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HELP IF                                            Steven Hardy Jan 1978
                                         Revised: Adrian Howard Mar 1992

-if- is the first word of a conditional expression, for example:

    if tough(steak) then

When this statement is executed, the POP-11 system first executes the
condition (i.e. "tough(steak)");  if this evaluates to -false- then
"grill(steak)" is executed, otherwise "fry(steak)" is executed. The full
syntax of allowable conditional commands is quite complex.  An -if-
statement has the form:

    if <CONDITION>
        then <CONSEQUENT>
    elseif <CONDITION>
        then <CONSEQUENT>
    [any number of additional -elseif-...-then-.... clauses]

The "else <CONSEQUENT>" can be omitted if desired. The "elseif
<CONDITION> then <CONSEQUENT>" can be either omitted or repeated as many
times as required.  A <CONDITION> is any expression, and a <CONSEQUENT>
is any sequence of POP-11 statements.

The condition should, when executed, produce a result. If the result is
-false-, then the corresponding consequent is ignored, and the program
moves on to the next -elseif-, or if there isn't one, to the -else-
clause. If there is no -elseif- or -else-, then the program moves on to
the instruction following the -endif-.

If the result of executing a condition is anything other than -false-,
then it is treated as if it were -true-, and the corresponding
consequent is executed, e.g.

    if 3 then "three" endif =>
    ** three

IF statements can be nested inside one another, i.e. a <CONDITION> or a
<CONSEQUENT> of an -if- statement may itself contain -if- statements.
For example:

    if isnumber(n) then
        if n > 0 then
            'less than or equal to zero'=>

It is permissible, but unusual, for a <CONSEQUENT> to contain no code at
all, for example:

    define prlist(list);
        lvars list;
        if  list == [] then
            hd(list) =>

It is clearer to use *UNLESS in this case, i.e.:

    define prlist(list);
        lvars list;
        unless list == [] then

The expression

    unless <CONDITION> then <CONSEQUENT>

is equivalent to:

    if not(<CONDITION>) then <CONSEQUENT>

See *UNLESS and *NOT for more details. There is also an -elseunless-
branch which is used in the same way as -elseif-. The expression

    ... elseunless <CONDITION> ...

is the same as doing

    ... elseif not(<CONDITION>) ...

If a conditional statement starts with -if- it must end with -endif-. If
it starts with -unless- it must end with -endunless-. Either form may
include -elseif-, -elseunless-, or -else- clauses.

For further information see:


    *BOOLEAN        --- Boolean values in POP-11.
    *CONDITIONALS   --- Summary of conditional statements in POP-11.
    *CONTROL        --- An overview of control facilities in POP-11.

        Summary of POP-11 syntax

--- C.all/help/if
--- Copyright University of Sussex 1992. All rights reserved. ----------