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

This is practice is recommended because the C99 standard 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- or plain-wide-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 standard C Standard does not require the letter characters to be in consecutive or in alphabetical or alphabetic order, the check might not work as expected.

Code Block

bgColor	#FFCCCC
lang	c


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

...

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.