Unit 5 cspc

CSPC-COMPUTER SYSTEMS
PROGRAMMING IN ‘C’
ANKUR SRIVASTAVA
DEPARTMENT OF COMPUTER SCIENCE
08/11/18ANKUR SRIVASTAVA ASSISTANT PROFESSOR JETGI
1

POINTERS: INTRODUCTION, DECLARATION, APPLICATIONS FILE HANDLING.
STANDARD C PREPROCESSORS, DEFINING AND CALLING MACROS.
CONDITIONAL COMPILATION, PASSING VALUES TO THE COMPILER.
 UNIT-5 TOPICS
2

UNIT-5 POINTERS
• In a generic sense, a “pointer” is anything that tells us where
something can be found.
– Addresses in the phone book
– URLs for webpages
-- Road signs
What actually ptr is?
 ptr is a variable storing an address
 ptr is NOT storing the actual value of i
3

EXAMPLE OF POINTER
4
 int i = 5; ptr
 int *ptr;
 ptr = &i;
 printf(“i = %dn”, i);
 printf(“*ptr = %dn”, *ptr);
 printf(“ptr = %pn”, ptr);
Output:
i = 5
*ptr = 5
ptr = effff5e0
address of i
5i
value of ptr
= address of
i
in memory

POINTER APPLICATIONS IN C PROGRAMMING
Passing Parameter by Reference
Accessing Array element
Dynamic Memory Allocation :
Reducing size of parameter
Some other pointer applications :
--Passing Strings to function
--Provides effective way of implementing the different data
structures such as tree, graph, linked list
5

FILE HANDLING: GOALS
By the end of this unit we should understand …
 … how to open a file to write to it.
 … how to open a file to read from it.
 … how to open a file to append data to it.
 … how to read strings from a file.
 … how to write strings to a file.
6

DEFINITION
 A file is a collection of related data that a computers treats as a
single unit.
 Computers store files to secondary storage so that the contents
of files remain intact when a computer shuts down.
 When a computer reads a file, it copies the file from the storage
device to memory; when it writes to a file, it transfers data from
memory to the storage device.
7

OPERATIONS ON FILE
 Opening a file
 Reading data from a file
 Writing data to a file
 Closing a file
8

OPENING A FILE
 A file must be “opened” before it can be used.
FILE *fp;
:
fp = fopen (filename, mode);
 fp is declared as a pointer to the data type FILE.
 filename is a string - specifies the name of the file.
 fopen returns a pointer to the file which is used in all
subsequent file operations.
 mode is a string which specifies the purpose of opening the
file:
9

EXAMPLES
““r”r” :: open the file for reading only
““w”w” :: open the file for writing only
““a”a” :: open the file for appending data to it.
FILE *in, *out ;
in = fopen (“mydata.dat”, “r”) ;
out = fopen (“result.dat”, “w”);
FILE *empl ;
char filename[25];
scanf (“%s”, filename);
empl = fopen (filename, “r”) ;
10

CLOSING A FILE
 After all operations on a file have been completed, it must be closed.
 Ensures that all file data stored in memory buffers are properly written to
the file.
 General format: fclose (file_pointer) ;
FILE *xyz ;
xyz = fopen (“test”, “w”) ;
…….
fclose (xyz) ;
11

STANDARD C PREPROCESSORS
 The C preprocessor executes before a program is compiled.
 Some actions it performs are the inclusion of other files in the file being compiled,
definition of symbolic constants and macros, conditional compilation of program code
and conditional execution of preprocessor directives.
 Preprocessor directives begin with # and only white-space characters and comments
may appear before a preprocessor directive on a line.
 Macro definition
 #define, #undef
 File inclusion
 #include
 Conditional Compilation
 #if, #ifdef, #ifndef, #elseif, #else
12

PREPROCESSOR DIRECTIVES
13

#INCLUDE PREPROCESSOR DIRECTIVE
 The #include preprocessor directive has been used
throughout this text.
 The #include directive causes a copy of a specified file to
be included in place of the directive.
 The two forms of the #include directive are:
 #include <filename>
#include "filename"
 The difference between these is the location the preprocessor
begins searches for the file to be included.
 If the file name is enclosed in quotes, the preprocessor starts
searches in the same directory as the file being compiled for
the file to be included (and may search other locations, too).
14

#DEFINE PREPROCESSOR DIRECTIVE: SYMBOLIC
CONSTANTS
 The #define directive creates symbolic constants—constants represented as symbols—and
macros—operations defined as symbols.
 The #define directive format is
 #define identifier replacement-text
 When this line appears in a file, all subsequent occurrences of identifier that do not appear in
string literals will be replaced by replacement-text automatically before the program is compiled.
 Consider the following macro definition with one argument for the area of a circle:
 #define CIRCLE_AREA ( x ) ( ( PI ) * ( x ) * ( x ) )
15

#ERROR AND #PRAGMA PREPROCESSOR DIRECTIVES:
 The #error directive
 #error tokens
prints an implementation-dependent message including the tokens
specified in the directive.
 The tokens are sequences of characters separated by spaces.
 For example,
 #error 1 - Out of range error
contains 6 tokens.
 When a #error directive is processed on some systems, the tokens in
the directive are displayed as an error message, preprocessing stops and
the program does not compile.
16

PREPROCESSOR DIRECTIVES
Directive Function
#define Defines a macro substitution
#undef Undefines a macro
#include Specifies the files to be included
#ifdef Test for a macro definition
#endif Specifies the end of #if
#ifndef Tests whether a macro is not defined
#if Test a compile time condition
#else Specifies alternatives when #if test fails
17## token pasting It combines two arguments

TYPES OF MACROS
 Macro substitution is a process where an identifier in a program is
replaced by a predefined string composed of one or more tokens.
 These directives can be divided into three categories:
 Macro substitution directives
 File inclusion directives
 Compiler control directives
18
Simple MS
Argumented MS
Nested MS

MACRO SUBSTITUTION
 General form:
#define identifier string
1. SIMPLE MS:
19
MACRO IDENTIFIER STRING
#define COUNT 200
#define FALSE 0
#define SUBJECTS 5
#define PI 3.1415
#define CAPITAL “LUCKNOW”

MACRO SUBSTITUTION
 2. Argumented MS
 The preprocessor permits us to define more complex & more useful
form of replacements.
 It takes the form:
#define identifier (f1, f2,………….fn) string
20
#define MAX(a, b) (((a)>(b))?(a):(b))
#define MIN (a, b) (((a)<(b))?(a):(b))
#define ABS(x) (((x)>(0))?(x):(-x))
#define STREQ(s1, s2) (strcmp((s1,)(s2))==0)
#define STRGT(s1, s2) (strcmp((s1,)(s2))>0)

MACRO SUBSTITUTION
 Nested MS:
We can also use one macro in the definition of another macro.
21
#define M 5
#define N M+1
#define SQUARE(x) ((x)* (x))
#define CUBE(x) (SQUARE (x) *(x))
#define SIXTH(x) (CUBE (x) * CUBE (x))

CONDITIONAL COMPILATION:
 Conditional compilation enables you to control the execution of
preprocessor directives and the compilation of program code.
 Each of the conditional preprocessor directives evaluates a constant
integer expression.
 Cast expressions, sizeof expressions and enumeration constants cannot
be evaluated in preprocessor directives.
 The conditional preprocessor construct is much like the if selection
statement.
22

CONDITIONAL COMPILATION
 Structure similar to if
#if !defined( NULL )
#define NULL 0
#endif
 Every #if must end with #endif
 #ifdef short for #if defined( name )
 #ifndef short for #if !defined( name )
 Other statements
 #elif – equivalent of else if in an if structure
 #else – equivalent of else in an if structure
23

PASSING VALUES TO THE COMPILER
CALL BY VALUE CALL BY REFERENCE
Formal parameters are local variables
to callee.
Values of the actual parameters are
copied to the formal parameters when
function is called.
Conceptually simple.
Values of actual parameters are
protected.
Costly copying if parameters are large.
Can only return 1 value.
Formal parameters are pointers to
actual parameters.
Every use of the formal parameter is
implicitly dereferenced.
Actual parameters must have l-values.
(If not, either prohibit, or create
temporary unnamed variable in
caller).
Aliasing problem.
Can’t use in remote procedure call.
24

CALL BY VALUE & CALL BY REFERENCE
08/11/18
ANKUR SRIVASTAVA ASSISTANT PROFESSOR JETGI
25
#include<stdio.h>
void interchange(int number1, int number2)
{
int temp;
temp = number1;
number1 = number2;
number2 = temp;
}
int main() {
int num1 = 50, num2 = 70;
interchange(num1, num2);
printf(“n Number 1 : %d”, num1);
printf(“n Number 2 : %d”,num2);
return(0); }

OUTPUT
26
Number 1 : 50
Number 2 : 70

FILE HANDLING FUNCTIONS
File handling functions Description
fopen () fopen () function creates a new file or opens an existing file.
fclose () fclose () function closes an opened file.
getw () getw () function reads an integer from file.
putw () putw () functions writes an integer to file.
fgetc () fgetc () function reads a character from file.
fputc () fputc () functions write a character to file.
gets () gets () function reads line from keyboard.
puts () puts () function writes line to o/p screen.
fgets () fgets () function reads string from a file, one line at a time.
fputs () fputs () function writes string to a file.
feof () feof () function finds end of file.
fgetchar () fgetchar () function reads a character from keyboard. 08/11/18ANKUR SRIVASTAVA ASSISTANT PROFESSOR JETGI
27

File handling functions Description
fprintf () fprintf () function writes formatted data to a file.
fscanf () fscanf () function reads formatted data from a file.
fputchar ()
fputchar () function writes a character onto the output screen
from keyboard input.
fseek () fseek () function moves file pointer position to given location.
SEEK_SET
SEEK_SET moves file pointer position to the beginning of the
file.
SEEK_CUR SEEK_CUR moves file pointer position to given location.
SEEK_END SEEK_END moves file pointer position to the end of file.
ftell () ftell () function gives current position of file pointer.
rewind ()
rewind () function moves file pointer position to the beginning
of the file.
fflush () fflush () function flushes a file.
08/11/18
28

getc () getc () function reads character from file.
getch () getch () function reads character from keyboard.
getche ()
getche () function reads character from keyboard and echoes to
o/p screen.
getchar () getchar () function reads character from keyboard.
putc () putc () function writes a character to file.
putchar () putchar () function writes a character to screen.
printf () printf () function writes formatted data to screen.
sprinf () sprinf () function writes formatted output to string.
scanf () scanf () function reads formatted data from keyboard.
remove () remove () function deletes a file. 08/11/18
29

Unit 5 cspc

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Unit 5 cspc