Unit - 5

String

In this tutorial you will learn about Initializing Strings, Reading Strings from the terminal, Writing strings to screen, Arithmetic operations on characters, String operations (string.h), strlen() function, strcat() function, strcmp function, strcmpi() function, strcpy() function, strlwr () function, strrev() function and strupr() function

A string is a sequence of characters. Any sequence or set of characters defined within double quotation symbols is a constant string. In c it is required to do some meaningful operations on strings they are:

· Reading string displaying strings

· Combining or concatenating strings

· Copying one string to another.

· Comparing string & checking whether they are equal

· Extraction of a portion of a string

Strings are stored in memory as ASCII codes of characters that make up the string appended with ‘\0’(ASCII value of null). Normally each character is stored in one byte, successive characters are stored in successive bytes.

Initializing Strings

Following the discussion on characters arrays, the initialization of a string must the following form which is simpler to one dimension array.

char month1[ ]={‘j’,’a’,’n’,’u’,’a’,’r’,’y’,’\0’};

Then the string month is initializing to January. This is perfectly valid but C offers a special way to initialize strings. The above string can be initialized char month1[]=”January”; The characters of the string are enclosed within a part of double quotes. The compiler takes care of string enclosed within a pair of a double quotes. The compiler takes care of storing the ASCII codes of characters of the string in the memory and also stores the null terminator in the end.

/*String.c string variable*/
#include < stdio.h >
main()
{
char month[15];
printf (“Enter the string”);
gets (month);
printf (“The string entered is %s”, month);
}

In this example string is stored in the character variable month the string is displayed in the statement

printf(“The string entered is %s”, month);

It is one dimension array. Each character occupies a byte. A null character (\0) that has the ASCII value 0 terminates the string. The figure shows the storage of string January in the memory recall that \0 specifies a single character whose ASCII value is zero.

Character string terminated by a null character ‘\0’.

A string variable is any valid C variable name & is always declared as an array. The general form of declaration of a string variable is

Char string_name[size];

The size determines the number of characters in the string name.

Example:

Char month[10];

char address[100];

Reading Strings from the terminal:

The function scanf with %s format specification is needed to read the character string from the terminal.

Example:

Char address[15];

scanf(“%s”,address);
Scanf statement has a draw back it just terminates the statement as soon as it finds a blank space, suppose if we type the string new york then only the string new will be read and since there is a blank space after word “new” it will terminate the string.

Note that we can use the scanf without the ampersand symbol before the variable name.
In many applications it is required to process text by reading an entire line of text from the terminal.

The function gets can be used repeatedly to read a sequence of successive single characters and store it in the array.

Writing strings to screen:

The printf statement along with format specifier %s to print strings on to the screen. The format %s can be used to display an array of characters that is terminated by the null character for example printf(“%s”,name); can be used to display the entire contents of the array name

Arithmetic operations on characters:

We can also manipulate the characters as we manipulate numbers in c language. When ever the system encounters the character data it is automatically converted into a integer value by the system. We can represent a character as a interface by using the following method.

X=’a’;
Printf(“%d\n”,x);

Will display 97 on the screen. Arithmetic operations can also be performed on characters for example x=’z’-1; is a valid statement. The ASCII value of ‘z’ is 122 the statement the therefore will assign 121 to variable x.

It is also possible to use character constants in relational expressions for example
ch>’a’ && ch < = ’z’ will check whether the character stored in variable ch is a lower case letter. A character digit can also be converted into its equivalent integer value suppose un the expression a=character-‘1’; where a is defined as an integer variable & character contains value 8 then a= ASCII value of 8 ASCII value ‘1’=56-49=7.

We can also get the support of the c library function to converts a string of digits into their equivalent integer values the general format of the function in x=atoi(string) here x is an integer variable & string is a character array containing string of digits.

String operations (string.h)

C language recognizes that string is a different class of array by letting us input and output the array as a unit and are terminated by null character. C library supports a large number of string handling functions that can be used to array out many o f the string manipulations such as:

· Length (number of characters in the string).

· Concatentation (adding two are more strings)

· Comparing two strings.

· Substring (Extract substring from a given string)

· Copy(copies one string over another)

To do all the operations described here it is essential to include string.h library header file in the program.

strlen() function:

This function counts and returns the number of characters in a string. The length does not include a null character.

Syntax n=strlen(string);

Where n is integer variable. Which receives the value of length of the string.

Example

length=strlen(“Hollywood”);

The function will assign number of characters 9 in the string to a integer variable length.

/*writr a c program to find the length of the string using strlen() function*/
#include < stdio.h >
include < string.h >
void main()
{
char name[100];
int length;
printf(“Enter the string”);
gets(name);
length=strlen(name);
printf(“\nNumber of characters in the string is=%d”,length);
}

strcat() function:

when you combine two strings, you add the characters of one string to the end of other string. This process is called concatenation. The strcat() function joins 2 strings together. It takes the following form

strcat(string1,string2)

string1 & string2 are character arrays. When the function strcat is executed string2 is appended to string1. the string at string2 remains unchanged.

Example

strcpy(string1,”sri”);
strcpy(string2,”Bhagavan”);
Printf(“%s”,strcat(string1,string2);

From the above program segment the value of string1 becomes sribhagavan. The string at str2 remains unchanged as bhagawan.

strcmp function:

In c you cannot directly compare the value of 2 strings in a condition like if(string1==string2)
Most libraries however contain the strcmp() function, which returns a zero if 2 strings are equal, or a non zero number if the strings are not the same. The syntax of strcmp() is given below:

Strcmp(string1,string2)

String1 & string2 may be string variables or string constants. String1, & string2 may be string variables or string constants some computers return a negative if the string1 is alphabetically less than the second and a positive number if the string is greater than the second.

Example:

strcmp(“Newyork”,”Newyork”) will return zero because 2 strings are equal.
strcmp(“their”,”there”) will return a 9 which is the numeric difference between

ASCII ‘i’ and ASCII ‘r’ strcmp(“The”, “the”) will return 32 which is the numeric difference between ASCII “T” & ASCII “t”.

strcmpi() function

This function is same as strcmp() which compares 2 strings but not case sensitive.

Example

strcmpi(“THE”,”the”); will return 0.

strcpy() function:

C does not allow you to assign the characters to a string directly as in the statement name=”Robert”;
Instead use the strcpy(0 function found in most compilers the syntax of the function is illustrated below.

strcpy(string1,string2);

Strcpy function assigns the contents of string2 to string1. string2 may be a character array variable or a string constant.

strcpy(Name,”Robert”);

In the above example Robert is assigned to the string called name.

strlwr () function:

This function converts all characters in a string from uppercase to lowercase.

syntax

strlwr(string);

For example:

strlwr(“EXFORSYS”) converts to exforsys.

strrev() function:

This function reverses the characters in a string.

Syntax

strrev(string);

For ex strrev(“program”) reverses the characters in a string into “margrop”.

strupr() function:

This function converts all characters in a string from lower case to uppercase.

Syntax

strupr(string);

For example strupr(“exforsys”) will convert the string to EXFORSYS.

/* Example program to use string functions*/
#include < stdio.h >
#include < string.h >
void main()
{
char s1[20],s2[20],s3[20];
int x,l1,l2,l3;
printf(“Enter the strings”);
scanf(“%s%s”,s1,s2);
x=strcmp(s1,s2);
if(x!=0)
{printf(“\nStrings are not equal\n”);
strcat(s1,s2);
}
else
printf(“\nStrings are equal”);
strcpy(s3,s1);
l1=strlen(s1);
l2=strlen(s2);
l3=strlen(s3);
printf(“\ns1=%s\t length=%d characters\n”,s1,l1);
printf(“\ns2=%s\t length=%d characters\n”,s2,l2);
printf(“\ns3=%s\t length=%d characters\n”,s3,l3);
}

File management in C

In this tutorial you will learn about C Programming - File management in C, File operation functions in C, Defining and opening a file, Closing a file, The getw and putw functions, The fprintf & fscanf functions, Random access to files and fseek function.

C supports a number of functions that have the ability to perform basic file operations, which include:

Naming a file
Opening a file
Reading from a file
Writing data into a file
Closing a file

File operation functions in C:

Function Name	Operation
fopen()	Creates a new file for use Opens a new existing file for use
fclose	Closes a file which has been opened for use
getc()	Reads a character from a file
putc()	Writes a character to a file
fprintf()	Writes a set of data values to a file
fscanf()	Reads a set of data values from a file
getw()	Reads a integer from a file
putw()	Writes an integer to the file
fseek()	Sets the position to a desired point in the file
ftell()	Gives the current position in the file
rewind()	Sets the position to the begining of the file

Defining and opening a file:

If we want to store data in a file into the secondary memory, we must specify certain things about the file to the operating system. They include the fielname, data structure, purpose.

The general format of the function used for opening a file is

FILE *fp;

fp=fopen(“filename”,”mode”);

The first statement declares the variable fp as a pointer to the data type FILE. As stated earlier, File is a structure that is defined in the I/O Library. The second statement opens the file named filename and assigns an identifier to the FILE type pointer fp. This pointer, which contains all the information about the file, is subsequently used as a communication link between the system and the program.
The second statement also specifies the purpose of opening the file. The mode does

this job.

R open the file for read only.

W open the file for writing only.

A open the file for appending data to it.

Consider the following statements

FILE *p1, *p2;

p1=fopen(“data”,”r”);
p2=fopen(“results”,”w”);

In these statements the p1 and p2 are created and assigned to open the files data and results respectively the file data is opened for reading and result is opened for writing. In case the results file already exists, its contents are deleted and the files are opened as a new file. If data file does not exist error will occur.

Closing a file:

The input output library supports the function to close a file; it is in the following format.
fclose(file_pointer);
A file must be closed as soon as all operations on it have been completed. This would close the file associated with the file pointer.

Observe the following program.

FILE *p1 *p2;

p1=fopen (“Input”,”w”);

p2=fopen (“Output”,”r”);

….
…
fclose(p1);
fclose(p2)

The above program opens two files and closes them after all operations on them are completed, once a file is closed its file pointer can be reversed on other file.
The getc and putc functions are analogous to getchar and putchar functions and handle one character at a time. The putc function writes the character contained in character variable c to the file associated with the pointer fp1. ex putc(c,fp1); similarly getc function is used to read a character from a file that has been open in read mode. c=getc(fp2).

The program shown below displays use of a file operations. The data enter through the keyboard and the program writes it. Character by character, to the file input. The end of the data is indicated by entering an EOF character, which is control-z. the file input is closed at this signal.

#include< stdio.h >

main()
{
file *f1;
printf(“Data input output”);
f1=fopen(“Input”,”w”); /*Open the file Input*/
while((c=getchar())!=EOF) /*get a character from key board*/
putc(c,f1); /*write a character to input*/
fclose(f1); /*close the file input*/
printf(“\nData output\n”);
f1=fopen(“INPUT”,”r”); /*Reopen the file input*/
while((c=getc(f1))!=EOF)
printf(“%c”,c);
fclose(f1);
}

The getw and putw functions:

These are integer-oriented functions. They are similar to get c and putc functions and are used to read and write integer values. These functions would be usefull when we deal with only integer data.

The fprintf & fscanf functions:

The fprintf and scanf functions are identical to printf and scanf functions except that they work on files. The first argument of theses functions is a file pointer which specifies the file to be used. The general form of fprintf is

fprintf(fp,”control string”, list);

Where fp id a file pointer associated with a file that has been opened for writing. The control string is file output specifications list may include variable, constant and string.

fprintf(f1,%s%d%f”,name,age,7.5);

Here name is an array variable of type char and age is an int variable

The general format of fscanf is

fscanf(fp,”controlstring”,list);

This statement would cause the reading of items in the control string.

Example:
fscanf(f2,”5s%d”,item,&quantity”);

Random access to files:

Sometimes it is required to access only a particular part of the and not the complete file. This can be accomplished by using the following function:

1 > fseek

fseek function:

The general format of fseek function is a s follows:

fseek(file pointer,offset, position);

This function is used to move the file position to a desired location within the file. Fileptr
is a pointer to the file concerned. Offset is a number or variable of type long, and position in an integer number. Offset specifies the number of positions (bytes) to be moved from the location specified bt the position. The position can take the 3 values.

Value Meaning
0 Beginning of the file
1 Current position
2 End of the file

The Preprocessor

A unique feature of c language is the preprocessor. A program can use the tools provided by preprocessor to make his program easy to read, modify, portable and more efficient.

Preprocessor is a program that processes the code before it passes through the compiler.

It operates under the control of preprocessor command lines and directives. Preprocessor directives are placed in the source program before the main line before the source code passes through the compiler it is examined by the preprocessor for any preprocessor directives. If there is any appropriate actions are taken then the source program is handed over to the compiler.

The preprocessor offers several features called Preprocessor directives .Preprocessor directives follow the special syntax rules and begin with the symbol # and do not require any semicolon at the end. A set of commonly used preprocessor directives

Preprocessor directives:

Directive	Function
#define	Defines a macro substitution
#undef	Undefined a macro
#include	Specifies a file to be included
#ifdef	Tests for macro definition
#endif	Specifies the end of #if
#ifndef	Tests whether the macro is not def
#if	Tests a compile time condition
#else	Specifies alternatives when # if test fails

The preprocessor directives can be divided into three categories
1. Macro substitution /expansion directive

2. File inclusion directive

3. Conditional Compiler directive

Macros:

Macro substitution is a process where an identifier in a program is replaced by a pre defined string composed of one or more tokens we can use the #define statement for the task.

It has the following form

#define identifier string

The preprocessor replaces every occurrence of the identifier the source code by a string. The definition should start with the keyword #define and should follow on identifier and a string with at least one blank space between them. The string may be any text and identifier must be a valid c name.

There are different forms of macro substitution. The most common form is
1.Simplemacrosubstitution
2.Argumentmacrosubstitution

Simple macro substitution:

Simple string replacement is commonly used to define constants example:

#define pi 3.1415926

Writing macro definition in capitals is a convention not a rule a macro definition can include more than a simple constant value it can include expressions as well. Following are valid examples:

#define AREA 12.36

#include<conio.h>

#include<stdio.h>

#define kimt 10

void main()

{

int i;

clrscr();

for(i=0;i<kimt;i++)

printf("%d\n",i);

getch();

}

Macros as arguments:

The preprocessor permits us to define more complex and more useful form of replacements it takes the following form.

# define identifier(f1,f2,f3…..fn) string.

Notice that there is no space between identifier and left parentheses and the identifier f1,f2,f3 …. Fn is analogous to formal arguments in a function definition.

There is a basic difference between simple replacement discussed above and replacement of macro arguments is known as a macro call

A simple example of a macro with arguments is

# define CUBE (x) (x*x*x)

If the following statements appears later in the program,

volume=CUBE(side);

The preprocessor would expand the statement to

volume =(side*side*side)

#include<conio.h>

#include<stdio.h>

#define cube(x) (x*x*x)

void main()

{

int i;

clrscr();

i=cube(5);

printf("%d\n",i);

getch();

}

Undefining a macro:

A defined macro can be undefined using the statement

# undef identifier.

This is useful when we want to restrict the definition only to a particular part of the program.

File inclusion:

The preprocessor directive "#include file name” can be used to include any file in to your program if the function s or macro definitions are present in an external file they can be included in your file

In the directive the filename is the name of the file containing the required definitions or functions alternatively the this directive can take the form

#include< filename >

Without double quotation marks. In this format the file will be searched in only standard directories.
The c preprocessor also supports a more general form of test condition #if directive. This takes the following form

#Ifconstantexpression
{
statement-1;
statemet2’
….
….
}
#endif
the constant expression can be a logical expression such as test < = 3 etc

If the result of the constant expression is true then all the statements between the #if and #endif are included for processing otherwise they are skipped. The names TEST LEVEL etc., may be defined as macros.

Conditional Compiler directive

C preprocessor provides a conditional compilation directive which is used to select alternate segment of code in a c program depending upon the condition.

1. #ifdef #else #endif

2. #ifndef #else #endif

3. #if #else #endif

The #ifdef #else #endif directive allows us to skip part of the source code.The genral form of this preprocessor directive is:

#ifdef macroname

statement1;

#else

statement2;

#endif

In case we have used #define for macro_name then statement1 will be compiled otherwise statement2.

#include<conio.h>

#include<stdio.h>

#define test

void main()

{

clrscr();

#ifdef test

printf("kimt");

#else

printf("mbd");

#endif

getch();

}

The #ifndef #else #endif directive works exactly opposite to the directive #ifdef #else #endif.The genral form of this preprocessor directive is:

#ifndef macroname

statement1;

#else

statement2;

#endif

In case we have not used #define for macro_name then statement1 will be compiled otherwise statement2.

#include<conio.h>

#include<stdio.h>

#define test

void main()

{

clrscr();

#ifdef test

printf("kimt");

#else

printf("mbd");

#endif

getch();

The #if #else #endif directive tests an expression for non zero value and performs the conditional compilation. The genral form of this preprocessor directive is:

#if constant expression

statement1;

#else

statement2;

#endif

In case the constant expression following #if is true (non-zero) then statement1 is compiled otherwise statement2.

#include<conio.h>

#include<stdio.h>

#define int test=6

void main()

{

clrscr();