Introduction to Programming in C

1
Introduction to Programming in C

• Class 19
• 04/02/2003
• Rohit Valsakumar

2
C Strings continued

• Important points to remember
• The size of the character array used to store a
  string should always be at least one greater than
  the no of characters in the string
• The string should be terminated by \0 (null a
  back slash followed by zero written together in
  single quotes) character
• When manipulating these strings be careful never
  to modify the \0 character otherwise the
  computer will crash

3
Using and with C Strings

• You cannot use and with C strings as they
  are character arrays
• E.g.,
• char str110, str2 Hello // str2 legal
• str1 Good Nite // illegal
• str2 assignment is legal as it is at point of
  declaration but for str1 using to assign a
  value is not allowed
• Use this instead strcpy(str1, Good Nite)
• strcpy() does not check size of char array
  before storing a string into it, so dangerous,
  use strncpy(str1, Good Nite, 9) instead if it
  is provided
• cannot be used to compare two C strings
  though you wont get an error from the compiler
  if you do so, however the program will behave
  strangely and the operation is not valid either

4
Using and with C Strings

• Use strcmp(str1, str2) instead returns true if
  the strings do not match otherwise it returns
  false remember this it is not what you would
  expect
• Strcmp() can also be used for lexicographic
  comparison, it returns 0 if the two strings
  match, -ve no if str1 is lexicographically less
  than str2 and ve no if it is greater than str2
• include if you want to use strcmp()
  and strcpy()

5
C string input and output

• cin and cout can be used to read and write a
  C string
• E.g., - cout
• char str110 Bon Jour
• cout
• Bon Jour will appear on the monitor
• For reading with cin take care to notice that
  all white spaces are skipped, moreover each
  reading of input stops at the next space or line
  break
• E.g.,
• char str222, str322
• cin str2 str3
• cout
• User types Bon Jour Mademoiselle
• BonJour End - this is what appear on the
  monitor

6
C string input and output

• This is not what we expected so use
  cin.getline(str2, 22) instead
• If size of char array is smaller than line then
  only that many characters will be read as will
  fit in the array, the \0 character is
  automatically inserted by cin.getline()
• If the char array is greater than the line to be
  read in then the reading will stop when the line
  is read
• Try all combinations yourself it is good practice

7
C string to number conversions

• include
• atoi converts C string of digits to
  corresponding integer value
• E.g.,
• int x atoi(1234)
• cout
• 1234 - printed on monitor
• atol converts strings to long value
• atof converts strings to double value
• If the conversion is not possible then the above
  functions return zero

8
The standard string class

• include
• Allows you to treat a string of characters as
  values of simple type
• E.g.,
• string s1, s2, s3
• s1 Bon
• s2 Jour
• s3 Mademoiselle
• string s4 s1 s2 s3
• cout
• Output will be
• BonJourMademoiselle
• Bon Jour Mademoiselle

9
The standard string class

• You can use the , , , and many other
  operators with objects of class string
• You can also use cout and cin to write and
  read values into objects of type string
• With cin you have to take the same care that
  you take in the case of character arrays if you
  want to read in white spaces i.e., use a function
  getline() as shown below
• string line
• cout
• getline(cin, line)
• cout
• With objects of class string we dont have to
  worry about size limitations or the null
  character either

10
String Processing with the Class string

• All the operators i.e., , , !, ,
  work in the same manner as when used with values
  of simple types
• is used to join to strings in the order they
  are written
• can also be used to concatenate one string
  to the other
• E.g.,
• string s1 Au, s2 revoir
• s1 s2
• cout s1
• Output
• Au revoir

11
String Processing with the Class string

• We can access individual characters of the string
  by indexing
• cout
• Output
• e
• - , , /, , - etc. cannot be used
  with objects of class string
• Read pages 674, 675, 676, 677 of the textbook

12
Converting between string and C Strings

• We can copy a C string to a string object but not
  vice versa
• char cstr Good day //legal
• string str cstr, str1 // legal
• str1 cstr // legal
• cstr str // illegal
• Strcpy(cstr, str) //illegal
• Explicit type casting is needed if you want to
  copy an object of string class into a C string
• This is done by stringc_str() member function
  of the string class
• Thus,
• strcpy(cstr, str.c_str()) // legal
• Will work