CCSA 221Programming in C

Chapter 10Character Strings

AlHanouf AlAmr1

Outline• Arrays of Characters• Variable-Length Character Strings

• Initializing and Displaying Character Strings • Testing Two Character Strings for Equality • Inputting Character Strings • Single-Character Input • The Null String

• Escape Characters • More on Constant Strings • Character Strings, Structures, and Arrays

• A Better Search Method • Character Operations

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What is a character string?

• In the statement: printf ("Programming in C is fun.\n"); the argument that is passed to the printf function is the character string "Programming in C is fun.\n“

• The double quotation marks are used to delimit the character string, which can contain any combinations of letters, numbers, or special characters.

• When introduced to the data type char , you learned that a variable that is declared to be of this type can contain only a single character.

• To assign a single character to such a variable, the character is enclosed within a pair of single quotation marks.

• Thus, the assignment plusSign = '+'; has the effect of assigning the character '+' to the variable plusSign.

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Arrays of Characters

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• If you want to be able to deal with variables that can hold more than a single character, you can use an array of characters.

• You can define an array of characters called word as follows:char word [] = { 'H', 'e', 'l', 'l', 'o', '!' };

To print out the contents of the array word, you run through each element in the array and display it using the %c format characters.

To do processings of the word (copy, concatenate two words, etc) you need to havethe actual length of the character array in a separate variable !

Variable-Length Character Strings

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• A method for dealing with character arrays without knowing the actual length of the character array:

Placing a special character at the end of every character string. In this manner, the function can then determine for itself when it has reached the end of a character string after it encounters this special character.

• In the C language, the special character that is used to signal the end of a string is known as the null character and is written as '\0'.

char word [] = { 'H', 'e', 'l', 'l', 'o', '!', '\0' };

• How these variable-length character strings are used? Using a function that counts the number of characters in a character string.

Variable-Length Character Strings

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// Function to count the number of characters in a string#include <stdio.h>

int stringLength (const char string[]){int count = 0;while ( string[count] != '\0' )

++count;return count;

}

int main (void) {const char word1[] = { 'a', 's', 't', 'e', 'r', '\0' };const char word2[] = { 'a', 't', '\0' };const char word3[] = { 'a', 'w', 'e', '\0' };printf ("%i %i %i\n", stringLength (word1),

stringLength (word2), stringLength (word3));return 0;

}

The function declares its argument as a const array of characters because it is not making any changes to the

array

Output:

5 2 3

Initializing and Displaying Character Strings• Initializing a string: char word[] = "Hello!";• Is equivalent to: char word[] = { 'H', 'e', 'l', 'l', 'o', '!', '\0'

};As you can see, the size of the array is not specified in the previous 2 examples.

• We can specify the size of the string as follows: char word[7] = { "Hello!" };

In this example, the compiler has enough room in the array to place the terminating null character.

• However, in: char word[6] = { "Hello!" };• the compiler can’t fit a terminating null character at the end of the array, and so it

doesn’t put one there.

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Initializing and Displaying Character Strings

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•In general, character-string constants in the C language are automatically terminated by the null character wherever they appear in your program.

In this example: printf ("Programming in C is fun\n");

Function printf can determine when the end of a character string has been reached when it finds the null character that is automatically placed after the newline character.

• . To display an array of characters, we use the special format characters %s. e.g.printf ("%s\n", word);

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String processing-Concatenate strings#include <stdio.h>// Function to concatenate two character stringsvoid concat (char result[], const char str1[], const char str2[]){

int i, j;// copy str1 to resultfor ( i = 0; str1[i] != '\0'; ++i )

result[i] = str1[i];// copy str2 to resultfor ( j = 0; str2[j] != '\0'; ++j )

result[i + j] = str2[j];// Terminate the concatenated string with a null characterresult [i + j] = '\0';

}

int main (void){

const char s1[] = "Test " ;const char s2[] = "works." ;char s3[20];concat (s3, s1, s2);printf ("%s\n", s3);return 0;

}

Output:

Test works.

Testing Two Character Strings for Equality

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• You cannot directly test two strings to see if they are equal with a statement such as

if ( string1 == string2 ) • We can develop a function that can be used to compare two character strings as

follows:bool equalStrings (const char s1[], const char s2[]){

int i = 0;bool areEqual;while ( s1[i] == s2 [i] && s1[i] != '\0' && s2[i] != '\0' )

++i;if ( s1[i] == '\0' && s2[i] == '\0' )

areEqual = true;else

areEqual = false;return areEqual;

}

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// Program to Test Strings for Equality#include <stdio.h>

/***** Insert equalStrings function here *****/int main (void){

const char stra[] = "string compare test";const char strb[] = "string";

printf ("%i\n", equalStrings (stra, strb));

printf ("%i\n", equalStrings (stra, stra));

printf ("%i\n", equalStrings (strb, "string"));

return 0;}

Comparing same string

Comparing same string

Output:

011

The function that we

declared in the previous slide

Inputting Character Strings

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• The scanf function can be used with the %s format characters to read in a string of characters up to a blank space, tab character, or the end of the line, whichever occurs first.

char string[81];scanf ("%s", string);

Note that unlike previous scanf calls, in the case of reading strings, the & is not placed before the array name !

If the following characters are entered: Shawshankthe string "Shawshank" is read in by the scanf function and is stored inside the string array.

If the following line of text is typed instead: iTunes playlist just the string "iTunes" is stored inside the string array because the blank space.

If the scanf call is executed again, this time the string "playlist" is stored inside the string array because the scanf function always continues

scanning from the most recent character that was read in.

& is not placed

Inputting Character Strings• If s1 , s2 , and s3 are defined to be character arrays of appropriate sizes, execution of

the statement: scanf ("%s%s%s", s1, s2, s3);

with the input line of text: micro computer system

results in the assignment of the string "micro" to s1 , "computer" to s2 , and "system“ to s3 .

• If the following line of text is typed instead:system expansion

it results in the assignment of the string "system" to s1 , and "expansion" to s2.

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// Program to illustrate the %s scanf format characters #include <stdio.h>

int main (void){char s1[81], s2[81], s3[81];printf ("Enter text:\n");scanf ("%s%s%s", s1, s2, s3);

printf ("\ns1 = %s\ns2 = %s\ns3 = %s\n", s1, s2, s3);

return 0;}

•If you type in more than 80 consecutive characters to the preceding program without pressing the spacebar, the tab key, or the Enter (or Return) key, scanf overflows the character array !

scanf has no way of knowing how large its character array parrameters are ! When handed a %s format, it simply continues to read and store characters until one of the noted terminator characters is reached.

•If you place a number after the % in the scanf format string, this tells scanf the maximum number of characters to read.

Correct use of scanf for reading strings: scanf ("%80s", string);

Output:

Enter text:system expansionbuss1 = systems2 = expansions3 = bus

Single-Character Input

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#include <stdio.h>// Function to read a line of text from the terminal void readLine (char buffer[]){char character;int i = 0;do{

character = getchar ();buffer[i] = character;++i;

}while ( character != '\n' );buffer[i - 1] = '\0';}

int main (void){ int i;char line[81];for ( i = 0; i < 3; ++i ){

readLine (line);printf ("%s\n\n", line);

}return 0;}

Reading 1 line by repeated calls to the getchar function return successive single characters from the input. When the end of the line is reached, the function returns the newline character '\n' .

Output:

This is a sample line of text.This is a sample line of text.

abcdefghijklmnopqrstuvwxyz Abcdefghijklmnopqrstuvwxyz

runtime library routinesruntime library routines

Read 3 lines of

text

Placing null at the end of every line

• The null string: A character string that contains no characters other than the null character.

• The null string length is zero.

• In any program that asked the user to enter a text, if the user presses Enter key without entering any character before, the string that has been read is a null string.

• Examples: char empty[]= ""; char buf[100]= "";

The Null String

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//Program to count words in a piece of text#include <stdio.h>#include <stdbool.h>

/***** Insert readLine function here *****/

// Function to determine if a character is alphabetic bool alphabetic (const char c){if ( (c >= 'a' && c <= 'z') || (c >= 'A' && c <= 'Z') )

return true;else

return false;}

// Function to count the number of words in a stringint countWords (const char string[]){int i, wordCount = 0;bool lookingForWord = true;for ( i = 0; string[i] != '\0'; ++i )

if ( alphabetic(string[i]) ){if ( lookingForWord ){

++wordCount;lookingForWord = false;

}}else

lookingForWord = true;

return wordCount;}

Function readLin

eSlide 15

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int main (void) {

char text[81];int totalWords = 0;bool endOfText = false;printf ("Type in your text.\n");printf ("When you are done, press ‘Enter'.\n\n");

while ( ! endOfText ){

readLine (text);

if ( text[0] == '\0' )endOfText = true;

elsetotalWords += countWords (text);

}printf ("\nThere are %i words in the above text.\n", totalWords);return 0;}

Function readLineSlide 15

tests the input line to see if just the Enter was pressed then it is a null

string.

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Escape Characters

\a Audible alert\b Backspace\f Form feed\n Newline\r Carriage return\t Horizontal tab\v Vertical tab\\ Backslash\" Double quotation mark\' Single quotation mark\? Question mark

• The backslash character has a special significance that extends beyond its use in forming the newline ( \n )and null ( \0 ) characters.

• Other characters can be combined with the backslash character to perform special functions. These are referred to as escape characters.

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Escape Characters

Examples:

• printf ("\aSYSTEM SHUT DOWN IN 5 MINUTES!!\n");sounds an alert and displays the indicated message.

• printf ("%i\t%i\t%i\n", a, b, c);displays the value of a , spaces over to the next tab setting, displays the value of b, spaces over to the next tab setting, and then displays the value of c.

• printf ("\\t is the horizontal tab character.\n");displays the following:\t is the horizontal tab character.Note that because the \\ is encountered first in the string, a tab is not displayed in this case.

• printf ("\"Hello,\" he said.\n");results in the display of the message:"Hello," he said.

• c = '\'';assigns a single quotation character to c .

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• Without the line continuation character, your C compiler generates an error message if you attempt to initialize a character string across multiple lines; for example:

char letters[] ={ "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ" };

By placing a backslash character at the end of each line to be continued, a character string constant can be written over multiple lines:

char letters[] ={ "abcdefghijklmnopqrstuvwxyz\ABCDEFGHIJKLMNOPQRSTUVWXYZ" };

More on Constant Strings

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Another way to break up long character strings is to divide them into two or more adjacent strings. Adjacent strings are constant strings separated by zero or more spaces, tabs, or newlines.

The compiler automatically concatenates adjacent strings together. So, the letters array can also be set to the letters of the alphabet by writingchar letters[] ={ "abcdefghijklmnopqrstuvwxyz""ABCDEFGHIJKLMNOPQRSTUVWXYZ" };

The three printf calls:printf ("Programming in C is fun\n");printf ("Programming" " in C is fun\n");printf ("Programming" " in C" " is fun\n");

all pass a single argument to printf because the compiler concatenates the strings together in the second and third calls.

More on Constant Strings

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Character Strings, Structures, and Arrays

• Suppose we want to write a computer program that acts like a dictionary. we could type the word into the program, and the program could then automatically “look up” the word inside the dictionary and tell us its definition.

• Because the word and its definition are logically related, we can represent the word

and its definition inside the computer as a structure. we can define a structure called entry to hold the word and its definition:struct entry{char word[15];char definition[50];};

• The following is an example of a variable defined to be of type struct entry that is initialized to contain the word “blob” and its definition.struct entry word1 = { "blob", "an amorphous mass" };

• Because we want to provide for many words inside our dictionary, it seems logical to define an array of entry structures, such as instruct entry dictionary[100];

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// Program to use the dictionary lookup program #include <stdio.h>#include <stdbool.h>

struct entry{char word[15];char definition[50];};

/***** Insert equalStrings function here *****/

// function to look up a word inside a dictionary int lookup (const struct entry dictionary[], const char search[],const int entries){int i;

for ( i = 0; i < entries; ++i )if ( equalStrings (search, dictionary[i].word) )

return i;return -1;

}

Function equalStrings Slide 10

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int main (void){

const struct entry dictionary[100] ={ { "aardvark", "a burrowing African mammal" },{ "abyss", "a bottomless pit" },{ "acumen", "mentally sharp; keen" },{ "addle", "to become confused" },{ "aerie", "a high nest" },{ "affix", "to append; attach" },{ "agar", "a jelly made from seaweed" },{ "ahoy", "a nautical call of greeting" },{ "aigrette", "an ornamental cluster of feathers" },{ "ajar", "partially opened" } };char word[10];int entries = 10;int entry;

printf ("Enter word: ");scanf ("%s", word);

entry = lookup (dictionary, word, entries);

if ( entry != -1 )printf ("%s\n", dictionary[entry].definition);

elseprintf ("Sorry, the word %s is not in my dictionary.\n", word);

return 0;}

Output:Enter word: agara jelly made from seaweed

Output (Rerun):Enter word: accedeSorry, the word accede is not in my dictionary.

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• The method used by the lookup function is perfectly fine for a small-sized dictionary. If you start dealing with large dictionaries containing hundreds or perhaps even thousands of entries, this approach might no longer be sufficient because of the time it takes to sequentially search through all of the entries.

• What you are really looking for is an algorithm that reduces the search time in most cases, not just in one particular case. Such an algorithm exists under the name of the binary search.

• Binary Search Algorithm

Search x in SORTED array M of size n.

Step 1: Set low to 0, high to n – 1.

Step 2: If low > high, x does not exist in M and the algorithm terminates.

Step 3: Set mid to (low + high) / 2.

Step 4: If M[mid] < x, set low to mid + 1 and go to step 2.

Step 5: If M[mid] > x, set high to mid – 1 and go to step 2.

Step 6: M[mid] equals x and the algorithm terminates.

A Better Search Method

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// Dictionary lookup program using binary search#include <stdio.h>

struct entry {char word[15];char definition[50];};

// Function to compare two character strings int compareStrings (const char s1[], const char s2[]) {int i = 0, answer;

while ( s1[i] == s2[i] && s1[i] != '\0'&& s2[i] != '\0' )++i;

if ( s1[i] < s2[i] )answer = -1; /* s1 < s2 */

else if ( s1[i] == s2[i] )answer = 0; /* s1 == s2 */

elseanswer = 1; /* s1 > s2 */

return answer;}

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// Function to look up a word inside a dictionary int lookup (const struct entry dictionary[], const char search[],const int entries){int low = 0;int high = entries - 1;int mid, result;

while ( low <= high ){

mid = (low + high) / 2;

result = compareStrings (dictionary[mid].word, search);

if ( result == -1 )low = mid + 1;

else if ( result == 1 )high = mid - 1;

elsereturn mid; /* found it */

}return -1; /* not found */

}

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int main (void){const struct entry dictionary[100] ={ { "aardvark", "a burrowing African mammal" },{ "abyss", "a bottomless pit" },{ "acumen", "mentally sharp; keen" },{ "addle", "to become confused" },{ "aerie", "a high nest" },{ "affix", "to append; attach" },{ "agar", "a jelly made from seaweed" },{ "ahoy", "a nautical call of greeting" },{ "aigrette", "an ornamental cluster of feathers" },{ "ajar", "partially opened" } };

int entries = 10;char word[15];int entry;

printf ("Enter word: ");scanf ("%s", word);

entry = lookup (dictionary, word, entries);

if ( entry != -1 )printf ("%s\n", dictionary[entry].definition);

elseprintf ("Sorry, the word %s is not in my dictionary.\n", word);

return 0;}

Output:Enter word: aigrettean ornamental cluster of feathers

Output (Rerun):Enter word: acerbSorry, that word is not in my dictionary.

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Character Operations• Character variables and constants are frequently used in relational and arithmetic

expressions.• Whenever a character constant or variable is used in an expression in C, it is automatically converted to an integer value.

If (c >= 'a' && c <= 'z‘) The first part of this expression is actually comparing the value of c against the internal numerical representation of the character 'a‘. In ASCII, the character 'a' has the value 97 , the character 'b' has the value 98, and so on.

printf ("%i \n", 'a'); displays 97.

c = 'a' + 1; printf ("%c \n", c);Because the value of 'a' is 97 in ASCII, c = 97+ 1 = 98. The value 98 represent 'b' in ASCII, so 'b' will be displayed by the printf call.

i = c - '0';The character '0' is not the same as the integer 0, the character '0' has the numerical value 48 in ASCII. Therefore, if you suppose that c contained the character '5' , which, in ASCII, is the number 53 so I = 53 – 48 = 5.which results in the integer value 5 being assigned to i .

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// Function to convert a string to an integer #include <stdio.h>

int strToInt (const char string[]){

int i, intValue, result = 0;

for ( i = 0; string[i] != ‘\0'; ++i ){

if ((string[i] >= '0') && (string[i] <= '9')){

intValue = string[i] - '0';result = result * 10 + intValue;

}}return result;}

int main (void){

printf ( "%i\n", strToInt("245") );printf ( "%i\n", strToInt("100") + 25 );printf ( "%i\n", strToInt("13x5") );

return 0;}

Output:24512513