EXPRESSIONS

From TNT

General syntax is similar to that in C. Precedence is checked from left to right (the parser works by calculating the values as it reads the expressions). Operators: arithmetic (+ - * /), comparisons ( == > < >= <= != ), logical ( && || ! ), and bitwise operations (& | ^). When a number is expected, within the context of a macro command, enclosing a series of numbers (0-29) in curly braces is equivalent to typing the integral number that corresponds to that bit set (that is, { 0 3 4 } = 20+23+24 = 1 + 8 + 16 = 25). Bitwise representation is used (as in other parsimony programs) to store sets of states at internal nodes or polymorphic terminal taxa; the expressions that access state-sets (such as states, see under "help+states;") return sets of states as sets of bits. The legal operations and comparisons are:

  a == b if a equals b, value is 1 (0 otherwise)
  a > b if a is greater than b, value is 1
        (same for <)
  a>= b if a is greater than or equal to b, value is 1
        (same for <=)
  a != b if a is different from b, value is 1
  a && b if conditions a and b fullfilled, value is 1
  a || b if conditions a or b fullfilled, value is 1
 !a if condition a is not fullfilled, value is 1
  a & b if a bit is 1 in a and in b, it is 1 in the result
  a | b if a bit is 1 in a or b, it is 1 in the result
  a ^ b if a bit is 1 in a or b (but not both), then it is
      1 in the result

A "condition" may consist of just the numerical value of a number. For example, the variable success in the example above, might have been checked as

if ( 'success' == 1 ) ...

but, given that success itself can take values 0 (zero, "false"), or different from 0 ("true"), this is unnecessary, and identical to the more economical expression used in the example:

if ( 'success' ) ...

Note that a negative number is different from 0, and therefore is also evaluated as "true".

Also, the value returned by a logical comparison is either 0 or 1, which sometimes facilitates writing more compact instructions. For example, placing in a variable numlongtrees the number of trees that are longer than (say) 1000 steps might be done as:

   loop 0 ntrees
      if ( length[0] > 1000 )
        set numlongtrees 'numlongtrees'+1 ;
      end
    stop
    QUOTE There are 'numlongtrees' above 1000 steps ;

But the same result would be achieved by the more compact:

    loop 0 ntrees
       set numlongtrees += ( length[0] > 1000 ) ;
    stop
    QUOTE There are 'numlongtrees' above 1000 steps ;