Control Statement
General Description
Control statements allow for the interruption of the normal sequence of execution of statements by causing execution to continue at a specified line, rather than at the one with the next higher line number.
The goto-statement
GO TO line-number
allows for an unconditional transfer.
The if-then-statement
IF expl rel exp2 THEN line-number
where “expl” and’texp2” are expressions and “rel” is a relational operator, allows for a conditional transfer.
The gosub and return statements
GO SUB line-number
RETURN
allow for subroutine calls.
The on-goto-statement
ON expression GO TO line-number, ..., line-number
allows control to be transferred to a selected line.
The stop-statement
STOP
allows for program termination.
Syntax
- goto-statement = GO space* TO line-number
- if-then-statement = IF relational-expression THEN line-number
- relational-expression = numeric-expression relation numeric-expression / string-expression equality-relation string-expression
- relation = equality-relation / less-than-sign / greater-than-sign / not-less / not-greater
- equality-relation = equals-sign / not-equals
- not-less = greater-than-sign equals-sign
- not-greater = less-than-sign equals-sign
- not-equals = less-than-sign greater-than-sign
- gosub-statement = GO space* SUB line-number
- return-statement = RETURN
- on-goto-statement = ON numeric-expression GO space* TO line-number (comma line-number)*
- stop-statement = STOP
Examples
GO TO 999 IF X > Y+83 then 200
IF A$ <> B$ THEN 550 ON L+1 GO TO 300,400,500
Semantics
A goto-statement indicates that execution of the program is to be continued at the specified line-number.
If the value of the relational-expression in an lf-then-statement is true, then execution of the program shall continue from the specified line-number; if the value of the relational-expression is false, then execution shall be continued in sequence i.e. with the statement on the line following that containing the if-then-statement.
The relation “less than or equal to” shall be denoted by <=. Similarly, “greater than or equal to” shall be denoted by >=, while “not equal to” shall be denoted by <>.
The relation of equality holds between two strings if and only if the two strings have the same length and contain identical sequences of characters.
The execution of the gosub-statement and the return-statement can be described in terms of a stack of line-numbers (but may be implemented in some other fashion). Prior to execution of the first gosub-statement by the program, this stack is empty. Each time a gosub-statement is executed, the line number the gosub-statement is placed on top of the stack and of the program is continued at the line specified in the gosub-statement. Each time a return-statement is executed, the line-number on top of the stack is removed from the stack and execution of the program is continued at the line following the one with that line-number.
It is not necessary that equal numbers of gosub-statements and return-statements be executed before termination of the program.
The expression in an on-goto-statement shall be evaluated and rounded to obtain an integer, whose value is then used to select a line-number from the list following the GOTO (the line-numbers in the list are indexed from left to right, starting 1). Execution of the program shall continue at the statement with the selected line-number.
All line-numbers in control-statements shall refer to lines in the program.
The stop-statement causes termination of the program.
Exceptions
- An attempt is made to execute a return-statement without having executed a corresponding gosub-statement (fatal).
- The integer obtained as the value of an expression in an on-goto-statement is less than one or greater than the number of line-numbers in the list (fatal).