20 Characters and Strings

Topic 20

Characters and Strings

Characters

In C, character constants are written surrounded

by single quotes e.g. 'a' or '?' or '4'.

A variable of type char can be used to store a

single character.

char ch;

ch = 'a';

When we want to store many characters, we can

use an array of characters.

Array of Characters

Example: array to store 5 characters.

char letters[5]; int i; letters[0] = 'a'; letters[1] = 'p'; letters[2] = 'p'; letters[3] = 'l'; letters[4] = 'e'; for (i=0; i<5; i++) printf("%c", letters[i]);

Character Manipulation Functions

C provides a set of functions for manipulating

characters.

To use these functions in your program,

remember to add this #include directive in your

program:

#include <ctype.h>


These functions are divided into 2 groups:

classifying functions and

converting functions

Classifying functions examine a character and tell

if it belongs to a given classification.

These functions names all start with ‘is’ and return

true or false.


Function Description

isalpha Checks if the character is an

alphabetic character (a-z or A-Z)

isdigit Checks if the character is a digit (0-9)

isalnum Checks if the character is an

alphabetic (a-z or A-Z) or numeric (0-

9) character

isupper Checks if the character is an

uppercase alphabetic character (A-Z)



islower Checks if the character is a lowercase

alphabetic character (a-z)

isspace Checks if the character is a

whitespace (blank space (‘ ‘),

horizontal tab (‘\t’), line feed (‘\n’),

vertical tab (‘\v’), form feed (‘\f’), or

carriage return (‘\r’))


Character conversion functions convert a

character from one case to another.


toupper Converts lowercase to uppercase. If

not lowercase, returns it unchanged.

tolower Converts uppercase to lowercase. If

not uppercase, returns it unchanged.

Character Function Examples

Example: To input a character and check if it is an

alphabet or digit.

char ch; printf("Enter a character: "); scanf("%c", &ch); if (isalpha(ch)) printf("It is an alphabet"); else if (isdigit(ch)) printf("It is a digit"); else printf("Not alphabet or digit");

Character Function Examples

Example: To convert all the characters in an array

to uppercase.

char ch[10]; int i; for (i=0; i<10; i++) ch[i] = toupper(ch[i]);

Characters and ASCII Codes

Remember that a character is represented in the

computer using ASCII code.

We can display the character using either %c (to

display the character) or %d (to display its ASCII

code) in function printf.

Example:

char ch; ch = 'a'; printf("Character is %c\n", ch); printf("Its ASCII code is %d\n", ch);

Displays

letter ‘a’ Displays

ASCII code

for ‘a’ i.e.

97

Characters and ASCII Codes

We can perform operations using the ASCII code.

Example: Given a character, generate the next

character in the alphabet.

char chIn, chOut; printf("Enter a character: "); scanf("%c", &chIn); if (isalpha(chIn)) // check if alphabetic { // character chOut = chIn + 1; printf("The next character is %c\n", ch); }

Note: Does not work if input character is z or Z

Strings

In C, string constants are written surrounded by

double quotes e.g. "Sales" or "Total\n".

Note: There is no type string in C.

But C allows us to use an array of characters to

store a string.

C also provides many functions to perform string

manipulation.

Strings

C treats a string differently from a normal array of

characters.

C uses a delimiter to indicate the end of the string.

The delimiter is a special character called a null

character.

Strings

The null character is represented by the character

sequence ‘\0’.

So a string is stored in C as an array of characters

but with a delimiter (‘\0’) to indicate the end of the

string.

Strings versus Array of Characters

An array of

A string characters

Strings

A string constant with one character is different

from a character constant.

An empty string is ""(two double quotes

without any space in between).

""

Strings

When we want to store a string, we declare an

array of characters.

The size of the array must be large enough to

store all the characters we want plus the null

character.

So the size is the maximum length of the string

plus one.

Example – to store a string with a maximum of 8

characters:

char str[9];

Initializing Strings

We can initialize a string variable using the index.

Example:

char str[9];

str[0] = ‘b’;

str[1] = ‘y’;

str[2] = ‘e’;

str[3] = ‘\0’;

b e y \0 ? ? ? ? ?

0 1 2 3 4 5 6 7 8

Part of array but

not part of string

Initializing Strings

We can also define and initialize it using a string

constant.

Example:

char str[9] = “bye”;

Note: the null character is added automatically.

b e y \0 ? ? ? ? ?

0 1 2 3 4 5 6 7 8

Using Function scanf for Strings

The function scanf can be used to input strings.

The conversion specification code for a string is s.

Example:

char str[9];

scanf("%s", str); // read a string

Note: the address operator (&) is not needed because the array name itself is a pointer type and therefore has the address of the first element of the array.


Function scanf adds the null character ‘\0’ at the

end of the string.

The maximum number of characters entered must

be one less than the size of the array.

If the number of characters entered is less than

the maximum number allowed, the remaining

spaces are undefined.


Example:

char str[9];

scanf("%s", str); // read a string

Suppose the user entered characters: good

The array str will contain:

g o o d \0 ? ? ? ?

0 1 2 3 4 5 6 7 8


Function scanf skips whitespace (blanks,

newlines, tabs) until it finds a character.

Then it reads the characters and stores them in

the array until it finds a whitespace.

Example Input Values What is stored in str

abc abc\0

12345678 12345678\0

abc def abc\0

abcd abcd\0

1234567890 Invalid input - error


To ensure that the user does not enter more than

the maximum number of characters allowed, we

can specify the maximum using the width

specification.

Example:

scanf("%8s", str); // read a string with a

// maximum of 8 characters

Example Input

Values

What is stored in str

abc abc\0

1234567890 12345678\0

Using Function printf for Strings

The function printf can be used to display strings.

The conversion specification code for a string is s.

The width can be used together with the

justification flag.

Examples: if str contains: good

Examples Output

printf("|%s|", str); |good|

printf("|%10s|", str); | good|

printf("|%-10s|", str); |good |


Note: we use the array name in the printf function

call. This passes the address of the first element

to the printf function.

We can also write this:

printf("|%s|", &str[0] );


We can also display part of the string.

Examples: if str contains: good

The function displays the characters starting from

the specified position until the end of the string.

Examples Output

printf(“%s", &str[0] ); good

printf(“%s", &str[1] ); ood

printf(“%s", &str[2] ); od

String Input with Spaces

The function scanf cannot be used to read a string that contains spaces.

We use the function gets instead.

Example:

char str[50];

gets(str);

The function reads characters (including spaces) until the end of line (user presses <enter> key which generates a newline) or end-of-file (user presses control-z ).

String Input with Spaces

If a newline character is read, function gets

discards the character and replaces it with a null

character.

Example:

String Manipulation Functions

C provides a set of functions for string

manipulation.

These function names start with ‘str’.

To use these functions in your program,

remember to add this #include directive in your

program:

#include <string.h>

String Functions – strlen

The function strlen returns the length of a string i..e the number of characters in the string excluding the null character.

If the string is empty, it returns 0.

Example: char str1[8] = “abc”;

strlen( str1 ) – returns 3

char str2[8] = “”; // empty string


char str3[8] = “ ”; // one space


String Functions – strlen

Note that we pass the whole array (representing

the string) to the function strlen.

This means the we are passing the address of the

first element.

So the function can also be called as:

strlen( &str[0] )

String Functions – strcpy

The function strcpy copies the contents of one

string to another.

Example: strcpy( s1, s2 )

String Functions – strcpy

We can also copy part of the word.

Example: strcpy( s1, &s2[5] )

String Functions – strncpy

The function strncpy is similar to function strcpy.

The difference is that we can specify the number

of characters to copy.

If the length of the string that is copied is less than

the number specified, the remaining places are

filled with the null character (‘\0’).

String Functions – strncpy

Example: strncpy( s1, s2, 8)

String Functions – strcmp

The function strcmp compares two strings to

determine if they are equal.

Example: strcmp( s1, s2 )

The strings are compared character by character

until unequal characters are found or until the end

of the strings is reached.

The function returns an integer to indicate the

results of the compare.


1) If the two strings are equal, the function returns

0.


2) If the first string is less than the second string,

the function returns a value less than 0 (i.e. a

negative value).


3) If the first string is greater than the second

string, the function returns a value greater than 0

(i.e. a positive value).


Example:

if (strcmp( s1, s2 ) == 0)

// strings are equal

else

// strings are not equal

if (strcmp( s1, s2 ) < 0)

//string s1 is less than string s2

if (strcmp( s1, s2 ) > 0)

// string s1 is greater than s2

String Functions – strcat

The function strcat appends or concatenates one

string to the end of another string.

Example: strcat( s1, s2 )

Note: The size of the destination string must be

large enough to hold the resulting string.

String Functions – strncat

The function strncat is similar to the function

strcat,

The difference is that we can specify the number

of characters to concatenate.

Example: strncat( s1, s2, 3 )

String Examples

Example: To input a word and display each

character of the word on a separate line.

char word[10]; int len, i; printf("Enter a word: "); scanf("%s", word); len = strlen(word); printf("The letters are:\n"); for (i=0; i<len; i++) printf("%c\n", word[i]);

String Examples

Example: To input a line of text and count the

number of spaces.

char line[80]; int spaces, len, i; printf("Enter a line of text:\n"); gets(line); spaces = 0; len = strlen(line); for (i=0; i<len; i++) if (line[i] == ' ') spaces++;

String Examples

Example: To input a word and display the word in

parts

Assume user enters: abc

char word[20]; int len, i; printf("Enter a word: "); scanf("%s", word); for (i=0; i<len; i++) printf("%s\n", &word[i]);

i &word[i] output

0 &word[0] abc

1 &word[1] bc

2 &word[2] c

Case Study 1

Problem:

Write a program that accepts a word from the

user and displays the word in pig-latin.

To form a word in pig-latin, the first letter of the

word is removed and added at the end of the

word and the letters ay are added after the added

letter.

However, if the word starts with a vowel (‘a’, ‘e’, ‘i’,

‘o’, or ‘u’), simply append the letters ay.

Case Study 1

Design the solution:

First we need to get the first letter of the word.

Next we copy the remaining letters to the new pig-

latin word.

If the first letter is not ‘a’, ‘e’, ‘i’, ‘o’ or ‘u’, we

append the first letter at the end of the new pig-

latin word.

Finally we append the letters “ay” to the end of the

new pig-latin word.

Case Study 1 – Complete Program #include <stdio.h> void pig_latin(char word[], char pigLatinWord[]); int main(void) { char inputWord[21]; // maximum 20-letter word char outputWord[23]; // need space for ay printf("Enter a word: "); scanf("%s", inputWord); pig_latin(inputWord, outputWord); printf("The word in pig latin is %s\n", outputWord); return 0 }

Case Study 1 – Complete Program

void pig_latin(char word[], char pigLatinWord[]); { char firstLetter; int len; firstLetter = word[0]; strcpy( pigLatinWord, &word[1] );

Case Study 1 – Complete Program

if (firstLetter != 'a' && firstLetter != 'e' && firstLetter != 'i' && firstLetter != 'o' && firstLetter != 'u' ) { len = strlen(pigLatinWord); pigLatinWord[len] = firstLetter; pigLatinWord[len+1] = '\0'; } strcat( pigLatinWord, "ay" ); return; }

20 Characters and Strings

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