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p := i = j

x := i + 1

k := log2(i+j)

F := log2 (* see 10.1 *)

s := {2, 3, 5, 7, 11, 13}

a[i] := (x+y) * (x-y)

t.key := i

w[i+1].name := "John"

t := c

9.2 Procedure Calls

A procedure call activates a procedure. It may contain a list of actual parameters which replace the corresponding formal parameters defined in the procedure declaration (see Ch. 10). The correspondence is established by the positions of the parameters in the actual and formal parameter lists. There are two kinds of parameters: variable and value parameters.

If a formal parameter is a variable parameter, the corresponding actual parameter must be a designator denoting a variable. If it denotes an element of a structured variable, the component selectors are evaluated when the formal/actual parameter substitution takes place, i.e. before the execution of the procedure. If a formal parameter is a value parameter, the corresponding actual parameter must be an expression. This expression is evaluated before the procedure activation, and the resulting value is assigned to the formal parameter (see also 10.1).

Examples:

WriteInt(i*2+1) (* see 10.1 *)

INC(w[k].count)

t.Insert("John") (* see 11 *)

9.3 Statement Sequences

Statement sequences denote the sequence of actions specified by the component statements which are separated by semicolons.

9.4 If Statements

If statements specify the conditional execution of guarded statement sequences. The Boolean expression preceding a statement sequence is called its guard. The guards are evaluated in sequence of occurrence, until one evaluates to TRUE, whereafter its associated statement sequence is executed. If no guard is satisfied, the statement sequence following the symbol ELSE is executed, if there is one.

Example:

IF (ch >= "A") & (ch <= "Z") THEN ReadIdentifier

ELSIF (ch >= "0") & (ch <= "9") THEN ReadNumber

ELSIF (ch = "'") OR (ch = '"') THEN ReadString

ELSE SpecialCharacter

END

9.5 Case Statements

Case statements specify the selection and execution of a statement sequence according to the value of an expression. First the case expression is evaluated, then that statement sequence is executed whose case label list contains the obtained value. The case expression must be of an integer or character type that includes the values of all case labels. Case labels are constants, and no value must occur more than once. If the value of the expression does not occur as a label of any case, the statement sequence following the symbol ELSE is selected, if there is one, otherwise the program is aborted.

Example:

CASE ch OF

"A" .. "Z": ReadIdentifier

| "0" .. "9": ReadNumber

| "'", '"': ReadString

ELSE SpecialCharacter

END

9.6 While Statements

While statements specify the repeated execution of a statement sequence while the Boolean expression (its guard) yields TRUE. The guard is checked before every execution of the statement sequence.

Examples:

WHILE i > 0 DO i := i DIV 2; k := k + 1 END

WHILE (t # NIL) & (t.key # i) DO t := t.left END

9.7 Repeat Statements

A repeat statement specifies the repeated execution of a statement sequence until a condition specified by a Boolean expression is satisfied. The statement sequence is executed at least once.

9.8 For Statements

A for statement specifies the repeated execution of a statement sequence while a progression of values is assigned to an integer variable called the control variable of the for statement.

The statement

FOR v := beg TO end BY step DO statements END

is equivalent to

temp := end; v := beg;

IF step > 0 THEN

WHILE v <= temp DO statements; v := v + step END

ELSE

WHILE v >= temp DO statements; v := v + step END

END

temp has the same type as v. step must be a nonzero constant expression. If step is not specified, it is assumed to be 1.

Examples:

FOR i := 0 TO 79 DO k := k + a[i] END

FOR i := 79 TO 1 BY -1 DO a[i] := a[i-1] END

9.9 Loop Statements

A loop statement specifies the repeated execution of a statement sequence. It is terminated upon execution of an exit statement within that sequence (see 9.10).

Example:

LOOP

ReadInt(i);

IF i < 0 THEN EXIT END;

WriteInt(i)

END

Loop statements are useful to express repetitions with several exit points or cases where the exit condition is in the middle of the repeated statement sequence.

9.10 Return and Exit Statements

A return statement indicates the termination of a procedure. It is denoted by the symbol RETURN, followed by an expression if the procedure is a function procedure. The type of the expression must be assignment compatible (see App. A) with the result type specified in the procedure heading (see Ch.10).

Function procedures require the presence of a return statement indicating the result value. In proper procedures, a return statement is implied by the end of the procedure body. Any explicit return statement therefore appears as an additional (probably exceptional) termination point.

An exit statement is denoted by the symbol EXIT. It specifies termination of the enclosing loop statement and continuation with the statement following that loop statement. Exit statements are contextually, although not syntactically associated with the loop statement which contains them.

9.11 With Statements

With statements execute a statement sequence depending on the result of a type test and apply a type guard to every occurrence of the tested variable within this statement sequence.

If v is a variable parameter of record type or a pointer variable, and if it is of a static type T0, the statement

WITH v: T1 DO S1 | v: T2 DO S2 ELSE S3 END

has the following meaning: if the dynamic type of v is T1, then the statement sequence S1 is executed where v is regarded as if it had the static type T1; else if the dynamic type of v is T2, then S2 is executed where v is regarded as if it had the static type T2; else S3 is executed. T1 and T2 must be extensions of T0. If no type test is satisfied and if an else clause is missing the program is aborted.

Example:

WITH t: CenterTree DO i := t.width; c := t.subnode END

10. Procedure Declarations

A procedure declaration consists of a procedure heading and a procedure body. The heading specifies the procedure identifier and the formal parameters. For methods it also specifies the receiver parameter and the attributes (see 10.2). The body contains declarations and statements. The procedure identifier is repeated at the end of the procedure declaration.

There are two kinds of procedures: proper procedures and function procedures. The latter are activated by a function designator as a constituent of an expression and yield a result that is an operand of the expression. Proper procedures are activated by a procedure call. A procedure is a function procedure if its formal parameters specify a result type. The body of a function procedure must contain a return statement which defines its result.

All constants, variables, types, and procedures declared within a procedure body are local to the procedure. Since procedures may be declared as local objects too, procedure declarations may be nested. The call of a procedure within its declaration implies recursive activation.

Local variables whose types are pointer types or procedure types are initialized to NIL before the body of the procedure is executed.

Objects declared in the environment of the procedure are also visible in those parts of the procedure in which they are not concealed by a locally declared object with the same name.

If a procedure declaration specifies a receiver parameter, the procedure is considered to be a method of the type of the receiver (see 10.2). A forward declaration serves to allow forward references to a procedure whose actual declaration appears later in the text. The formal parameter lists of the forward declaration and the actual declaration must match (see App. A) and the names of corresponding parameters must be equal.

10.1 Formal Parameters

Formal parameters are identifiers declared in the formal parameter list of a procedure. They correspond to actual parameters specified in the procedure call. The correspondence between formal and actual parameters is established when the procedure is called. There are two kinds of parameters, value and variable parameters, the latter indicated in the formal parameter list by the presence of one of the keywords VAR, IN, or OUT. Value parameters are local variables to which the value of the corresponding actual parameter is assigned as an initial value. Variable parameters correspond to actual parameters that are variables, and they stand for these variables. Variable parameters can be used for input only (keyword IN), output only (keyword OUT), or input and output (keyword VAR). Inside the procedure, input parameters are read-only. Like local variables, output parameters of pointer types and procedure types are initialized to NIL. Other output parameters must be considered as undefined prior to the first assignment in the procedure. The scope of a formal parameter extends from its declaration to the end of the procedure block in which it is declared. A function procedure without parameters must have an empty parameter list. It must be called by a function designator whose actual parameter list is empty too. The result type of a procedure can be neither a record nor an array.

Let f be the formal parameter and a the corresponding actual parameter. If f is an open array, then a must be array compatible to f and the lengths of f are taken from a. Otherwise a must be parameter compatible to f (see App. A)

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