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For portable applications, use only the assignment = operator, the equality operators == and !=, and the unary & operator on plain character character–typed or plain wide character-typed character–typed expressions.

This is recommended because the C99 C standard requires only the digit characters ('0' - '9'0–9) to have consecutive numerical values. Consequently, operations that rely on expected values for plain character character– or plain wide character-typed character–typed expressions can lead to unexpected behavior.

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• The binary + operator may be used to add integer values from 0 to through 9 to ' 0'.
• The binary - operator may be used to subtract character ' 0'.
• Relational operators <, <=, >, and >= can be used to check whether a character or wide character is a digit.

Character types should be chosen and used in accordance with recommendation STR04-C. Use plain char for characters in the basic character set.

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This noncompliant code example attempts to determine if the value of a character variable is between 'a' and 'c' inclusive. However, because the C99 C standard does not require the letter characters to be consecutive or in alphabetical order, the check might not work as expected.

char ch = 'b';
if ((ch >= 'a') && (ch <= 'c')) {
  /* ... */
}

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char ch = 't';
if ((ch == 'a') || (ch == 'b') || (ch == 'c')) {
  /* ... */
}

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