In Addition . . .

1
In Addition . . .

• To the string class from the standard library
  accessed by include ltstringgt
• C also has another library of string functions
  for C strings that can be accessed by include
  ltcstringgt

1
2
What is a C String?

• A C string is a char array terminated by the null
  character \0 ( with ASCII value 0 ).
• A C string variable can be initialized in its
  declaration in two equivalent ways.
• char message 8 H, e, l, l,
  o, \0
• char message 8 Hello

3
char vs. C string

• A has data type char
• and is stored in 1 byte
• A is a C string of 2 characters
• and is stored in 2 bytes

4
Recall that . . .

• char message8 // declaration allocates
  memory
• To the compiler, the value of the identifier
  message alone is the base address of the array.
  We say message is a pointer (because its value is
  an address). It points to a memory location.

5
Aggregate C String I/O in C

• I/O of an entire C string is possible using the
  array identifier with no subscripts and no
  looping.

EXAMPLE char message 8 cin gtgt
message cout ltlt message HOWEVER
. . .
6
Extraction operator gtgt

• When using the extraction operator ( gtgt ) to
  read input characters into a string variable,
• the gtgt operator skips any leading whitespace
  characters such as blanks and newlines
• it then reads successive characters into the
  array, and stops at the first trailing whitespace
  character (which is not consumed, but remains
  waiting in the input stream)
• the gtgt operator adds the null character to the
  end of the string

7
Example Using gtgt

• char name 5
• cin gtgt name
• Suppose input stream looks like this
• J o e

total number of elements in the array
7000
J o e \0
name 0 name 1 name 2 name 3
name 4
null character is added
8
Function get( )

• because the extraction operator stops reading at
  the first trailing whitespace, gtgt cannot be used
  to input a string with blanks in it
• if your strings declared size is not large
  enough to hold the input characters and add the
  \0, the extraction operator stores characters
  into memory beyond the end of the array
• use get function with 2 parameters to overcome
  these obstacles
• EXAMPLE
• char message 8
• cin.get ( message, 8 ) // inputs at most 7
  characters plus \0

9
inFileStream.get ( str, count 1)

• get does not skip leading whitespace characters
  such as blanks and newlines
• get reads successive characters (including
  blanks) into the array, and stops when it either
  has read count characters, or it reaches the
  newline character \n, whichever comes first
• get appends the null character to str
• if it is reached, newline is not consumed by get,
  but remains waiting in the input stream

10
Function ignore( )

• can be used to consume any remaining characters
  up to and including the newline \n left in the
  input stream by get
• EXAMPLE
• cin.get ( string1, 81 ) // inputs at most
  80 characters
• cin.ignore ( 30, \n ) // skips at most
  30 characters // but stops if \n
  is read
• cin.get ( string2, 81 )

11
Another Example Using get( )

• char ch
• char fullName 31
• char address 31
• cout ltlt Enter your full name
• cin.get ( fullName, 31 )
• cin.get (ch) // to
  consume the newline
• cout ltlt Enter your address
• cin.get ( address, 31 )

N e l l D a
l e \0 . . .
fullName 0
A u s t i n
T X \0 . . .
address 0
12
String Function Prototypes inltcstring gt

• int strlen (char str )
• // FCTNVAL integer length of string str (
  not including \0 )
• int strcmp ( char str1 , char str2 )
• // FCTNVAL negative, if str1 precedes str2
  lexicographically
• // positive, if str1 follows str2
  lexicographically
• // 0, if str1 and str2 characters same
  through \0
• char strcpy ( char toStr , char fromStr
  )
• // FCTNVAL base address of toStr ( usually
  ignored )
• // POSTCONDITION characters in string fromStr
  are copied to
• // string toStr, up to and including \0,
• // overwriting contents of string toStr

13

• include ltcstring gt
• .
• .
• .
• char author 21
• int length
• cin.get ( author , 21 )
• length strlen ( author ) // What is the
  value of length ?

14

• char myName 21 Huang // WHAT IS
  OUTPUT?
• char yourName 21
• cout ltlt Enter your last name
• cin.get ( yourName, 21 )
• if ( strcmp ( myName, yourName ) 0 )
• cout ltlt We have the same name!
• else if ( strcmp ( myName, yourName ) lt 0 )
• cout ltlt myName ltlt comes before ltlt
  yourName
• else if ( strcmp ( myName, yourName ) gt 0 )
• cout ltlt yourName ltlt comes before ltlt
  myName

H u a n g \0
. . .
myName 0
H e a d i n g t
o n \0 . . .
yourName 0
15

• char myName 21 Huang
• char yourName 21
• if ( myName yourName ) // compares
  addresses only!
• // That is, 4000 and 6000 here.
• . // DOES NOT COMPARE CONTENTS!
• .
• .

4000
H u a n g \0
. . .
myName 0
6000
H e a d i n g t
o n \0 . . .
yourName 0
16

• char myName 21 Huang
• char yourName 21
• cin.get ( yourName, 21 )
• yourName myName // DOES NOT COMPILE!
• // What is the value of myName ?

4000
H u a n g \0
. . .
myName 0
6000
H e a d i n g t
o n \0 . . .
yourName 0
17

• char myName 21 Huang
• char yourName 21
• cin.get ( yourName, 21 )
• strcpy ( yourName, myName ) // changes
  string yourName
• // OVERWRITES CONTENTS!

4000
H u a n g \0
. . .
myName 0
6000
u n g \0
H e a d i n g t
o n \0 . . .
yourName 0
18

• There is a difference between 'A' and "A". The
  first one is character A and the second is string
  A.
• Since strings are null terminated, "A" represents
  two characters, 'A' and '\0'.
• "Hello" represents six characters, 'H', 'e', 'l',
  'l', 'o', and '\0'.
• To store 'A' we need only one memory cell of the
  type char, while to store "A", we need two memory
  cells of the type char, one for 'A' and the other
  for '\0'.
• To store the string "HELLO" in computer we need
  six memory cells of the type char.
• Consider the statement.
• char name16
• Since C strings are null terminated and name has
  sixteen components, the largest string that can
  be stored in name is 15.
• If you store a string of length, say 10 in name,
  the first 11 components of name are used and the
  last 5 are left unused.

19

• char name16 'J', 'o', 'h', 'n', '\0'
• During char array variable declaration, C
  allows the string notation to be used in the
  initialization statement.
• char name16 "John" //Line A
• The statement
• char name "John" //Line B
• declares a string variable name of length large
  enough, that is, 5 (here) and stores "John" in
  it.
• There is a difference between the last two
  statements. Both statements stores "John" in
  name.
• The size of name in the statement in Line A is 16
  while its size in the statement in Line B is 5.

20

• Most of the rules that apply to other arrays also
  apply to character arrays.
• Consider the statement
• char studentName26
• studentName "Lisa L. Johnson" //Illegal

21

• String Comparison
• In C, C-strings are compared character by
  character using the collating sequence of the
  system.
• Let us assume that we are using the ASCII
  character set.
• 1. The string "Air" is smaller than the string
  "Boat".
• 2. The string "Air" is smaller than the string
  "An".
• 3. The string "Bill" is smaller than the string
  "Billy".
• 4. The string "Hello" is smaller than "hello".

22

• Example 9-7
• char studentName21
• char myname16
• char yourname16
• Statement Effect
• a. strcpy(myname,"John Robinson") mynameJohn
  Robinson
• b. strlen("John Robinson") Returns 13, the
  length
• of the string
• c. int len
• len strlen ("Sunny Day") Stores 9 into len
• d. strcpy(yourname,"Lisa Miller") yourname
  Lisa Miller
• strcpy(studentName,yourname) studentName
  Lisa Miller
• e. strcmp("Bill", "Lisa") Returns a value lt
  0
• f. strcpy(yourname, "Kathy Brown") yourname
  Kathy Brown
• strcpy(myname, "Mark G. Clark") myname
  Mark G. Clark
• strcmp(myname,yourname) Return a value gt 0

23

• Reading and Writing Strings
• char name31
• String Input
• Since aggregate operations are allowed for string
  input, the statement
• cingtgtname
• will store the next input string into name.
• Strings that contain blanks cannot be read using
  the extraction operator gtgt.

24

• To read strings with blanks, the general form
  (syntax) of get function together with an input
  stream variable, such as cin is
• cin.get(str,m1)
• This statement stores the next m characters or
  until the newline character '\n' is found into
  str. The newline character is not stored in str.
• If the input string has fewer than m characters,
  then the reading stops at the newline character.

25

• Consider the following statements
• char str31
• cin.get(str,31)
• If the input is
• William T. Johnson
• then "William T. Johnson" will be stored in str
  and
• If the input is
• Hello there. My name is Mickey Mouse.
• then the string
• "Hello there. My name is Mickey"

26

• char str126
• char str226
• char discard
• Input is, say
• Summer is warm.
• Winter will be cold.
• The following statements store line 1 in str1 and
  line 2 in str2.
• cin.get(str1,26)
• cin.get(discard)
• cin.get(str2,26)

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• String Output
• The statement
• coutltltname
• will output the content of name on the screen.
• The insertion operator ltlt continues to write the
  contents of name until it finds the null
  character.
• If name does not contain the null character, then
  we will see strange output since ltlt continues to
  output data from memories adjacent to name until
  '\0' is found.

28

• Specifying Input/Output Files at the Execution
  Time
• ifstream infile
• ofstream outfile
• char fileName51
• coutltlt"Enter the input file name "
• cingtgtfileName
• infile.open(fileName) //open the input file
• .
• .
• .
• coutltlt"Enter the output file name "
• cingtgtfileName
• outfile.open(fileName) /open the output file

29

• The string type and Input/Output Files
• Values (that is, strings) of the type string are
  not null terminated.
• Variables of type string can also be used to read
  and store the names of input/output files.
• The argument to the function open must be a null
  terminated string, that is a C string.
• if we use a variable of the type string to read
  the name of an input/output file and then use
  this variable to open a file, the value of the
  variable must (first) be converted to a C-string
  (that is, a null-terminated string).

30

• The header file string contains the function
  c_str, which converts a value of the type string
  to a null-terminated character array (that is,
  C-string).
• The syntax to use the function c_str is
• strVar.c_str()
• where strVar is a variable of the type string.

31

• ifstream infile
• string fileName
• coutltlt"Enter the input file name "
• cingtgtfileName
• infile.open(fileName.c_str()) //open the input
  file