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4. Character Handling

Programs that work with characters and strings often need to classify a character--is it alphabetic, is it a digit, is it whitespace, and so on--and perform case conversion operations on characters. The functions in the header file `ctype.h' are provided for this purpose.

Since the choice of locale and character set can alter the classifications of particular character codes, all of these functions are affected by the current locale. (More precisely, they are affected by the locale currently selected for character classification--the LC_CTYPE category; see Categories of Activities that Locales Affect.)

The ISO C standard specifies two different sets of functions. The one set works on char type characters, the other one on wchar_t wide characters (see section Introduction to Extended Characters).


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4.1 Classification of Characters

This section explains the library functions for classifying characters. For example, isalpha is the function to test for an alphabetic character. It takes one argument, the character to test, and returns a nonzero integer if the character is alphabetic, and zero otherwise. You would use it like this:

 
if (isalpha (c))
  printf ("The character `%c' is alphabetic.\n", c);

Each of the functions in this section tests for membership in a particular class of characters; each has a name starting with `is'. Each of them takes one argument, which is a character to test, and returns an int which is treated as a boolean value. The character argument is passed as an int, and it may be the constant value EOF instead of a real character.

The attributes of any given character can vary between locales. See section Locales and Internationalization, for more information on locales.

These functions are declared in the header file `ctype.h'.

Function: int islower (int c)

Returns true if c is a lower-case letter. The letter need not be from the Latin alphabet, any alphabet representable is valid.

Function: int isupper (int c)

Returns true if c is an upper-case letter. The letter need not be from the Latin alphabet, any alphabet representable is valid.

Function: int isalpha (int c)

Returns true if c is an alphabetic character (a letter). If islower or isupper is true of a character, then isalpha is also true.

In some locales, there may be additional characters for which isalpha is true--letters which are neither upper case nor lower case. But in the standard "C" locale, there are no such additional characters.

Function: int isdigit (int c)

Returns true if c is a decimal digit (`0' through `9').

Function: int isalnum (int c)

Returns true if c is an alphanumeric character (a letter or number); in other words, if either isalpha or isdigit is true of a character, then isalnum is also true.

Function: int isxdigit (int c)

Returns true if c is a hexadecimal digit. Hexadecimal digits include the normal decimal digits `0' through `9' and the letters `A' through `F' and `a' through `f'.

Function: int ispunct (int c)

Returns true if c is a punctuation character. This means any printing character that is not alphanumeric or a space character.

Function: int isspace (int c)

Returns true if c is a whitespace character. In the standard "C" locale, isspace returns true for only the standard whitespace characters:

' '

space

'\f'

formfeed

'\n'

newline

'\r'

carriage return

'\t'

horizontal tab

'\v'

vertical tab

Function: int isblank (int c)

Returns true if c is a blank character; that is, a space or a tab. This function was originally a GNU extension, but was added in ISO C99.

Function: int isgraph (int c)

Returns true if c is a graphic character; that is, a character that has a glyph associated with it. The whitespace characters are not considered graphic.

Function: int isprint (int c)

Returns true if c is a printing character. Printing characters include all the graphic characters, plus the space (` ') character.

Function: int iscntrl (int c)

Returns true if c is a control character (that is, a character that is not a printing character).

Function: int isascii (int c)

Returns true if c is a 7-bit unsigned char value that fits into the US/UK ASCII character set. This function is a BSD extension and is also an SVID extension.


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4.2 Case Conversion

This section explains the library functions for performing conversions such as case mappings on characters. For example, toupper converts any character to upper case if possible. If the character can't be converted, toupper returns it unchanged.

These functions take one argument of type int, which is the character to convert, and return the converted character as an int. If the conversion is not applicable to the argument given, the argument is returned unchanged.

Compatibility Note: In pre-ISO C dialects, instead of returning the argument unchanged, these functions may fail when the argument is not suitable for the conversion. Thus for portability, you may need to write islower(c) ? toupper(c) : c rather than just toupper(c).

These functions are declared in the header file `ctype.h'.

Function: int tolower (int c)

If c is an upper-case letter, tolower returns the corresponding lower-case letter. If c is not an upper-case letter, c is returned unchanged.

Function: int toupper (int c)

If c is a lower-case letter, toupper returns the corresponding upper-case letter. Otherwise c is returned unchanged.

Function: int toascii (int c)

This function converts c to a 7-bit unsigned char value that fits into the US/UK ASCII character set, by clearing the high-order bits. This function is a BSD extension and is also an SVID extension.

Function: int _tolower (int c)

This is identical to tolower, and is provided for compatibility with the SVID. See section SVID (The System V Interface Description).

Function: int _toupper (int c)

This is identical to toupper, and is provided for compatibility with the SVID.


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4.3 Character class determination for wide characters

Amendment 1 to ISO C90 defines functions to classify wide characters. Although the original ISO C90 standard already defined the type wchar_t, no functions operating on them were defined.

The general design of the classification functions for wide characters is more general. It allows extensions to the set of available classifications, beyond those which are always available. The POSIX standard specifies how extensions can be made, and this is already implemented in the GNU C library implementation of the localedef program.

The character class functions are normally implemented with bitsets, with a bitset per character. For a given character, the appropriate bitset is read from a table and a test is performed as to whether a certain bit is set. Which bit is tested for is determined by the class.

For the wide character classification functions this is made visible. There is a type classification type defined, a function to retrieve this value for a given class, and a function to test whether a given character is in this class, using the classification value. On top of this the normal character classification functions as used for char objects can be defined.

Data type: wctype_t

The wctype_t can hold a value which represents a character class. The only defined way to generate such a value is by using the wctype function.

This type is defined in `wctype.h'.

Function: wctype_t wctype (const char *property)

The wctype returns a value representing a class of wide characters which is identified by the string property. Beside some standard properties each locale can define its own ones. In case no property with the given name is known for the current locale selected for the LC_CTYPE category, the function returns zero.

The properties known in every locale are:

"alnum"

"alpha"

"cntrl"

"digit"

"graph"

"lower"

"print"

"punct"

"space"

"upper"

"xdigit"

This function is declared in `wctype.h'.

To test the membership of a character to one of the non-standard classes the ISO C standard defines a completely new function.

Function: int iswctype (wint_t wc, wctype_t desc)

This function returns a nonzero value if wc is in the character class specified by desc. desc must previously be returned by a successful call to wctype.

This function is declared in `wctype.h'.

To make it easier to use the commonly-used classification functions, they are defined in the C library. There is no need to use wctype if the property string is one of the known character classes. In some situations it is desirable to construct the property strings, and then it is important that wctype can also handle the standard classes.

Function: int iswalnum (wint_t wc)

This function returns a nonzero value if wc is an alphanumeric character (a letter or number); in other words, if either iswalpha or iswdigit is true of a character, then iswalnum is also true.

This function can be implemented using

 
iswctype (wc, wctype ("alnum"))

It is declared in `wctype.h'.

Function: int iswalpha (wint_t wc)

Returns true if wc is an alphabetic character (a letter). If iswlower or iswupper is true of a character, then iswalpha is also true.

In some locales, there may be additional characters for which iswalpha is true--letters which are neither upper case nor lower case. But in the standard "C" locale, there are no such additional characters.

This function can be implemented using

 
iswctype (wc, wctype ("alpha"))

It is declared in `wctype.h'.

Function: int iswcntrl (wint_t wc)

Returns true if wc is a control character (that is, a character that is not a printing character).

This function can be implemented using

 
iswctype (wc, wctype ("cntrl"))

It is declared in `wctype.h'.

Function: int iswdigit (wint_t wc)

Returns true if wc is a digit (e.g., `0' through `9'). Please note that this function does not only return a nonzero value for decimal digits, but for all kinds of digits. A consequence is that code like the following will not work unconditionally for wide characters:

 
n = 0;
while (iswdigit (*wc))
  {
    n *= 10;
    n += *wc++ - L'0';
  }

This function can be implemented using

 
iswctype (wc, wctype ("digit"))

It is declared in `wctype.h'.

Function: int iswgraph (wint_t wc)

Returns true if wc is a graphic character; that is, a character that has a glyph associated with it. The whitespace characters are not considered graphic.

This function can be implemented using

 
iswctype (wc, wctype ("graph"))

It is declared in `wctype.h'.

Function: int iswlower (wint_t wc)

Returns true if wc is a lower-case letter. The letter need not be from the Latin alphabet, any alphabet representable is valid.

This function can be implemented using

 
iswctype (wc, wctype ("lower"))

It is declared in `wctype.h'.

Function: int iswprint (wint_t wc)

Returns true if wc is a printing character. Printing characters include all the graphic characters, plus the space (` ') character.

This function can be implemented using

 
iswctype (wc, wctype ("print"))

It is declared in `wctype.h'.

Function: int iswpunct (wint_t wc)

Returns true if wc is a punctuation character. This means any printing character that is not alphanumeric or a space character.

This function can be implemented using

 
iswctype (wc, wctype ("punct"))

It is declared in `wctype.h'.

Function: int iswspace (wint_t wc)

Returns true if wc is a whitespace character. In the standard "C" locale, iswspace returns true for only the standard whitespace characters:

L' '

space

L'\f'

formfeed

L'\n'

newline

L'\r'

carriage return

L'\t'

horizontal tab

L'\v'

vertical tab

This function can be implemented using

 
iswctype (wc, wctype ("space"))

It is declared in `wctype.h'.

Function: int iswupper (wint_t wc)

Returns true if wc is an upper-case letter. The letter need not be from the Latin alphabet, any alphabet representable is valid.

This function can be implemented using

 
iswctype (wc, wctype ("upper"))

It is declared in `wctype.h'.

Function: int iswxdigit (wint_t wc)

Returns true if wc is a hexadecimal digit. Hexadecimal digits include the normal decimal digits `0' through `9' and the letters `A' through `F' and `a' through `f'.

This function can be implemented using

 
iswctype (wc, wctype ("xdigit"))

It is declared in `wctype.h'.

The GNU C library also provides a function which is not defined in the ISO C standard but which is available as a version for single byte characters as well.

Function: int iswblank (wint_t wc)

Returns true if wc is a blank character; that is, a space or a tab. This function was originally a GNU extension, but was added in ISO C99. It is declared in `wchar.h'.


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4.4 Notes on using the wide character classes

The first note is probably not astonishing but still occasionally a cause of problems. The iswXXX functions can be implemented using macros and in fact, the GNU C library does this. They are still available as real functions but when the `wctype.h' header is included the macros will be used. This is the same as the char type versions of these functions.

The second note covers something new. It can be best illustrated by a (real-world) example. The first piece of code is an excerpt from the original code. It is truncated a bit but the intention should be clear.

 
int
is_in_class (int c, const char *class)
{
  if (strcmp (class, "alnum") == 0)
    return isalnum (c);
  if (strcmp (class, "alpha") == 0)
    return isalpha (c);
  if (strcmp (class, "cntrl") == 0)
    return iscntrl (c);
  …
  return 0;
}

Now, with the wctype and iswctype you can avoid the if cascades, but rewriting the code as follows is wrong:

 
int
is_in_class (int c, const char *class)
{
  wctype_t desc = wctype (class);
  return desc ? iswctype ((wint_t) c, desc) : 0;
}

The problem is that it is not guaranteed that the wide character representation of a single-byte character can be found using casting. In fact, usually this fails miserably. The correct solution to this problem is to write the code as follows:

 
int
is_in_class (int c, const char *class)
{
  wctype_t desc = wctype (class);
  return desc ? iswctype (btowc (c), desc) : 0;
}

See section Converting Single Characters, for more information on btowc. Note that this change probably does not improve the performance of the program a lot since the wctype function still has to make the string comparisons. It gets really interesting if the is_in_class function is called more than once for the same class name. In this case the variable desc could be computed once and reused for all the calls. Therefore the above form of the function is probably not the final one.


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4.5 Mapping of wide characters.

The classification functions are also generalized by the ISO C standard. Instead of just allowing the two standard mappings, a locale can contain others. Again, the localedef program already supports generating such locale data files.

Data Type: wctrans_t

This data type is defined as a scalar type which can hold a value representing the locale-dependent character mapping. There is no way to construct such a value apart from using the return value of the wctrans function.

This type is defined in `wctype.h'.

Function: wctrans_t wctrans (const char *property)

The wctrans function has to be used to find out whether a named mapping is defined in the current locale selected for the LC_CTYPE category. If the returned value is non-zero, you can use it afterwards in calls to towctrans. If the return value is zero no such mapping is known in the current locale.

Beside locale-specific mappings there are two mappings which are guaranteed to be available in every locale:

"tolower"

"toupper"

These functions are declared in `wctype.h'.

Function: wint_t towctrans (wint_t wc, wctrans_t desc)

towctrans maps the input character wc according to the rules of the mapping for which desc is a descriptor, and returns the value it finds. desc must be obtained by a successful call to wctrans.

This function is declared in `wctype.h'.

For the generally available mappings, the ISO C standard defines convenient shortcuts so that it is not necessary to call wctrans for them.

Function: wint_t towlower (wint_t wc)

If wc is an upper-case letter, towlower returns the corresponding lower-case letter. If wc is not an upper-case letter, wc is returned unchanged.

towlower can be implemented using

 
towctrans (wc, wctrans ("tolower"))

This function is declared in `wctype.h'.

Function: wint_t towupper (wint_t wc)

If wc is a lower-case letter, towupper returns the corresponding upper-case letter. Otherwise wc is returned unchanged.

towupper can be implemented using

 
towctrans (wc, wctrans ("toupper"))

This function is declared in `wctype.h'.

The same warnings given in the last section for the use of the wide character classification functions apply here. It is not possible to simply cast a char type value to a wint_t and use it as an argument to towctrans calls.


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