Transcript Database Systems

C Programming Language
 Developed in 1972 by Dennis Ritchie at
AT&T Bell Laboratories
 Used to rewrite the UNIX operating system
 Widely used on UNIX systems
 ANSI standardized in order to prevent the
fragmentation of the language
C Language Elements
 Preprocessor directives
– Commands processed by the preprocessor
before compilation
– #include (especially for standard header files)
• #include <stdio.h>
– The text of the included file is replaces the
#include
– #define (especially for constants)
• #define PI 3.14
– Every instance of (e.g.) PI is replaced with 3.14
C Language Elements
 Comments (begin with ‘/*’ end with ‘*/’ )
– /* HELLO THERE! */
 Comments have no effect on the program
and are ignored by the compiler
 Be careful to close all comments!
 Used carefully, comments can help
document a program
C Language Elements
 A program is divided into one or more
functions
– This makes a program easier to understand (we
can ignore the details of a function)
– We can also reuse someone else’s functions
 Every C program contains a function called
main
– Execution of the program begins with the main
function
C Language Elements
 The main function begins with the heading:
int main (void)
 The remainder of the function is called the
body of the function
– It is enclosed in braces {, }
– The body consists of declarations and
executable statements
– Declarations names memory cells needed by
the function and tells what kind of data will be
stored in each cell
C Language Elements
 Example: int my_age, your_age;
 Executable statements cause operations to
be performed
int
main(void)
{
printf(“Hello world\n”);
return(0);
}
C Language Elements
 Reserved words have special meanings in
C and cannot be used for other purposes
– Examples: int, void, return
 Standard identifiers are words defined in
the standard libraries and known to C.
– They can be used, but shouldn’t be!
 User-defined identifiers are words that you
define in your program
C Language Elements
 Identifiers are used to name memory cells
and operations that we define
 Identifiers must respect the following three
rules
– An identifier must consist only of letters, digits,
and underscores
– An identifier cannot begin with a digit
– A C reserved word cannot be used as an
identifier
C Language Elements
 Identifiers
– The case of the letters is significant in C! (cat
and CAT are two different identifiers)
– Choose meaningful names for your identifiers.
The people who maintain your programs will
be eternally grateful!
– 1. Unless there is a good reason, don’t use the
names of standard identifiers
– 2. There are no good reasons! Don’t do it.
Variable Declarations and Data
Types
 The memory cells which contain a
program’s input, intermediate results, and
final results are called variables
 Variable declarations tell the C compiler
the names of all of the variables which will
be used in a program and the kinds of
values they will store
Variable Declarations and Data
Types
 The form of a variable declaration is: type
variable_name;
– If we have more than one variable of the same
type, we can give the type just once and
separate the variables with commas: type var1,
var2, var3;
 The type must be one of the predefined C
data types (greatly simplified explanation!)
Variable Declarations and Data
Types
 Data types are a set of values and a set of
operations on those values
– The set of integers is one such set of values.
What are the operations defined on this set?
– Among the predefined data types of C are
char, double, and int which represent
characters, real numbers, and integers
– The range of data type int is at least -32767
through 32767
Variable Declarations and Data
Types
 Values of type double have both an integral
and a fractional part, separated by a decimal
point: 3.141, 0.1111, etc.
– We can also represent real numbers using
scientific notation: 5.001e3 or 5.001E3 which
means
– Either one of these representations can be used
in a C program
Variable Declarations and Data
Types
 Data type char represents an individual
character (not a string!)
– Each character is enclosed in apostrophes: ‘A’,
‘b’, ‘ ‘
 The values of a data type: ‘a’, 12, 4.0e3 are
called literals when used in a program
Executable Statements
 The executable statements follow the
declarations in a function. (Why?)
 These statements implement the algorithm
previously developed
 The statements must be translated into
machine language prior to execution
 We will now examine several common
types of statements
Executable Statements
 Assignment statements are perhaps the
most common of all statements. They assign
a value to a variable (a named memory cell)
– The syntax for an assignment statement is:
variable = expression;
– Note that an assignment statement will always
have a variable on the left hand side, never a
literal
Executable Statements
 Examples of assignment statements:
– a = 2.0;
– b = ‘c’;
– c = a;
– a = 2.0 + 7.0 * e;
– ‘b’ = ‘c’;
– j = k/l;
Executable Statements
 Input/Output operations are performed by
special-purpose functions in C
– scanf is an input operation
– printf is an output operation
– These functions are part of the standard i/o
library
– In order to use them we must have the
following statement in our program:
#include <stdio.h>
Executable Statements
 In order to activate a function like printf or
scanf we use a statement called a function
call
– This statement causes the executable statements
contained in the function to be executed.
 The function printf is called with two
arguments, a format string and a print list.
– printf(“That equals %f kilometers.\n”, kms);
Executable Statements
 The arguments to a function are pieces of
information that the function needs in order
to be executed
 The arguments to a function are enclosed in
parentheses and separated by commas.
 In the printf function, for each variable
whose value we want to print, we must have
a corresponding placeholder in the format
string
Executable Statements
 The placeholders for the data types we have
seen are given below:
–
–
–
–
–
%c
char
%d
int
%f
double (used in printf only)
%lf
double (used in scanf only)
Example:
• printf(“%c %d %f That’s all!\n”, ch, real, integer);
• Note that the ordering of the placeholders is
important!
Executable Statements
 The \n pair of symbols is called an escape
sequence (used to represent a non-printing
character) and causes the cursor to go to the
next line in the output
– Example: printf(\nline two\nline three\n”);
 The printf statement can be used to prompt
the user to enter some input