J3/04-112

Date:    08 Dec. 2003
To:      J3
From:    Dan Nagle
Subject: DO UNTIL


While some may argue that one more form of do loop
is redundant, nevertheless several compilers support it.
This argues in favor of its inclusion in a future standard.

Number:

Title: DO UNTIL

Submitted By: J3

Status: For Consideration

References: Section 8.1.6

Basic Functionality: DO UNTIL

Rationale:  Many programming languages have a post-loop-test iteration form,
            several Fortran compilers support the DO UNTIL form, and
            some programming problems fit naturally into a post-loop-test
            scheme, for example, reading until end-of-file.

            The argument against DO UNTIL is that the same functionality
            is available via the appropriate combination of do forever
            and exit.  However, any loop may be replaced by do forever
            and exit.  Furthermore, Fortran is an array-and-iteration
            oriented language, it seems difficult to argue against
            Fortran being rich in iteration structures.

Estimated Impact: For many compilers, the impact is none (they already
                  support DO UNTIL), for others, the impact will be minor,
                  they support the functionality using a different syntax.

Detailed Specification: DO UNTIL has the form:

                        [name:] DO UNTIL( s-l-e )
                           ! loop body here
                        ENDDO [name]

                        The scalar logical expression is evaluated
                        "at the enddo statement", so it's operands
                        need not be defined prior to the DO UNTIL statement.
                        That is, the operands may get their values during
                        execution of the loop body.

History: