procedure declaration

Syntax A procedureDeclaration is:
procedure id [(paramDeclaration {, paramDeclaration })]
statementsAndDeclarations
end id



Description A procedure declaration creates (but does not run) a new procedure. The name of the procedure (id) is given in two places, just after procedure and just after end.

Example
procedure greetings
put "Hello world"
end greetings

greetings % This outputs Hello world

procedure sayItAgain ( msg : string, n : int )
for i : 1 .. n
put msg
end for
end sayItAgain

sayItAgain ("Toot", 2 ) % Toot is output twice

procedure double ( var x : real )
x := 2 * x
end double

var y : real := 3.14
double ( y ) % This doubles the value of y

Details The set of parameters declared with the procedure are called formal parameters. In the double procedure, for example, x is a formal parameter. A procedure is called (invoked) by a procedure call statement which consists of the procedure's name followed by the parenthesized list of actual parameters (if any). For example, double(y) is a call having y as an actual parameter. If there are no parameters (see the greet procedure above), the call does not have parentheses. The keyword procedure can be abbreviated to proc.
Ordinarily, a procedure returns (finishes and goes back to the place where it was called) by reaching its end. However, the return statement in a procedure causes it to return immediately. Note that return can also be used in the main program to cause it to halt immediately.

Only parameters declared using var may be changed in the procedure, for example, x is changed in the double procedure. The upper bounds of arrays and strings that are parameters may be declared to be an asterisk (*). This means that the bound is that of the actual parameter. See paramDeclaration for details about parameters.

Procedures and functions cannot be declared inside other procedures and functions.

The syntax of a procedureDeclaration presented above has been simplified by leaving out the optional import list, pre condition, init clause, post condition and exception handler. The full syntax is


procedure [ pervasive ] id
[ ( [ paramDeclaration {,paramDeclaration } ] ) ]
[ : deviceSpecification ]
[ pre trueFalseExpn ]
[ init id := expn {, id := expn } ]
[ post trueFalseExpn ]
[ exceptionHandler ]
statementsAndDeclarations
end id
A procedure must be declared before being called. To allow