CCSA 221 Programming in C

1
CCSA 221Programming in C

• Chapter 10
• Character Strings

2
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

3
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.

4
Arrays of Characters

• 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 have
• the actual length of the character array in a
• separate variable !

5
Variable-Length Character Strings

• 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.

6
Variable-Length Character Strings

• // Function to count the number of characters in
  a string
• include ltstdio.hgt
• int stringLength (const char string)
• int count 0
• while ( stringcount ! '\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
7
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 word7 "Hello!"
• In this example, the compiler has enough room in
  the array to place the terminating null
  character.
• However, in char word6 "Hello!"
• the compiler cant fit a terminating null
  character at the end of the array, and so it
  doesnt put one there.

8
Initializing and Displaying Character Strings

• 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)

9
String processing-Concatenate strings
include ltstdio.hgt // Function to concatenate two
character strings void concat (char result,
const char str1, const char str2) int i,
j // copy str1 to result for ( i 0 str1i !
'\0' i ) resulti str1i // copy str2 to
result for ( j 0 str2j ! '\0' j
) resulti j str2j // Terminate the
concatenated string with a null character result
i j '\0' int main (void) const char
s1 "Test " const char s2 "works."
char s320 concat (s3, s1, s2) printf
("s\n", s3) return 0
Output Test works.
10
Testing Two Character Strings for Equality

• 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 ( s1i s2 i s1i ! '\0'
  s2i ! '\0' )
• i
• if ( s1i '\0' s2i '\0' )
• areEqual true
• else
• areEqual false
• return areEqual

11
// Program to Test Strings for Equality include
ltstdio.hgt / 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
The function that we declared in the previous
slide
Comparing same string
Comparing same string
Output 0 1 1
12
Inputting Character Strings

• 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 string81
• 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
  Shawshank
• the 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
13
Inputting Character Strings

• If s1 , s2 , and s3 are defined to be character
  arrays of appropriate sizes, execution of the
  statement
• scanf ("sss", 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.

14

• // Program to illustrate the s scanf format
  characters
• include ltstdio.hgt
• int main (void)
• char s181, s281, s381
• printf ("Enter text\n")
• scanf ("sss", 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.

Output Enter text system expansion bus s1
system s2 expansion s3 bus
15
Single-Character Input
include ltstdio.hgt // Function to read a line of
text from the terminal void readLine (char
buffer) char character int i
0 do character getchar () bufferi
character i while ( character ! '\n'
) bufferi - 1 '\0' int main (void) int
i char line81 for ( i 0 i lt 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' .
Placing null at the end of every line
Output This is a sample line of text. This is a
sample line of text. abcdefghijklmnopqrstuvwxyz
Abcdefghijklmnopqrstuvwxyz runtime library
routines runtime library routines
Read 3 lines of text
16
The Null String

• 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 buf100 ""

17
//Program to count words in a piece of
text include ltstdio.hgt include
ltstdbool.hgt / Insert readLine function here
/ // Function to determine if a character
is alphabetic bool alphabetic (const char
c) if ( (c gt 'a' c lt 'z') (c gt 'A'
c lt 'Z') ) return true else return
false // Function to count the number of words
in a string int countWords (const char
string) int i, wordCount 0 bool
lookingForWord true for ( i 0 stringi !
'\0' i ) if ( alphabetic(stringi) ) if (
lookingForWord ) wordCount lookingForWord
false else lookingForWord true return
wordCount
Function readLine Slide 15
18
int main (void) char text81 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 ( text0 '\0'
) endOfText true else totalWords countWords
(text) printf ("\nThere are i words in the
above text.\n", totalWords) return 0
Function readLine Slide 15
tests the input line to see if just the Enter was
pressed then it is a null string.
19
Escape Characters

• 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.

\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
20
Escape Characters

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

21
More on Constant Strings

• 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
• "abcdefghijklmnopqrstuvwxyz
• ABCDEFGHIJKLMNOPQRSTUVWXYZ"
• 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"

22
More on Constant Strings

• 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 writing
• char 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.

23
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 word15
• char definition50
• 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 in
• struct entry dictionary100

24
// Program to use the dictionary lookup program
include ltstdio.hgt include ltstdbool.hgt struct
entry char word15 char
definition50 / 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 lt entries i ) if ( equalStrings
(search, dictionaryi.word) ) return i return
-1
Function equalStrings Slide 10
25
int main (void) const struct entry
dictionary100 "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 word10 int
entries 10 int entry printf ("Enter
word ") scanf ("s", word) entry lookup
(dictionary, word, entries) if ( entry ! -1
) printf ("s\n", dictionaryentry.definition) e
lse printf ("Sorry, the word s is not in my
dictionary.\n", word) return 0
Output Enter word agar a jelly made from seaweed
Output (Rerun) Enter word accede Sorry, the
word accede is not in my dictionary.
26
A Better Search Method

• 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 gt high, x does not exist in M and
  the algorithm terminates.
• Step 3 Set mid to (low high) / 2.
• Step 4 If Mmid lt x, set low to mid 1 and go
  to step 2.
• Step 5 If Mmid gt x, set high to mid 1 and go
  to step 2.
• Step 6 Mmid equals x and the algorithm
  terminates.

27
// Dictionary lookup program using binary
search include ltstdio.hgt struct entry char
word15 char definition50 // Function
to compare two character strings int
compareStrings (const char s1, const char
s2) int i 0, answer while ( s1i
s2i s1i ! '\0' s2i ! '\0' ) i if
( s1i lt s2i ) answer -1 /
s1 lt s2 / else if ( s1i s2i ) answer
0 / s1 s2 / else answer
1 / s1 gt s2 / return answer
28
// 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 lt high ) mid (low
high) / 2 result compareStrings
(dictionarymid.word, search) if ( result
-1 ) low mid 1 else if ( result 1 ) high
mid - 1 else return mid / found it
/ return -1 / not found /
29
int main (void) const struct entry
dictionary100 "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 word15 int entry printf ("Enter
word ") scanf ("s", word) entry lookup
(dictionary, word, entries) if ( entry ! -1
) printf ("s\n", dictionaryentry.definition) e
lse printf ("Sorry, the word s is not in my
dictionary.\n", word) return 0
Output Enter word aigrette an ornamental
cluster of feathers
Output (Rerun) Enter word acerb Sorry, that
word is not in my dictionary.
30
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 gt 'a' c lt '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 .

31
// Function to convert a string to an integer
include ltstdio.hgt int strToInt (const char
string) int i, intValue, result 0 for
( i 0 stringi ! \0' i ) if
((stringi gt '0') (stringi lt
'9')) intValue stringi - '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 245 125 13