C-tutorial - Part II

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C-tutorial - Part II

• Prakash Linga
• linga_at_cs.cornell.edu
• 4110 Upson

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All about Strings!

• Array of characters (chars)
• Definition char str
• str is a pointer to a memory location which
  stores a character (the first in the string).
• How we know the end of the string?
• The character \0 (null character) indicates the
  end of the string.
• Where on earth is the string stored?

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Strings

• In memory right!
• So allocate memory to store a string
• char str (char ) malloc(sizeof(char)MAXSTRLE
  N)
• How do I store the string now?
• Brute-force Store character by character.
• Better Approach Use strcpy
• Note What follows is mainly from SunOS man pages.

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Sssss

• strcpy
• strcpy(chardestn, char source)
• include ltstring.hgt
• char str (char ) malloc(sizeof(char)MAXSTRLE
  N)
• strcpy(str, That was easy)
• What is the length of my string?
• Use strlen
• int len strlen(str)

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Strings in Java

• Strings are Objects
• Creating a String is a call to one of the
  constructors.
• String str new String(This is ridiculously
  easy! Why cant I do this in C?)
• Host of methods in the String class
• length, trim, compareTo, substring, charAt,
  concat,
• indexOf , endsWith, replace
• to name a few.

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Back to the Obscure World!

• We have a host of functions for string
  manipulation in Ctoo
• strcpy, strncpy, strlcpy
• strcat, strncat, strlcat
• strcmp, strncmp, strcasecmp, strncasecmp,
• strchr, strrchr, strstr, strlen
• strtok, strtok_r, strcspn, strspn, strdup, strpbrk

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Copies

• char strcpy(char s1, const char s2)
• char strncpy(char s1, const char s2, size_t
  n)
• size_t strlcpy(char dst, const char src,
  size_t dstsize)
• strncpy copies over n bytes from the source to
  the destination
• (many questions here!!!???)
• strlcpy copies at most dstsize-1 characters
  from src to dst,
• truncating src if necessary. Result is always
  null terminated.
• (again many small questions!)

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Cats

• Cats are similar to copies!
• char strcat(char s1, const char s2)
• char strncat(char s1, const char s2, size_t
  n)
• size_t strlcat(char dst, const char src,
  size_t dstsize)
• Any interesting questions here?
• strncat result is \0 terminated (always). This
  is not the case
• with strncpy

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Compares

• int strcmp(const char s1, const char s2)
• int strncmp(const char s1, const char s2,
  size_t n)
• strcasecmp and strncasecmp are case insensitive
  versions of
• strcmp.
• Return values for strcmp
• Returns 0 if equal 1 if s1 gt s2 and 1
  otherwise.
• Note Use includeltstrings.hgt for strcasecmp and
  strncasecmp

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strchr, strrchr and strstr

• char strchr(const char s, int c)
• char strrchr(const char s, int c)
• char strstr(const char s1, const char s2)
• strchr
• Return a pointer to the first occurrence of c in
  s
• strrchr
• Return a pointer to the last occurrence of c in
  s
• strstr
• Return a pointer to the first occurrence of
  string s2 in s1
• (any tricky case?)

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Toks

• char strtok(char s1, const char s2)
• char strtok_r(char s1, const char s2,
  char lasts)
• The strtok() function can be used to break
  the string pointed to by s1 into a sequence of
• tokens, each of which is delimited by one or more
  characters from the string pointed to by
• s2.
• The first call (with pointer s1 specified)
  returns a pointer to the first character of the
  first
• token, and will have written a null character
  into s1 immediately following the returned
• token.
• The function keeps track of its position in the
  string between separate calls, so that
• subsequent calls (which must be made with the
  first argument being a null pointer) will
• work through the string s1 immediately
  following that token.

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Strtok example

• include ltstring.hgt
• int main ()
• char str100
• char tmp
• strcpy(str, "This is a sample string,just
  testing.")
• printf ("Splitting \"s\" into tokens\n",str)
• tmp strtok (str," ,.")
• while (tmp ! NULL)
• printf ("s\n",tmp)
• tmp strtok (NULL, " ,.")
• return 0

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Output of tok program

• Splitting "This is a sample string,just testing."
  into tokens
• This
• is
• a
• sample
• string
• just
• testing

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Same example with strtok_r

• include ltstring.hgt
• int main ()
• char str100
• char tmp, last
• strcpy(str, "This is a sample string,just
  testing.")
• printf ("Splitting \"s\" into tokens\n",str)
• tmp strtok_r (str," ,. " , last)
• while (tmp ! NULL)
• printf ("s\n",tmp)
• tmp strtok_r (NULL, " ,. " , last)
• return 0

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strspn and strcspn

• size_t strcspn(const char s1, const char s2)
• size_t strspn(const char s1, const char s2)
• The strcspn() function returns the length of
  the initial
• segment of string s1 that consists entirely of
  characters
• not from string s2.
• The strspn() function returns the length of the
  initial segment
• of string s1 that consists entirely of
  characters from string
• s2.

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strdup

• char strdup(const char s1)
• Make a copy (allocates memory using malloc) of
  the
• parameter string and return a pointer to this
  copy
• char strpbrk(const char s1, const char s2)
• The strpbrk() function returns a pointer to
  the first occurrence in
• string s1 of any character from string s2, or a
  null pointer if no character
• from s2 exists in s1.