Expression Syntax in BitchX  This document describes the "expression" context of a statement, as used with the ${} expando, the @ operator, and the FOR, IF, UNLESS, UNTIL, and WHILE commands. The normal context is "text", meaning everything is assumed to be plain text, unless it is preceded by a '$'. With "expression" context, everything is assumed to be a variable unless contained in square brackets '[]'. To enter expression context explicitly while in text context, use the ${} notation. Unlike text context, whitespace is not significant in expression context (but see below). The @ operator is a special case that allows any arbitrary expression to be evaluated. Unlike the other commands listed above, however, the return value of the expression is always thrown away. In many cases, the behavior of @ is much like that of EVAL. The following statements are equivalent: @ foo = 3 @ ::foo = 3 # two colons declare GLOBAL variables eval (foo = 3) Also note the use of variables local to a script's current scope. These could be assigned with the local(5) command as opposed to assign(5) for global variables. They can also be created with the @ operator (as shown above). The following statements are equivalent: @ :foo = 3 # one colon declares LOCAL variables local foo 3 BitchX's expression syntax is modeled after that of C++, and BitchX's operators (in general) hold the same precedence as they do in C++. Most of the C/C++ operator set is represented in BitchX, and their operation is the same. The following operators, from highest to lowest precedence, are supported:  1: () []  2: ! ~ ++ --  3: * / % **  4: + - ##  5: < <= > >= << >>  6: == !=  7: &  8: ^  9: | 10: && 11: ^^ 12: || 13: ?: 14: = += -= *= /= %= &= ^= |= #= #~ =~ !~ 15: , The string concatenation operators, ##, #=, and #~, are a special case, as they are not present in C or C++. As their name indicates, they are used to join two or more strings together, end to end. For example: @ foo = [foo] ## [bar] /* sets $foo to "foobar" */ @ foo #= [blah] /* sets $foo to "foobarblah" */ @ foo #~ [hmm] /* sets $foo to "hmmfoobarblah" */ Also like C/C++, parentheses may be used to force certain parts of the expression to be evaluated first (mainly in the event that the user wishes for it to evaluate in an order other than that of operator precedence). Parentheses may be nested. For example, if some variable $foo is set to 3: foo * 4 + 5 /* returns 17 */ foo * (4 + 5) /* returns 27 */ 4 + ((foo + 9) / 3) /* returns 8 */ All assignment operators always return the value assigned, which allows for the assignment of multiple variables at once. Keep in mind that expressions are evaluated right to left. For example, if $foo is 12 and $bar is 11: @ foo += bar *= 2 /* $bar is 22, $foo is 34 */ Since the release of the EPIC4 pre-betas, the client has been growing ever more perlish. Like perl, the =~ and !~ operators match with wildcards. =~ is a direct opposite of !~, where it returns true if the patterns patch, while !~ returns false. In this example, $bar is "epic": @ foo = bar =~ [*pi*] /* returns 1 */ @ foo = bar !~ [*z*] /* returns 1 */ The various bitwise operators are of special interest also. Assuming $foo is 12 and $bar is 11: foo & bar /* returns 8 */ foo | bar /* returns 15 */ foo ^ bar /* returns 7 */ The exponential operator takes numbers to various powers. It is especially useful, since many script writers create a $power() function for this purpose. It supports negative and fractional exponents as long as the system's math library (libm) does. Assuming $foo is 9: foo ** 2 /* returns 81 */ foo ** 0.5 /* returns 3 */ The {pre,post}fix {in,de}crement operators are big timesavers that C and C++ users everywhere swear by. They have also been known to swear at them, for reasons you will soon see. Assume $foo is 5, each column shows 3 ways of doing the same thing, from least efficient to most efficient: @ foo = foo + 1 @ foo = foo - 1 @ foo += 1 @ foo -= 1 @ foo++ @ foo-- However, these operators have pitfalls, which are mainly discovered by those who do not understand how they work. Both may either prefix or postfix a variable; prefix causes it to evaluate before the operation, postfix causes it to evaluate aster. For the examples shown above, it makes no difference. However, it does make a difference in this example: while ( foo++ < 10 ) { ... } The expression is evaluated for whether $foo is less than 10, and then $foo is incremented. If the autoincrement operator was instead used in prefix form, $foo would be incremented before the expression was evaluated, which would cause the loop to have one less iteration. Another pitfall of the autoincrement and decrement operators is the ambiguity introduced by insufficient whitespace when used in conjunction with addition and subtraction operators. Consider the following: @ foo = 4 @ bar = 8 @ foobar = foo+++bar How should one interpret the last assignment? Should it really look like ${foo++ + bar} or ${foo + ++bar}? It's hard to tell. The best solution is to not write code that looks so silly and unreadable. Add a couple spaces, and there is no ambiguity. (The answer is, the first one.) Another popular operator familiar to most C/C++ programmers is the tertiary operator (sometimes referred to as the alternation operator). It performs a function similar to IF, except is much more compact and efficient. We'll let $foo be 5 again: @ bar = foo > 3 ? 1 : 0 /* sets $bar to 1 */ @ bar = foo > 8 ? 1 : 0 /* sets $bar to 0 */ Functions (builtin and scripted) can also be used within expressions. The function will be evaluated, and its return value is used in the expression: @ foo = pattern(b* foo bar blah) /* sets $foo to "bar blah" */ All functions implicitly use a special operator, (). That is, the pair of parentheses themselves compose an operator, though of course it is somewhat different in nature from more traditional operators like '+' or '<' or '&'. Functions (aliases with return values) require the () to function properly. A similar operator is [], which is used for alias and variable structures. We've already seen that it can be used to explicitly switch the evaluation context to text. This can be extended to structure elements, such that they can be expanded on the fly: @ foo.1.1 = foo @ foo.1.2 = bar alias blah echo $foo[1][$0] /blah 2 /* expands to $foo.1.2 -> "bar" */ The same can be applied to aliases and functions as well. Because of the nature of the [] operator, anything may be expanded inside it, variables and functions alike. Operator parse tree: NU_POIX = varexp++ | varexp-- | NU_EXPR NU_EXPR = NU_ASSN NU_ASSN = varexp = NU_ASSN | varexp += NU_ASSN | varexp -= NU_ASSN | varexp *= NU_ASSN | varexp /= NU_ASSN | varexp %= NU_ASSN | varexp &= NU_ASSN | varexp ^= NU_ASSN | varexp |= NU_ASSN | varexp #= NU_ASSN | NU_TERT NU_TERT = NU_COMP ? NU_COMP : NU_COMP | NU_CONJ NU_CONJ = NU_CONJ && NU_CONJ | NU_CONJ || NU_CONJ | NU_CONJ ^^ NU_CONJ | NU_BITW NU_BITW = NU_COMP & NU_COMP | NU_COMP | NU_COMP | NU_COMP ^ NU_COMP | NU_COMP NU_COMP = NU_COMP == NU_COMP | NU_COMP != NU_COMP | NU_COMP > NU_COMP | NU_COMP >= NU_COMP | NU_COMP < NU_COMP | NU_COMP <= NU_COMP | NU_ADD NU_ADD = NU_ADD + NU_ADD | NU_ADD - NU_ADD | NU_ADD ## NU_ADD | NU_MULT NU_MULT = NU_MULT * NU_MULT | NU_MULT / NU_MULT | NU_MULT % NU_MULT | NU_UNIT NU_UNIT = token NUX_MODIF | unaryop token | ( NU_EXPR ) | [ expression ] NUX_MODIF | NU_PRIX NU_PRIX = ++varexp | --varexp NUX_MODIF = ( expression ) NUX_MODIF | [ expression ] NUX_MODIF unaryop = ! | ~ See Also: Patterns(7); Programming(7); Special_Vars(7)