Strings

1
Strings
2
String Literals

• String literals are enclosed in double quotes
• "Put a disk in drive A, then press any key to
  continue\n
• A string literal may be extended over more than
  one line by writing \ immediately followed by the
  end of the line
• printf("Put a disk in drive A, then \
• press any key to continue\n")
• A string literal may be divided into two or
  more shorter strings the compiler will join
  these together into one string
• printf("Put a disk in drive A, then "
• "press any key to continue\n")

3
How String Literals Are Stored
The string literal "abc" is represented by the
three characters a, b, and c, followed by a null
character (\0) Like any array, a string
literal is represented by a pointer (reference)
to the first character in the string. A string
literal of length 1 is different from a character
constant. A string literal of length 1 ("a", for
example) is represented by a pointer. A character
constant ('a', for example) is represented by an
8 bit integer value.
4
How String Literals Are Stored

• Warning Dont ever use a character constant
  when a string literal is required (or
  vice-versa). The call
• printf("\n")
• is legal, because printf expects a string as its
  first parameter, but
• printf('\n')
• is not. \n is a character, not a string!

5
String Variables

• A string variable is just a one-dimensional
  array of characters
• define STR_LEN 80
• char strSTR_LEN1
• The array should be one character longer than the
  string it will hold, to leave space for the null
  character.
• Warning Failure to leave room for the null
  character may cause unpredictable results when
  using string-handling functions in the C library.

6
String Variables

• A string variable (character array) can be
  initialized
• char date18 "June 14"
• The date array will have the following
  appearance
• A string initializer need not completely fill
  the array
• char date29 "June 14"
• The leftover array elements are filled with null
  characters

7
String Variables

• If the length of the array is omitted, the
  compiler will compute it, if you give it an
  initial value
• char date3 "June 14" / date3 is 8
  characters long /

8
Reading and Writing Strings

• To read or write a string, use scanf or printf
  with the s conversion specification
• scanf("s", str)
• printf("s", str)
• scanf skips white space, then reads characters
  into str until it encounters a white-space
  character.
• No ampersand is needed when using scanf to read
  into a string variable. Since a string variable
  is an array, the name of the variable is already
  a pointer (to the
• beginning of the array).

9
Reading and Writing Strings

• A faster alternative Use gets and puts instead
  of scanf and printf
• gets(str)
• puts(str)
• gets reads characters into str until it
  encounters a new-line character. puts prints str,
  followed by a new-line character.
• scanf and gets automatically put a null character
  at the end of the input string. printf and puts
  assume that the output string ends with a null
  character.

10
Reading and Writing Strings
Warning Both scanf and gets assume that the
string variable is large enough to contain the
input string (including the null character at the
end). Failing to make the variable long enough
will have unpredictable results. To make scanf
safer, use the conversion specification ns,
where n specifies the maximum number of
characters to be read. You can use fgets instead
of gets for greater safety (see their
documentation).
11
Accessing the Characters in a String

• Because of the close relationship between
  arrays and pointers, strings can be accessed by
  array subscripting.
• Array version of a function that counts the
  number of spaces in a string
• int count_spaces(char s)
• int count, i
• count 0
• for (i 0 si ! '\0' i)
• if (si ' ') count
• return count

12
Using the C String Library

• C provides little built-in support for strings.
  Since strings are treated as arrays, they are
  restricted in the same ways as arraysin
  particular, strings cannot be
• copied by assignment or compared with for
  equality.
• Warning Attempts to copy or compare two
  strings using Cs built-in operators will fail
• char str110, str210
• str1 str2 / illegal /
• if (str1 str2) ... / will produce the wrong
  result /
• The C library provides a set of functions for
  performing operations on strings. Declarations
  for these functions reside in ltstring.hgt.

13
The strcpy Function

• strcpy copies one string into another
• char str110, str210
• strcpy(str1, "abcd") / str1 now contains "abcd"
  /
• strcpy(str2, str1) / str2 now contains "abcd"
  /
• strcpy calls can be chained
• strcpy(str2, strcpy(str1, "abcd"))
• / both str1 and str2 now contain "abcd" /
• Warning strcpy has no way to check that the
  second string will fit in the first one.

14
The strcat Function

• strcat appends the contents of one string to
  the end of another
• char str10 "abc"
• strcat(str, "def") / str now contains "abcdef"
  /
• Warning strcat has no way to check that the
  first string can accommodate the added
  characters. The programmer must make certain
  that sufficient space exists in the destination
  string.

15
The strcmp Function

• strcmp compares two strings
• if (strcmp(str1, str2) lt 0) ...
• strcmp returns a value less than, equal to, or
  greater than 0, depending on whether str1 is less
  than, equal to, or greater than str2.
• strcmp considers str1 to be less than str2 if
• The first i characters of str1 and str2 match,
  but the (i1)st character of str1 is less than
  the (i1)st character of str2 (for example, "abc"
  is less than "acc", and "abc" is less than
  "bcd"), or
• All characters of str1 match str2, but str1 is
  shorter than str2 (for example, "abc" is less
  than "abcd")

16
The strlen Function

• strlen returns the length of a string
• int i
• char str10
• i strlen("abc") / i is now 3 /
• i strlen("") / i is now 0 /
• strcpy(str, "abc")
• i strlen(str) / i is now 3 /
• When given an array of characters as its
  argument, strlen does not measure the length of
  the array itself instead, it returns the length
  of the string stored
• inside the array. The length of a string does
  not include the null terminating character.

17
Writing the strlen Function

• int strlen(char s)
• int n
• for (n 0 sn ! '\0' n)
• return n

18
First use of pointers
Array names are pointers (references) to their
first element. The syntax char means pointer
to a character. The asterisk operator () when
used in an expression dereferences a pointer
This means its value is the thing being pointed
to. Thus, dereferencing a character pointer
produces the character it is pointing
at. Pointers may be incremented or decremented
to go to the next (or previous element). char
strcat(char s1, const char s2) char p
s1 while (p) --p while (p
s2) return s1