String Handling, String Manipulation Functions, Character Handling Functions, String Conversion Functions

<< Multi-dimensional Arrays, Pointers to Pointers, Command-line Arguments

Files: Text File Handling, Output File Handling >>

CS201 Introduction to Programming

Lecture Handout

Introduction to Programming

Lecture No. 17

Reading Material

Deitel & Deitel - C++ How to Program

Chapter 5

5.29,5.30, 5.31, 5.32, 5.33, 5.34

Chapter 16

16.16 16.33 ( Pages 869 884)

Summary

String Handling

String Manipulation Functions

Character Handling Functions

Sample Program

String Conversion Functions

String Functions

Search Functions

Examples

Exercises

String Handling

We have briefly talked about 'Strings' in some of the previous lectures. In this lecture,

you will see how a string may be handled. Before actually discussing the subject, it is

pertinent to know how the things were going on before the evolution of the concept of

'strings'.

When C language and UNIX operating system were being developed in BELL

Laboratories, the scientists wanted to publish the articles. They needed a text editor to

publish the articles. What they needed was some easy mechanism by which the articles

could be formatted and published. We are talking about the times when PCs and word

Page 194

CS201 Introduction to Programming

processors did not exist. It may be very strange thing for you people who can perform

the tasks like making the characters bold, large or format a paragraph with the help of

word processors these days. Those scientists had not such a facility available with them.

The task of writing article and turning into publishable material was mainly done with the

help of typewriters. Then these computer experts decided to develop a program, which

could help in the processing of text editing in an easy manner. The resultant efforts led to

the development of a program for editing the text. The process to edit text was called text

processing. The in- line commands were written as a part of the text and were processed

on out put. Later, such programs were evolved in which a command was inserted for the

functions like making the character bold. The effect of this command could be preview

and then modified if needed.

Now coming to the topic of strings again, we will discuss in detail the in-built functions

to handle the strings.

String Manipulation Functions

C language provides many functions to manipulate strings. To understand the functions,

let's consider building block (or unit) of a string i.e., a character. Characters are

represented inside the computers in terms of numbers. There is a code number for each

character, used by a computer. Mostly the computers use ASCII (American Standard

Code for Information Interchange) code for a character to store it. This is used in the

computer memory for manipulation. It is used as an output in the form of character. We

can write a program to see the ASCII values.

We have a data type char to store a character. A character includes every thing, which we

can type with a keyboard for example white space, comma, full stop and colon etc all are

characters. 0, 1, 2 are also characters. Though, as numbers, they are treated differently,

yet they are typed as characters. Another data type is called as int, which stores whole

numbers. As we know that characters are stored in side computer as numbers so these can

be manipulated in the same form. A character is stored in the memory in one byte i.e. 8

bits. It means that 28 (256) different combinations for different values can be stored. We

want to ascertain what number it stores, when we press a key on the board. In other

words, we will see what character will be displayed when we have a number in memory.

The code of the program, which displays the characters and their corresponding integer,

values (ASCII codes) as under.

In the program the statement c = i ; has integer value on right hand side (as i is an int)

while c has its character representation. We display the value of i and c. It shows us the

characters and their integer values.

//This program displays the ASCII code table

# include <iostream.h>

main ( )

{

Page 195

CS201 Introduction to Programming

int i, char c ;

for (i = 0 ; i < 256 ; i ++)

{

c=i;

cout << i << "\t" << c << "\n" ;

}

In the output of this program, we will see integer numbers and their character

representation. For example, there is a character, say white space (which we use between

two words). It is a non-printable character and leaves a space. From the ASCII table, we

can see that the values of a-z and A-Z are continuos. We can get the value of an alphabet

letter by adding 1 to the value of its previous letter. So what we need to remember as a

baseline is the value of `a' and `A'.

Character Handling Functions

C language provides many functions to perform useful tests and manipulations of

character data. These functions are found in the header file ctype.h. The programs that

have character manipulation or tests on character data must have included this header file

to avoid a compiler error. Each function in ctype.h receives a character (an int ) or EOF

(end of file; it is a special character) as an argument. ctype.h has many functions, which

have self-explanatory names.

Of these, int isdigit (int c) takes a simple character as its argument and returns true or

false. This function is like a question being asked. The question can be described whether

it is a character digit? The answer may be true or false. If the argument is a numeric

character (digit), then this function will return true otherwise false. This is a useful

function to test the input. To check for an alphabet (i.e. a-z), the function isalpha can be

used. isalpha will return true for alphabet a-z for small and capital letters. Other than

alphabets, it will return false. The function isalnum (is alphanumeric) returns true if its

argument is a digit or letter. It will return false otherwise. All the functions included in

ctype.h are shown in the following table with their description.

Prototype

Description

Returns true if c is a digit and false otherwise.

int isdigit( int c )

int isalpha( int c )

Returns true if c is a letter and false otherwise.

int isalnum( int c ) Returns true if c is a digit or a letter and false otherwise.

Returns true if c is a hexadecimal digit character and false

int isxdigit( int c )

otherwise.

Page 196

CS201 Introduction to Programming

Returns true if c is a lowercase letter and false otherwise.

int islower( int c )

int isupper( int c ) Returns true if c is an uppercase letter; false otherwise.

int tolower( int c ) If c is an uppercase letter, tolower returns c as a lowercase letter.

Otherwise, tolower returns the argument unchanged.

int toupper( int c ) If c is a lowercase letter, toupper returns c as an uppercase letter.

Otherwise, toupper returns the argument unchanged.

Returns true if c is a white-space character--newline ('\n'), space

int isspace( int c )

(' '), form feed ('\f'), carriage return ('\r'), horizontal tab ('\t'), or

vertical tab ('\v')--and false otherwise

Returns true if c is a control character and false otherwise.

int iscntrl( int c )

Returns true if c is a printing character other than a space, a digit,

int ispunct( int c )

or a letter and false otherwise.

Returns true value if c is a printing character including space (' ')

int isprint( int c )

and false otherwise.

int isgraph( int c ) Returns true if c is a printing character other than space (' ') and

false otherwise.

The functions tolower and toupper are conversion functions. The tolower function

converts its uppercase letter argument into a lowercase letter. If its argument is other than

uppercase letter, it returns the argument unchanged. Similarly the toupper function

converts its lowercase letter argument into uppercase letter. If its argument is other than

lowercase letter, it returns the argument without effecting any change.

Sample Program

Let's consider the following example to further demonstrate the use of the functions of

ctype.h. Suppose, we write a program which prompts the user to enter a string. Then

the string entered is checked to count different types of characters (digit, upper and

lowercase letters, white space etc). We keep a counter for each category of character

entered. When the user ends the input, the number of characters entered in different

types will be displayed. In this example we are using a function getchar(), instead of cin

to get the input. This function is defined in header file as stdio.h. While carrying out

character manipulation, we use the getchar() function. This function reads a single

character from the input buffer or keyboard. This function can get the new line character

`\n' (the ENTER key) so we run the loop for input until user presses the ENTER key. As

soon as the getchar() gets the ENTER key pressed (i.e. new line character `\n'), the loop

is terminated. We know that, every C statement returns a value. When we use an

assignment statement ( as used in our program c = getchar()), the value assigned to the

left hand side variable is the value of the statement too. Thus, the statement (c =

getchar()) returns the value that is assigned to char c. Afterwards, this value is

compared with the new line character `\n'. If it is not equal inside the loop, we apply the

Page 197

CS201 Introduction to Programming

tests on c to check whether it is uppercase letter, lowercase letter or a digit etc. In this

program, the whole string entered by the user is manipulated character.

Following is the code of this program.

// Example: analysis of text using <ctype.h> library

#include <iostream.h>

#include <stdio.h>

#include <ctype.h>

main()

{

char c;

int i = 0, lc = 0, uc = 0, dig = 0, ws = 0, pun = 0, oth = 0;

cout << "Please enter a character string and then press ENTER: ";

// Analyse text as it is input:

while ((c = getchar()) != '\n')

{

if (islower(c))

lc++;

else if (isupper(c))

uc++;

else if (isdigit(c))

dig++;

else if (isspace(c))

ws++;

else if (ispunct(c))

pun++;

else

oth++;

}

// display the counts of different types of characters

cout << "You typed:"<< endl;

cout<< "lower case letters = "<< lc<< endl;

cout << "upper case letters = " << uc <<endl;

cout<< "digits = " << dig << endl;

cout<< "white space = "<< ws << endl;

cout<< "punctuation = "<< pun<< endl;

cout<< "others = "<< oth;

}

Page 198

CS201 Introduction to Programming

A sample output of the program is given below.

Please enter a character string and then press ENTER: Sixty Five = 65.00

You typed:

lower case letters = 7

upper case letters = 2

digits = 4

white space = 3

punctuation = 2

others = 0

String Conversion Functions

The header file stdlib.h includes functions, used for different conversions. When we get

input of a different type other than the type of variable in which the value is being stored,

it warrants the need to convert that type into another type. These conversion functions

take an argument of a type and return it after converting into another type. These

functions and their description are given in the table below.

Prototype

Description

Converts the string nPtr to double.

double atof( const char *nPtr )

Converts the string nPtr to int.

Int atoi( const char *nPtr )

Converts the string nPtr to long int.

long atol( const char *nPtr )

Converts the string nPtr to double.

double strtod( const char *nPtr, char

**endPtr )

Converts the string nPtr to long.

long strtol( const char *nPtr, char

**endPtr, int base )

Converts the string nPtr to unsigned long.

unsigned long strtoul( const char

*nPtr, char **endPtr, int base )

Use of these functions:

While writing main () in a program, we can put them inside the parentheses of main. `int

arg c, char ** arg v are written inside the parentheses. The arg c is the count of number

of arguments passed to the program including the name of the program itself while arg v

is a vector of strings or an array of strings. It is used while giving command line

arguments to the program. The arguments in the command line will always be character

strings. The number in the command line (for example 12.8 or 45) are stored as strings.

While using the numbers in the program, we need these conversion functions.

Following is a simple program which demonstrate the use of atoi function. This program

prompts the user to enter an integer between 10-100, and checks if a valid integer is

entered.

//This program demonstrate the use of atoi function

Page 199

CS201 Introduction to Programming

# include <iostream.h>

# include <stdlib.h>

main( )

{

int anInteger;

char myInt [20]

cout << "Enter an integer between 10-100 : ";

cin >> myInt;

if (atoi(myInt) == 0)

cout << "\nError : Not a valid input"; // could be non numeric

else

{

anInteger = atoi(myInt);

if (anInteger < 10 || anInteger > 100)

cout << "\nError : only integers between 10-100 are allowed!";

else

cout << "\n OK, you have entered " << anInteger;

}

The output of the program is as follows.

Enter an integer between 10-100 : 45.5

OK, you have entered 45

String Functions

We know a program to guess a number, stored in the computer. To find out a name

(which is a character array) among many names in the memory, we can perform string

comparison on two strings by comparing a character of first string with the corresponding

character of the second string. Before doing this, we check the length of both the strings

to compare. C library provides functions to compare strings, copy a string and for other

string manipulations.

The following table shows the string manipulation functions and their description. All

these functions are defined in the header file string.h, in the C library.

Function prototype

Function description

Copies string s2 into character array s1.

char *strcpy( char *s1, const char *s2 )

The value of s1 is returned.

Copies at most n characters of string s2

char *strncpy( char *s1, const char *s2,

into array s1. The value of s1 is

size_t n )

Page 200

CS201 Introduction to Programming

returned.

Appends string s2 to array s1. The first

char *strcat( char *s1, const char *s2 )

character of s2 overwrites the

terminating null character of s1. The

value of s1 is returned.

Appends at most n characters of string

char *strncat( char *s1, const char *s2,

s2 to array s1. The first character of s2

size_t n )

overwrites the terminating null character

of s1. The value of s1 is returned.

int strcmp( const char *s1, const char *s2) Compares string s1 to s2. Returns a

negative number if s1 < s2, zero if s1 ==

s2 or a positive number if s1 > s2

int strncmp( const char *s1, const char *s2, Compares up to n characters of string s1

to s2. Returns a negative number if s1 <

size_t n )

s2, zero if s1 == s2 or a positive number

if s1 > s2.

int strlen ( const char *s)

Determines the length of string s. The

number of characters preceding the

terminating null character is returned.

Let's look at the string copy function which is strcpy. The prototype of this function is

char *strcpy( char *s1, const char *s2 )

Here the first argument is a pointer to a character array or string s1 whereas the second

argument is a pointer to a string s2. The string s2 is copied to string s1 and a pointer to

that resultant string is returned. The string s2 remains the same. We can describe the

string s1 as the destination string and s2 as the source string. As the source remains the

same during the execution of strcpy and other string functions, the const keyword is used

before the name of source string. The const keyword prevents any change in the source

string (i.e. s2). If we want to copy a number of characters of a string instead of the entire

string, the function strncpy is employed. The function strncpy has arguments a pointer

to destination strings (s1), a pointer to source string (s2) . The third argument is int n.

Here n is the number of characters which we want to copy from s2 into s1. Here s1 must

be large enough to copy the n number of characters.

The next function is strcat (string concatenation). This function concatenates (joins) two

strings. For example, in a string, we have first name of a student, followed by another

string, the last name of the student is found. We can concatenate these two strings to get a

string, which holds the first and the last name of the student. For this purpose, we use the

strcat function. The prototype of this function is char *strcat( char *s1, const char *s2

). This function writes the string s2 (source) at the end of the string s1(destination). The

characters of s1 are not overwritten. We can concatenate a number of characters of s2 to

s1 by using the function strncat. Here we provide the function three arguments, a

character pointer to s1, a character pointer to s2 while third argument is the number of

characters to be concatenated. The prototype of this function is written as

char *strncat( char *s1, const char *s2, size_t n )

Page 201

CS201 Introduction to Programming

Examples

Let's consider some simple examples to demonstrate the use of strcpy, strncpy, strcat

and strncat functions. To begin with, we can fully understand the use of the function

strcpy and strncpy.

Example 1

//Program to display the operation of the strcpy() and strncpy()

# include<iostream.h>

# include<string.h>

void main()

{

char string1[15]="String1";

char string2[15]="String2";

cout<<"Before the copy :"<<endl;

cout<<"String 1:\t"<<string1<<endl;

cout<<"String 2:\t"<<string2<<endl;

//copy the whole string

strcpy(string2,string1); //copy string1 into string2

cout<<"After the copy :"<<endl;

cout<<"String 1:\t"<<string1<<endl;

cout<<"String 2:\t"<<string2<<endl;

//copy three characters of the string1 into string3

strncpy(string3, string1, 3);

cout << "strncpy (string3, string1, 3) = " << string3 ;

}

Following is the output of the program.

Before the copy :

String 1:

String1

String 2:

String2

After the copy :

String 1:

String1

String 2:

String1

Strncpy (string3, string1, 3) = Str

Example 2 (strcat and strncat)

Page 202

CS201 Introduction to Programming

The following example demonstrates the use of function strcat and strncat.

//Program to display the operation of the strcat() and strncat()

#include <iostream.h>

#include <string.h>

int main()

{

char s1[ 20 ] = "Welcome to ";

char s2[] = "Virtual University ";

char s3[ 40 ] = "";

cout<< "s1 = " << s1 << endl << "s2 = " << s2 << endl << "s3 = " << s3 << endl;

cout<< "strcat( s1, s2 ) = "<< strcat( s1, s2 );

cout << "strncat( s3, s1, 6 ) = " << strncat( s3, s1, 6 );

}

The output of the program is given below.

s1 = Welcome to

s2 =

s3 =

strcat( s1, s2 ) = Welcome to Virtual University

strncat( s3, s1, 7 ) = Welcome

Now we come across the function strcmp. This function compares two strings, and

returns an integer value depending upon the result of the comparison. The prototype of

this function is

int strcmp( const char *s1, const char *s2)

This function returns a number less than zero (a negative number), if s1 is less than s2. It

returns zero if s1 and s2 are identical and returns a positive number (greater than zero) if

s1 is greater than s2. The space character in a string and lower and upper case letters are

also considered while comparing two strings. So the strings "Hello", "hello" and "He llo"

are three different strings these are not identical.

Similarly there is a function strncmp, which can be used to compare a number of

characters of two strings. The prototype of this function is

int strncmp( const char *s1, const char *s2, size_t n )

Here s1 and s2 are two strings and n is the number upto which the characters of s1 and s2

are compared. Its return type is also int. It returns a negative number if first n characters

of s1 are less than first n characters of s2. It returns zero if n characters of s1 and n

characters of s2 are identical. However, it returns a positive number if n characters of s1

are greater than n characters of s2.

Now we will talk about the function, `strlen' (string length) which is used to determine

the length of a character string. This function returns the length of the string passed to it.

The prototype of this function is given below.

int strlen ( const char *s)

Page 203

CS201 Introduction to Programming

This function determines the length of string s. the number of characters preceding the

terminating null character is returned.

Search Functions

C provides another set of functions relating to strings, called search functions. With the

help of these functions, we can do different types of search in a string. For example, we

can find at what position a specific character exists. We can search a character starting

from any position in the string. We can find the preceding or proceeding string from a

specific position. We can find a string inside another string. These functions are given in

the following table.

Function prototype

Function description

Locates the first occurrence of character c in string

char *strchr( const char *s, int c

s. If c is found, a pointer to c in s is returned.

);

Otherwise, a NULL pointer is returned.

Determines and returns the length of the initial

size_t strcspn( const char *s1,

segment of string s1 consisting of characters not

const char *s2 );

contained in string s2.

Determines and returns the length of the initial

size_t strspn( const char *s1,

segment of string s1 consisting only of characters

const char *s2 );

contained in string s2.

Locates the first occurrence in string s1 of any

char *strpbrk( const char *s1,

character in string s2. If a character from string s2

const char *s2 );

is found, a pointer to the character in string s1 is

returned. Otherwise, a NULL pointer is returned.

char *strrchr( const char *s, int c Locates the last occurrence of c in string s. If c is

found, a pointer to c in string s is returned.

);

Otherwise, a NULL pointer is returned.

char *strstr( const char *s1, const Locates the first occurrence in string s1 of string s2.

If the string is found, a pointer to the string in s1 is

char *s2 );

returned. Otherwise, a NULL pointer is returned.

char *strtok( char *s1, const char A sequence of calls to strtok breaks string s1 into

"tokens"--logical pieces such as words in a line of

*s2 );

text--separated by characters contained in string

s2. The first call contains s1 as the first argument,

and subsequent calls to continue tokenizing the

same string contain NULL as the first argument. A

pointer to the current token is returned by each call.

If there are no more tokens when the function is

called, NULL is returned.

Page 204

CS201 Introduction to Programming

Example 3

Here is an example, which shows the use of different string manipulation functions.

The code of the program is given below.

//A program which shows string manipulation using <string.h> library

#include <iostream.h>

#include <string.h>

#include <stdlib.h>

main()

{

char s1[] = "Welcome to " ;

char s2[] = "Virtual University" ;

char s3[] = "Welcome to Karachi" ;

char city[] = "Karachi";

char province[] = "Sind";

char s[80];

char *pc;

int n;

cout << "s1 = " << s1 << endl << "s2 = " << s2 << endl ;

cout << "s3 = " << s3 << endl ;

// function for string length

cout << "The length of s1 = " << strlen(s1) << endl ;

cout << "The length of s2 = " << strlen(s2) << endl ;

cout << "The length of s3 = " << strlen(s3) << endl ;

strcpy(s, "Hyderabad"); // string copy

cout<< "The nearest city to "<< city << " is " << s << endl ;

strcat(s, " and "); // string concatenation

strcat(s,city);

strcat(s, " are in ");

strcat(s, province);

strcat(s, ".\n");

cout << s;

if (!(strcmp (s1,s2))) // ! is used as zero is returned if s1 & s2 are equal

cout << "s1 and s2 are identical" << endl ;

else

cout << "s1 and s2 are not identical" << endl ;

if (!(strncmp (s1,s3,7)))

// ! is used as zero is returned for equality

cout << "First 7 characters of s1 and s3 are identical" << endl ;

Page 205

CS201 Introduction to Programming

else

cout << "First 7 characters of s1 and s3 are not identical" << endl ;

}

Following is the output of the program.

S1 = Welcome to

S2 =

S3 = Welcome to Karachi

The length of s1 = 11

The length of s2 = 18

The length of s3 = 18

The nearest city to Karachi is Hyderabad

Hyderabad and Karachi are in Sind.

S1 and s2 are not identical

First 7 characters of s1 and s3 are identical

Exercises

1: Write a program that displays the ASCII code set in tabular form on the screen.

2: Write your own functions for different manipulations of strings.

3: Write a program, which uses different search functions.

Page 206

Table of Contents: