C for Java Programmers

1
C for Java Programmers

• Chapter 8
• Characters and Strings

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Characters and Literal Strings

• Char in C is normally an 8-bit quantity,
  whereas in Java it is a 16-bit value.
• Char can be declared either as signed or
  unsigned.
• wchar_t represents a wide character, 16-bit
  character.

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Character Literals andthe cctype Library

• isalpha(c) True if c is alphabetic
• isupper(c) True if c is upper case
• islower(c) True if c is lower case
• isdigit(c) True if c is decimal digit char
• isxdigit(c) True if c is hexidecimal digit
• isalnum(c) True if c is alphabetic or numeric
• isspace(c) True if c is whitespace (space, tab
  or newline)
• isprint(c) True if c is a printable character
  (8 bit only)

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String Literals

• A literal string value has type array of
  character in C, whereas it has type string in
  Java.
• Always remember the null character at the end of
  a string literal.
• Char a abc
• // a has 4 elements, not 3

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String Literals

• char text "A Literal Text"
• int vowelCount (char const p) // procedure to
  count vowels in textint sum 0while (1) //
  will break out of loop in switch
  statement switch (p) // case '\0'
  return sum case 'a' case 'e' case 'i' case
  'o' case 'u' sum break return sum

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The cstring Library

• defined by header file string.h, should not be
  confused with the header file string.
• strcpy(dest, src) Copy characters from
  source to destination
• strncpy(dest, src, n) Copy exactly n characters
• strcat(dest, src) Append characters from
  source onto destination
• strncat(dest, src, n) Append only n characters
• strcmp(s1, s2) Compare strings s1 and
  s2
• strncmp(s1, s2, n) Compare first n
  characters in s1 and s2
• strlen(s) Count number of
  characters in s

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The strcmp function

• Comparison between pointer values is determined
  by the relative placement in memory of the
  locations being pointed to.
• To determine a lexicographic comparison of two
  character pointer values, an explicit comparison
  function must be used.
• If the first string is lexicographically smaller,
  return less than zero, if equal, returns zero,
  otherwise larger than zero.

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Definition of strcmp

• int strcmp
• (const unsigned char p, const unsigned char
  q) while (p (p q)) p q
  return p - q
• // classic (and cryptic) example of
• // character pointer manipulation

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strcpy, strcat - copy strings

• char buffer20
• char text "literal"
• strcpy(buffer, literal) // copy literal into
  buffer
• for (int i 0 i lt 5 i)strcat(buffer,
  literal) // append literal to buffer
• char buffer7
• strcpy (buffer, "literal") // error! - copies
  eight values, not seven

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Constant and Mutable Values

• String literals in Java are immutable, the same
  is not true in C.
• char text "literal " // note space at
  endtext0 'm' // change l to mtext2-- //
  t becomes sstrcpy (text 6, "ble") // replace
  l with ble
• // this sort of thing makes a literal miserable

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Constant and Mutable Values

• Immutable strings can be formed with the const
  modifier.
• But, this modifier can imply multiple meanings,
  depending on its placement, and the actual
  meaning may not be intuitive.

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Const strings

• Placing the const at the front cannot be
  modified, but it can be reassigned to a new
  constant string
• const char a "literal"a2 'z'
  // error -- cannot modify aa "new literal"
  // ok -- can change to new //
  constant value

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A different type of const

• Placing the const in the middle the pointer
  cannot be changed, but the value it references
  can.
• char const b "literal"b2 'z'
  // ok -- can change what it points tob "new
  literal" // error -- cannot change pointer itself

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Really constant

• A value cannot be changed and be reassigned, can
  only be formed by using two occurrences of the
  const modifier
• const char const c "literal"c2 'z'
  // error -- cannot be modifiedc "new
  literal" // error -- cannot change pointer
  itself

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The string Data Type

• The string data type is a recent addition to C
  and is still not widely used.
• The string abstraction is similar to combination
  of the String and StringBuffer data types in Java.

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Creating strings

• string a
• string b "initial text"
• string c("more text")
• string d(b) // copy of b
• a "some text" // assignment to a
• Subscript index values are not checked for
  validity.

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Table 8.1 Comparison of string functionality in
C and Java
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Test your understanding

• How many bits in a C char?
• What functions are found in cctype?
• What is the type of a string literal?
• What functions are found in cstring?

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Test your understanding

• Why is the following probably in error
• char x here now
• char y there now
• if (x lt y)

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Test your understanding

• Why is the following probably in error
• char buffer10
• strcpy(buffer, now is the winter of our
  discontent)