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To use the predicates in the following table you will need to include <cctype>.

Note: Predicates such as isdigit and isalpha can return integer results other than 0 or 1, with 0 meaning false and any other integer meaning true. The coding standard requiring all tests to yield either true or false are relaxed for these expressions.

Note: Functions such as isdigit and isalpha are actually pseudo-functions. They are converted into expressions that select a value from an array, so that no function call is done for them. It is important for an array index not to be negative. So convert a character to type unsigned or unsigned char to use it as a parameter to these predicates.

c − '0'

Use this to convert a digit to the number that it represents. For example, '8' − '0' yields 8.

d + '0'

Use this to convert a single-digit number d to a character. For example, 8 + '0' yields '8'.

c − ('a' − 'A')

This is the result of converting letter c from lower case to upper case. For example, 'q' − ('a' − 'A') yields 'Q'. This only works if c is a lower case letter.

c + ('a' − 'A')

This is the result of converting letter c from upper case to lower case. For example, 'Q' + ('a' − 'A') yields 'q'. This only works if c is an upper case letter.

(int)(c)

This is character c treated as having type int.

(char)(n)

This is integer n treated as having type char.

isdigit((unsigned) c)

This is nonzero if c is a digit.

isalpha((unsigned) c)

This is nonzero if c is a letter

isupper((unsigned) c)

This is nonzero if c is an upper-case letter

isupper((unsigned) c)

This is nonzero if c is a lower-case letter

isspace((unsigned) c)

This is nonzero if c is a white-space character (space, tab, newline, carriage-return, etc.).

isprint((unsigned)c)

isprint((unsigned) c) is mostly nonzero if character c can be printed. For example, isprint('x') is nonzero, but isprint('\0') is 0.

But isprint has a peculiarity. Isprint(c) is nonzero if c is a space, but is 0 for other characters, such as '\n' and '\t', where isspace(c) is nonzero.


Exercises

  1. Write an expresion that is true (or nonzero) if character c is a letter. Answer

  2. Write an expresion that is true if character c is an upper case letter. Answer

  3. Suppose that c is a digit (one of the characters '0', '1', ..., '9'). Write an expression that yields the integer that corresponds to that digit. For example, if c is '4' then your expression should yield 4. Answer

  4. Look at the coding standards for writing constants. Suppose that c has type char. Is 

      if(c == 10)
  {
    …
  }
    acceptable by the coding standards? If not, how should it be written? Answer

 

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