[ prev | next | top ]

16. Control-flow constructs

The pic language provides conditionals and looping. For example,

pi = atan2(0, -1);
for i = 0 to 2 * pi by 0.1 do {
    "-" at (i/2, 0);
    "." at (i/2, sin(i)/2);
    ":" at (i/2, cos(i)/2);
}

which yields this:

Image grohtml-2197043.png

Figure 16-1: Plotting with a for loop

The syntax of the for statement is:

for variable = expr1 to expr2 [by [*]expr3] do X body X

The semantics are as follows: Set variable to expr1. While the value of variable is less than or equal to expr2, do body and increment variable by expr3; if by is not given, increment variable by 1. If expr3 is prefixed by * then variable will instead be multiplied by expr3. X can be any character not occurring in body; or the two Xs may be paired braces (as in the sh command).

The syntax of the if statement is as follows:

if expr then X if-true X [else Y if-false Y]

Its semantics are as follows: Evaluate expr; if it is non-zero then do if-true, otherwise do if-false. X can be any character not occurring in if-true. Y can be any character not occurring in if-false.

Eithe or both of the X or Y pairs may instead be balanced pairs of braces ({ and }) as in the sh command. In either case, the if-true may contain balanced pairs of braces. None of these delimiters will be seen inside strings.

All the usual relational operators my be used in conditional expressions; ! (logical negation, not factorial), &&, ||, ==, !=, >=, <=, <, >.

String comparison is also supported using == and !=. String comparisons may need to be parenthesized to avoid syntactic ambiguities.


[ prev | next | top ]