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FIRST PRINCIPLES.
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Turbo C is a general-purpose programming language. It is structured and
typed and uses a free format (ALGOL-like). It features an economy of
expression and has a rich set of operators. C is not a big language,
but by using functions, which are stored in library and header files for
inclusion in the user program, it becomes as effective as many
supposedly more powerful languages.
To appreciate these opening statements, a brief historical review of
high-level languages is helpful.
1957 FORTRAN numerically orientated - fixed format.
1960 ALGOL numerically orientated - structured and typed.
1965 BASIC simple for teaching in schools and colleges.
1971 Pascal for teaching program concepts and allowing
efficient implementation of large programs.
ALGOL-like.
1972 C a small but flexible language developed at the
Bell Laboratories.
1980 Ada designed for US Dept. of Defense. Pascal-like.
1983 Turbo Pascal an enhanced version of Pascal.
FORTRAN and BASIC have fixed format, such that each statement is located
at a (fixed) point on a line after a line number, obligatory in BASIC,
optional in FORTRAN. ALGOL introduced free format, which allows the
statements to be indented by varying amounts, so making programs more
readable.
ALGOL also introduced 'compound statements', such that several
statements can form the body of a 'for loop' provided that they are
surrounded in 'begin-end' brackets.
ALGOL also uses a hierarchical structure (nesting), whereby compound
statements can be executed repeatedly under the control of the 'for
loop', which itself is embedded or nested within other statements.
Whereas with FORTRAN and BASIC, 'if' statements require the use of GOTO,
ALGOL eliminates the need for the GOTO-statement, by its implementation
of compound statements. The 'if-then-else' statement can be similarly
accommodated and with suitable use of free format can produce a very
readable program structure, as shown below.
IF condition THEN
BEGIN
statement 1;
......
statement m
END
ELSE
BEGIN
statement n;
......
statement r
END;
To quote Edsger Dijkstra (1968), 'The difficulty in reading programs,
which make much use of GOTO statements is a result of the "conceptual
gap" between the static structure of the program (spread out on the
page) and the dynamic structure of the corresponding compuations
(spread out in time)'.
From this observation, the Structure Principle was defined as:
'The static structure of the program should correspond in a simple way
with the dynamic structure of the corresponding computations'.
Whereas ALGOL introduced the concept of separate data types such as
integer, real and Boolean, C is even more strongly typed, including
character and pointer types, etc. Integer and real data types are
further subdivided by size and range. The purpose of data typing is to
avoid errors and ambiguities at run-time, when for example 5.999 should
really be 6 for correct decision making within the program.
It follows that all variables must be declared and typed at the start of
the program. In fact, all constants, user-defined types and variables
must be predeclared.
In C, variable names can be of any length up to 32 characters and are
chosen to be meaningful to the reader. An assignment (i.e. giving a
value to a memory location defined by a variable name) is indicated by =
All statements are separated by a semi-colon (;). The text of the
program is 'case' sensitive, lower case being used in most instances.
All implementations of C should conform to the ANSI standard.
Syntactic errors are found at compile time, semantic errors at run
time and logical errors are detected in Turbo C by using the 'watch'
window and 'tracing', facilities which are available in the 'Integrated
Development Environment', which is one of the most helpful features of
Turbo C.