introduction to programming in C / PDF

Programming in C / PDF

Sample of the pdf document :

1. Introduction:

C is a computer language available on the GCOS and UNIX operating systems at Murray Hill and (in
preliminary form) on OS/360 at Holmdel. C lets you write your programs clearly and simply _ it has decent
control flow facilities so your code can be read straight down the page, without labels or GOTO’s; it
lets you write code that is compact without being too cryptic; it encourages modularity and good program
organization; and it provides good datastructuring facilities.

This memorandum is a tutorial to make learning C as painless as possible. The first part concentrates
on the central features of C; the second part discusses those parts of the language which are useful (usually
for getting more efficient and smaller code) but which are not necessary for the new user. This is not a reference manual. Details and special cases will be skipped ruthlessly, and no attempt will be made to cover
every language feature. The order of presentation is hopefully pedagogical instead of logical. Users who
would like the full story should consult the C Reference Manual by D. M. Ritchie [1], which should be read
for details anyway. Runtime support is described in [2] and [3]; you will have to read one of these to learn
how to compile and run a C program.

We will assume that you are familiar with the mysteries of creating files, text editing, and the like in
the operating system you run on, and that you have programmed in some language before.

2. A Simple C Program:

main( ) {
printf("hello, world");
}

A C program consists of one or more functions, which are similar to the functions and subroutines of
a Fortran program or the procedures of PL/I, and perhaps some external data definitions. main is such a
function, and in fact all C programs must have a main. Execution of the program begins at the first statement
of main. main will usually invoke other functions to perform its job, some coming from the same
program, and others from libraries.

One method of communicating data between functions is by arguments. The parentheses following
the function name surround the argument list; here main is a function of no arguments, indicated by ( ).
The {} enclose the statements of the function. Individual statements end with a semicolon but are otherwise
freeformat.printf is a library function which will format and print output on the terminal (unless some other destination is specified). In this case it prints


hello,
world

A function is invoked by naming it, followed by a list of arguments in parentheses. There is no CALL statement as in Fortran or PL/I.

3. A Working C Program; Variables; Types and Type Declarations

Here’s a bigger program that adds three integers and prints their sum.
main( ) {
int a, b, c, sum;
a = 1; b = 2; c = 3;
sum = a + b + c;
printf("sum is %d", sum);
}
Arithmetic and the assignment statements are much the same as in Fortran (except for the semicolons)
or PL/I. The format of C programs is quite free. We can put several statements on a line if we want,
or we can split a statement among several lines if it seems desirable. The split may be between any of the
operators or variables, but not in the middle of a name or operator. As a matter of style, spaces, tabs, and
newlines should be used freely to enhance readability.
C has four fundamental types of variables:
int integer (PDP11:
16 bits; H6070: 36 bits; IBM360: 32 bits)
char one byte character (PDP11,
IBM360: 8 bits; H6070: 9 bits)
float singleprecision
floating point
double doubleprecision
floating point
There are also arrays and structures of these basic types, pointers to them and functions that return them,
all of which we will meet shortly.
All variables in a C program must be declared, although this can sometimes be done implicitly by
context. Declarations must precede executable statements. The declaration
int a, b, c, sum;
declares a, b, c, and sum to be integers.
Variable names have one to eight characters, chosen from AZ,
az,
09,
and _, and start with a nondigit.
Stylistically, it’s much better to use only a single case and give functions and external variables
names that are unique in the first six characters. (Function and external variable names are used by various
assemblers, some of which are limited in the size and case of identifiers they can handle.) Furthermore,
keywords and library functions may only be recognized in one case.

4. Constants

We have already seen decimal integer constants in the previous example _ 1, 2, and 3. Since C is often
used for system programming and bitmanipulation,
octal numbers are an important part of the language.
In C, any number that begins with 0 (zero!) is an octal integer (and hence can’t have any 8’s or 9’s
in it). Thus 0777 is an octal constant, with decimal value 511.
A ‘‘character’’ is one byte (an inherently machinedependent
concept). Most often this is expressed
as a character constant, which is one character enclosed in single quotes. However, it may be any quantity
that fits in a byte, as in flags below:
char quest, newline, flags;
quest = ¢?¢;
newline = ¢\n¢;

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Programming in C / PDF