 
that it is <I>inside</I> the definition of <TT>main</TT>.  Also, notice that 
the statement is indented, which helps to show visually which lines are inside 
the definition.</P>

<P>At this point it would be a good idea to sit down in front of a computer and
compile and run this program.  The details of how to do that depend on your 
programming environment, but from now on in this book I will assume that you 
know how to do it.</P>

<P>As I mentioned, the C++ compiler is a real stickler for syntax.  If you make
any errors when you type in the program, chances are that it will not compile 
successfully.  For example, if you misspell <TT>iostream</TT>, you might get an
error message like the following:</P>

<PRE>
hello.cpp:1: oistream.h: No such file or directory
</PRE>

<P>There is a lot of information on this line, but it is presented in a dense 
format that is not easy to interpret.  A more friendly compiler might say 
something like:</P>

<BLOCKQUOTE>
``On line 1 of the source code file named hello.cpp, you tried to include a 
header file named oistream.h.  I didn't find anything with that name, but I did
find something named iostream.h.  Is that what you meant, by any chance?''
</BLOCKQUOTE>

<P>Unfortunately, few compilers are so accomodating.  The compiler is not 
really very smart, and in most cases the error message you get will be only a 
hint about what is wrong.  It will take some time to gain facility at 
interpreting compiler messages.</P>

<P>Nevertheless, the compiler can be a useful tool for learning the syntax 
rules of a language.  Starting with a working program (like hello.cpp), modify 
it in various ways and see what happens.  If you get an error message, try to 
remember what the message says and what caused it, so if you see it again in 
the future you will know what it means.</P>

<BR><BR>
<H3>1.6 Glossary</H3>

<DL>
  <DT>problem-solving:</DT><DD>The process of formulating a problem, finding a 
  solution, and expressing the solution.</DD>

  <DT>high-level language:</DT><DD>A programming language like C++ that is 
  designed to be easy for humans to read and write.</DD>

  <DT>low-level language:</DT><DD>A programming language that is designed to be
  easy for a computer to execute.  Also called ``machine language'' or 
  ``assembly language.''</DD>

  <DT>portability:</DT><DDT>A property of a program that can run on more than 
  one kind of computer.</DD>

  <DT>formal language:</DT><DD>Any of the languages people have designed for 
  specific purposes, like representing mathematical ideas or computer programs.
  All programming languages are formal languages.</DD>

  <DT>natural language:</DT><DD>Any of the languages people speak that have 
  evolved naturally.</DD>

  <DT>interpret:</DT><DD>To execute a program in a high-level language by 
  translating it one line at a time.</DD>

  <DT>compile:</DT><DD>To translate a program in a high-level language into a 
  low-level language, all at once, in preparation for later execution.</DD>

  <DT>source code:</DT><DD>A program in a high-level language, before being 
  compiled.</DD>

  <DT>object code:</DT><DD>The output of the compiler, after translating the 
  program.</DD>

  <DT>executable:</DT><DD>Another name for object code that is ready to be 
  executed.</DD>

  <DT>algorithm:</DT><DD>A general process for solving a category of problems.
  </DD>

  <DT>bug:</DT><DD>An error in a program.</DD>

  <DT>syntax:</DT><DD>The structure of a program.</DD>

  <DT>semantics:</DT><DD>The meaning of a program.</DD>

  <DT>parse:</DT><DD>To examine a program and analyze the syntactic structure.
  </DD>

  <DT>syntax error:</DT><DD>An error in a program that makes it impossible to 
  parse (and therefore impossible to compile).</DD>

  <DT>run-time error:</DT><DD>An error in a program that makes it fail at 
  run-time.</DD>

  <DT>logical error:</DT><DD>An error in a program that makes it do something 
  other than what the programmer intended.</DD>

  <DT>debugging:</DT><DD>The process of finding and removing any of the three 
  kinds of errors.</DD>
</DL>

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