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6.3.2 Evaluation of logical expressions

All arguments of a logical expression are first evaluated and then the value of the logical expression is determined. For example, the logical expressions (a || b) is evaluated by first evaluating a and b, even though the value of b has no influence on the value of (a || b), if a evaluates to true.

Note, that this evaluation is different from the left-to-right, conditional evaluation of logical expressions (as found in most programming languages). For example, in these other languages, the value of (1 || b) is determined without ever evaluating b. This causes some problems with boolean tests on variables, which might not be defined at evaluation time. For example, the following results in an error, if the variable i is undefined:

if (defined(i) && i > 0) {} // WRONG!!!

This must be written instead as:

if (defined(i))
  if (i > 0) {}

However, there are several short work-arounds for this problem:

  1. If a variable (say, i) is only to be used as a boolean flag, then define (value is TRUE) and undefine (value is FALSE) i instead of assigning a value. Using this scheme, it is sufficient to simply write

    if (defined(i))

    in order to check whether i is TRUE. Use the command kill to undefine a variable, i.e. to assign it a FALSE value (see kill).

  2. If a variable can have more than two values, then define it, if necessary, before it is used for the first time. For example, if the following is used within a procedure

    if (! defined(DEBUG)) { int DEBUG = 1;}
    if (DEBUG == 3)  {...}
    if (DEBUG == 2)  {...}

    then a user of this procedure does not need to care about the existence of the DEBUG variable -- this remains hidden from the user. However, if DEBUG exists globally, then its local default value is overwritten by its global one.