Character Data and Strings

Tom Kelliher, CS17

May 13, 1996

• Don't forget the basics: parameter passing, variable scope and visibility.
• The final is cumulative.
• A review session is scheduled for Thursday, May 16 from 8:15--9:15pm in Hoyt 166.
• The final is Monday, May 20 at 11:30am in Hoyt 150.

Character Data

• Represented internally as a small (one byte) integer.
• ASCII code.
• EBCDIC code.
• Whether a ``plain'' char is signed or unsigned depends upon the compiler, operating system, and hardware.
• For us, char is signed char.

The sizeof() function.

Collating sequence:

How the characters in a character set ``compare'' to each other.

ASCII collating sequence:

  0 nul    1 soh    2 stx    3 etx    4 eot    5 enq    6 ack    7 bel
  8 bs     9 ht    10 nl    11 vt    12 np    13 cr    14 so    15 si
 16 dle   17 dc1   18 dc2   19 dc3   20 dc4   21 nak   22 syn   23 etb
 24 can   25 em    26 sub   27 esc   28 fs    29 gs    30 rs    31 us
 32 sp    33  !    34  "    35  #    36  $    37  %    38  &    39  '
 40  (    41  )    42  *    43  +    44  ,    45  -    46  .    47  /
 48  0    49  1    50  2    51  3    52  4    53  5    54  6    55  7
 56  8    57  9    58  :    59  ;    60  <    61  =    62  >    63  ?
 64  @    65  A    66  B    67  C    68  D    69  E    70  F    71  G
 72  H    73  I    74  J    75  K    76  L    77  M    78  N    79  O
 80  P    81  Q    82  R    83  S    84  T    85  U    86  V    87  W
 88  X    89  Y    90  Z    91  [    92  \    93  ]    94  ^    95  _
 96  `    97  a    98  b    99  c   100  d   101  e   102  f   103  g
104  h   105  i   106  j   107  k   108  l   109  m   110  n   111  o
112  p   113  q   114  r   115  s   116  t   117  u   118  v   119  w
120  x   121  y   122  z   123  {   124  |   125  }   126  ~   127 del

char Conversions

Traditionally, character data is stored in char variables, but int formal parameters are used.

The conversion is automatic.

chars can be used alongside other types in arithmetic expressions.

tolower()

This function, if passed an upper case character, returns the lower case equivalent. Any other character is returned as-is.

Since a--z and A--Z are adjacent in the collating sequence and the lowercase letters are after the uppercase letters, we can convert an uppercase letter to the corresponding lowercase letter just by adding the difference of 'a' and 'A'.

int tolower(int c)
{
   if ('A' <= c && c <= 'Z')
      return c + 'a' - 'A';
   else
      return c;
}

isdigit()

Return 1 if int argument is an ASCII digit, otherwise return 0.

int isdigit(int c)
{
   return '0' <= c && c <= '9';
}

Reading a Number

A number must be converted from a character sequence to a numeric value.

Numbers are read from most significant digit to least significant digit, so we multiply the ``number so far'' by 10 every time we read a digit.

To convert the ASCII code for a digit to a numeric value, subtract '0' from the ASCII character.

int getNumber(void)
{
   int value = 0;
   char c;

   // cin.get() returns one character from cin.
   while (isdigit(c = cin.get()))
      value = 10 * value + c - '0';

   cin.unget();   // Puts last character back on cin.

   return value;
}

Character I/O

Assume:

char c;

• cin >> c; --- skips whitespace.
• cin.get(c);, c = cin.get(); --- doesn't skip whitespace.
• cout << c --- prints the ASCII character corresponding to c's value.
• cout.put(c) --- ditto.
• cin.eof() --- end of file.

Character Handling Functions

Must include ctype.h.

• int isdigit(int c)
• int isalpha(int c)
• int isspace(int c)
• int toupper(int c)
• ... See pg. 663 for more.

Character Strings

A sequence of characters terminated by a null character ('\0').

Assume:

const int LINE_LENGTH = 80;

char s[LINE_LENGTH];

• cin >> s; --- reads next word into s.
• cin.getline(s, LINE_LENGTH); --- reads an entire line (upto LINE_LENGTH characters) into s.
• cin.get() similar, does not remove newline at end of line.
• cout << s; --- output a string.

Details of getline()

Prototype:

istream::getline(char in[], int max, int term = '\n');

• in --- the actual parameter should be a char array.
• max --- the number of elements in in.
• term --- an optional parameter. When encountered, cease reading characters.

String Function Exercises

These functions are provided in string.h.

Write the functions:

• int strlen(const char s[]) (same as int strlen(const char* s))
• char* strcat(char s[], const char append[])
• char* strchr(const char* s, int c)
• int strcmp(const char s1[], const char s2[])