Overview of C
2.1
C Language Elements
Preprocessor
Directives
preprocessor
directive a
C program line beginning with # that provides an instruction to the preprocessor. preprocessor a system program that modifies a C program
prior to its compilation library a collection of useful functions and symbols that may be
accessed by a program
/*
*
Converts distances from miles to kilometers.
*/
#include
<stdio.h>
/* printf, scanf definitions */
#define
KMS_PER_MILE 1.609
/* conversion constant */
int
main(void)
{
double
miles,
/*
distance in miles kms;
/* equivalent distance in kilometers */
/*
Get the distance in miles. */
printf("Enter
the distance in miles> ");
scanf("%lf",
&miles);
/*
Convert the distance to kilometers. */
kms
= KMS_PER_MILE * miles;
/*
Display the distance in kilometers. */
printf("That
equals %f kilometers.\n", kms);
return
(0);
constant macro a name that is
replaced by a particular constant value before the program is sent to the compiler
comment
text
beginning with /* and ending with */ that provides supplementary information
but is ignored by the preprocessor and compiler.
Syntax Displays for Preprocessor Directives
#include Directive for Defining Identifiers from
Standard Libraries
SYNTAX: #include <standard header file>
EXAPLES: #include
<stdio.h>
#include
<math.h>
INTERPRETATION: #include
directives tell the preprocessor where to find the
meanings of standard identifiers used in the program. These meanings are
collected in files called standard header files. ThMe header
file stdio.h contains information about standard input and output
functions such as scanf and printf. Descriptions of common
mathematical functions are found in the header file math.h .
We will investigate header files associated with other standard libraries in
later chapters.
#define
Directive for Creating Constant Macros
SYNTAX: #define
NAME value
EXAMPLES: #define
MILES_PER_KM 0.62137
#define PI 3.141593
#define MAX_LENGTH 100
INTERPRETATION: The C
preprocessor is notified that it is to replace each use of the identifier
NAME
by value . C program statements cannot
change the value associated with NAME .
Function main
declarations
the
part of a program that tells the compiler the names of memory cells in a program
executable statements program executable statements
program lines that are
converted
to
machine language instructions and executed by the computer
EXAMPLE: int
main(void)
{
printf("Hello
world\n");
return (0);
}
Reserved Words
reserved word a word that has
special meaning in C
Standard Identifiers
standard identifier
a
word having special meaning but one that a programmer may redefine (but redefinition is
not
recommended!)
2.2 Variable Declarations and Data Types
Variable Declarations
variable a name associated
with a memory cell whose value can change variable declarations statements that communicate
to the compiler the names of variables in the program and the kind of
information stored in each variable.
Data Types
data type a set of values
and operations that can be performed on those values.
Data Type
double A
real number has an integral part and a fractional part that
are separated by a decimal point. In
C, the data type double is used to represent
real numbers (for example, 3.14159 , 0.0005 , 150.0 ). You can store a
real number
in a type double variable, perform
the common arithmetic operations (add,
subtract, multiply, and divide), and
compare them.
The ASCII Code
ASCII code a particular code
that specifies the integer representing each char value.
Assignment Statements
assignment
statement an
instruction that stores a value or a computational result in a variable
Input/Output Operations and Functions
input operation an instruction
that copies data from an input device into memory output operation an instruction
that displays information stored in memory input/output function a C function that
performs an input or output operation. function call calling or
activating a function. function argument enclosed in
parentheses
following
the function
name;
provides
information
needed by
the
function
format string in a call to printf, a string of characters enclosed in quotes ("), which specifies the form
of the
output
line
print list in
a call to
printf, the variables or expressions
whose
values
are displayed
placeholder a
symbol
beginning
with % in
a
format string that indicates where to display the output value newline escape sequence
the character sequence \n, which is used
in a format string to terminate an
output
line.
Syntax
Display for printf Function Call
SYNTAX: printf(
format string, print list );
printf( format
string );
EXAMPLES: printf("I
am %d years old, and my gpa is %f\n",
age, gpa);
printf("Enter
the object mass in grams> ");
1. Every C program has preprocessor
directives and a main function. The main
function contains variable
declarations and executable statements.
2. Variable names must begin with a
letter or an underscore (the latter not recommended)
and consist of letters, digits, and
underscore symbols. A reserved
word cannot be used as an identifier.
3. C’s data types enable the compiler
to determine how to store a particular
value in memory and what operations
can be performed on that value.
Three standard data types are int , d ouble , and char . The data type of
each
variable must be declared.
4. The executable statements are
derived from the algorithm and are translated
into machine language. Assignment
statements are used to perform
computations and store results in
memory. Function calls are used to get
data (function scanf ) and to display
values stored in memory (function
printf ).
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