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

This practice is recommended because the C Standard requires only the digit characters (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 0 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.

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In this example, the specific check is enforced using compliant operations on character expressions.:

Code Block

bgColor	#CCCCFF
lang	c

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

Exceptions

STR09-C-EX0EX1: Consecutive values for characters like a~z can be assumed on platforms where ASCII or Unicode is used. This recommendation is primarily concerned with platform portability, for example, if code is migrated from ASCII systems to non-ASCII systems.