Lect 16P. 1

1
Strings

• Lecture 16

2
Character Strings

• Up until now, we have dealt mostly with simple
  character variables. A variable of type char may
  hold a single character and is assigned a value
  using a pair of single quotes
• Example char onechar 'z'
• On the other hand, character strings are arrays
  of simple characters with a special character
  inserted into the string at the very end. They
  are assigned values with a pair of double quotes
• Example char arraychar6 "abcde"

3
Character Strings

• This array of characters, or string, ends in the
  special null character ( '\0' ).
• Strings are normally accessed by a pointer to the
  first character in the string. This means that
  the value of a string is the address of it's
  first character. Thus we say that a string is a
  pointer, mostly because we often use the string
  name in manipulating the string. Since the
  string name is the name of the array of
  characters, it is a pointer like the name of any
  other array in C is a pointer.

4
Character Strings

• Declaration and initialization
• char color "scarlet"
• or
• char colorPtr "scarlet"
• or
• char color 8 's', 'c', 'a', 'r', 'l', 'e',
  't', '\0'
• NOTE Allowance must always be made for the
  terminating null character.

5
String I/O Library Routines

• include ltstdio.hgt
• / The following are function prototypes for
  some of the String I/O and Handling Library
  Routines /
• / Input next character as an integer /
• int getchar (void)
• / Input string into array s until newline /
• char gets (char s)

6
String I/O Library Routines

• include ltstdio.hgt
• / Print character stored in character variable
  c /
• int putchar (int c)
• / Print character string s followed by \n /
• int puts (const char s)
• / Performs scanf function on string s /
• int sscanf (char s, const char format, )

7
String I/O Library Routines

• include ltstdio.hgt ltnotice!!! from stdio lib
  !!!
• / To input a number of characters from a
  stream /
• char fgets (char string1, int n, FILE
  stream)
• / where up to (n - 1) characters are accepted,
  including newline /
• / To output a string of characters to a stream
  /
• char fputs (char string1, FILE stream)
• / where all the characters are placed into the
  I/O stream /

8
String Handling Library Routines

• include ltstring.hgt lt Notice !!! from string
  lib!!!
• / To copy string 2 into string 1 /
• char strcpy (char string1, const char
  string2)
• / To append string 2 to the end of string 1 /
• char strcat (char string1, const char
  string2)
• / where first character of string 2 replaces
  the null in string 1 /
• / To find out length of string 1 ( of
  characters) /
• size_t strlen (const char string1)
• / where "size_t" is either an "unsigned int" or
  "unsigned long" /

9
String Handling Library Routines

• include ltstring.hgt
• / Compare string 1 to string 2. The function
  returns 0, less than 0, or greater than 0 if
  string 1 is equal to, less than, or greater than
  string 2 respectively. /
• int strcmp (const char string1, const char
  string2 )
• / In this context, less than and greater than
  depend on the integer values assigned to each of
  the the individual characters. In the ASCII
  scheme of things, the integer values (or
  character codes) are assigned in an order so that
  we can do things alphabetically, i.e., 'a' is
  less than 'b' is less than 'c', and so on. /

10
Arrays, Strings, and Pointers

• / Passing an array to a function /
• include ltstdio.hgt
• include ltstring.hgt
• void mysub ( char )
• int main ( )
• char name20 "Richard J. Freuler"
• mysub (name)
• return 0

11
Arrays, Strings, and Pointers

• void mysub ( char text )
• int len, k
• len strlen (text)
• printf ("d\n", len )
• for (k 0 k lt len k) printf ("c",
  text k )
• /Program Output /
• 18
• RichardJ.Freuler

blanks
12
Sorting Strings with Pointers

• A character string is actually just a
  one-dimensional array of simple character
  variables.
• So, an array of character strings is really an
  array of arrays, that is, a two-dimensional
  array.
• It might be declared as follows
• char mystrings m n
• where m is the number of rows (strings) in the
  array and n is the maximum number of characters
  in each string.

13
Sorting Strings with Pointers

• Now, C has to know how many characters are
  actually stored in each string. Recall that a
  string termination character, or null, ('\0') is
  inserted after the last character.
• Note that a string termination character takes
  one array element to store, therefore the maximum
  string length that could be stored in one row of
  the array mystrings is n - 1.
• Often, a string is stored with a newline
  character ('\n'), which also takes a character
  space. Thus, the max string length can be
  reduced to n - 2.

14
Sorting Strings with Pointers

• The address of the beginning of the array
  mystrings is just the array name mystrings
  without any subscripts.
• The address of any string (any row) in the array
  is mystringsk where k is the row number.
• Since all subscripts start at zero in C,
  mystrings and mystrings0 are effectively the
  same address.

15
Sorting Strings with Pointers

• Strings can be sorted using an array of pointers.
  Since the first element of each string (each
  row) is the address of the string, that address
  can be assigned to one of the pointers.
• When address of each string (or row) is assigned
  to a pointer, the array of pointers can be sorted
  and used to retrieve the strings in the new
  order.
• The order (location) of the actual strings in
  memory will NOT be changed.

16
Sorting Strings with Pointers

• / This program sorts strings using array of
  pointers /
• include ltstdio.hgt
• include ltstring.hgt
• define SIZE 4
• int main ( )
• int k, swaps
• char nameSIZE30, nptrSIZE, temp
• char filename "namelist.dat"
• FILE fptr
• fptr fopen (filename,"r")

17
Sorting Strings with Pointers

• printf ("\n Original list\n")
• for (k 0 k lt SIZE k) / Assign
  addresses /
• /
  to pointers in array /
• nptrk namek
• fgets (namek, 30, fptr)
• printf ("s", namek) / Print
  original list /

18
Sorting Strings with Pointers
So, the name array might look like
0 1 2 3 4 5
29
0 1 2 3
And, assuming name array started at memory
location xxx200, the nptr array would look like
0 1 2 3
19
Sorting Strings with Pointers

• do / Sort the pointers in ascending
  alphabetic /
• / order of the strings /
• swaps 0
• for (k 0 k lt (SIZE - 1) k)
• if (strcmp (nptrk, nptrk1) gt 0)
  
• temp nptrk
• nptrk nptrk1
• nptrk1 temp
• swaps1
• while (swaps)

20
Sorting Strings with Pointers
So, the name array would still look like
0 1 2 3 4 5
29
0 1 2 3
And, the nptr array would end up as
0 1 2 3
21
Sorting Strings with Pointers

• printf ("\n New list\n")
• for (k 0 k lt SIZE k) printf ("s",
  nptrk)
• printf ("\n Original list again\n")
• for (k 0 k lt SIZE k) printf ("s",
  namek)
• printf ("\n")

22
Sorting Strings with Pointers

• Original list
• Smith, Robert A.
• Sanderson, John T.
• Alberts, Mary C.
• Dreese, Edward A.
• New list
• Alberts, Mary C.
• Dreese, Edward A.
• Sanderson, John T.
• Smith, Robert A.
• Original list again
• Smith, Robert A.
• Sanderson, John T.
• Alberts, Mary C.
• Dreese, Edward A.

23
Assignment G14

• The previous example provides the essential
  elements for solving G14.
• Suggestions
• Copy file to your directory
• Use more to see what is in the file
• First write your program to read and print the
  contents of the file
• Then develop your sorting routine for the file.