[pspp] / doc / flow-control.texi

@c PSPP - a program for statistical analysis.
@c Copyright (C) 2017, 2020 Free Software Foundation, Inc.
@c Permission is granted to copy, distribute and/or modify this document
@c under the terms of the GNU Free Documentation License, Version 1.3
@c or any later version published by the Free Software Foundation;
@c with no Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts.
@c A copy of the license is included in the section entitled "GNU
@c Free Documentation License".
@c

@node Conditionals and Looping
@chapter Conditional and Looping Constructs
@cindex conditionals
@cindex loops
@cindex flow of control
@cindex control flow

This chapter documents @pspp{} commands used for conditional execution,
looping, and flow of control.

@menu
* BREAK::                       Exit a loop.
* DEFINE::                      Define a macro.
* DO IF::                       Conditionally execute a block of code.
* DO REPEAT::                   Textually repeat a code block.
* LOOP::                        Repeat a block of code.
@end menu

@node BREAK
@section BREAK
@vindex BREAK

@display
BREAK.
@end display

@cmd{BREAK} terminates execution of the innermost currently executing
@cmd{LOOP} construct.

@cmd{BREAK} is allowed only inside @cmd{LOOP}@dots{}@cmd{END LOOP}.
@xref{LOOP}, for more details.

@node DEFINE
@section DEFINE
@vindex DEFINE
@cindex macro

@display
DEFINE macro_name([argument[/argument]@dots{}])
@dots{}body@dots{}
!ENDDEFINE.

Each argument takes the following form:
  @{!arg_name =,!POSITIONAL@} [!DEFAULT(default)] [!NOEXPAND]
  @{!TOKENS(count),!CHAREND('token'),!ENCLOSE('start','end'),!CMDEND@}

The following directives may be used within the body:
  !OFFEXPAND
  !ONEXPAND

The following functions may be used within the body:
  !BLANKS(count)
  !CONCAT(arg@dots{})
  !EVAL(arg)
  !HEAD(arg)
  !INDEX(haystack, needle)
  !LENGTH(arg)
  !NULL
  !QUOTE(arg)
  !SUBSTR(arg, start[, count])
  !TAIL(arg)
  !UNQUOTE(arg)
  !UPCASE(arg)

The body may also include the following constructs:

!IF (condition) !THEN true-expansion !ENDIF
!IF (condition) !THEN true-expansion !ELSE false-expansion !ENDIF
@end display

The DEFINE command defines a macro that can later be called any number
of times within a syntax file.  Each time it is called, the macro's
body is @dfn{expanded}, that is, substituted, as if the body had been
written instead of the macro call.  A macro may accept arguments,
whose values are specified at the point of invocation and expanded in
the body where they are referenced.  Macro bodies may also use various
directives and functions, which are also expanded when the macro is
called.

Many identifiers associated with macros begin with @samp{!}, a
character not normally allowed in identifiers.  These identifiers are
reserved only for use with macros, which helps keep them from being
confused with other kinds of identifiers.

@node Macro Basics
@subsection Macro Basics

The simplest macros have no arguments.  The following defines a macro
named @code{!vars} that expands to the variable names @code{v1 v2 v3},
along with a few example uses.  The macro's name begins with @samp{!},
which is optional for macro names.  The @code{()} following the macro
name are required:

@example
DEFINE !vars()
v1 v2 v3
!ENDDEFINE.

DESCRIPTIVES !vars.
FREQUENCIES /VARIABLES=!vars.
@end example

Macros can also expand to entire commands.  For example, the following
example performs the same analyses as the last one:

@example
DEFINE !commands()
DESCRIPTIVES v1 v2 v3.
FREQUENCIES /VARIABLES=v1 v2 v3.
!ENDDEFINE.

!commands
@end example

The body of a macro can call another macro.  For example, we could
combine the two preceding examples, with @code{!commands} calling
@code{!vars} to obtain the variables to analyze.  The following shows
one way that could work:

@example
DEFINE !commands()
DESCRIPTIVES !vars.
FREQUENCIES /VARIABLES=!vars.
!ENDDEFINE.

DEFINE !vars() v1 v2 v3 !ENDDEFINE.
!commands

* We can redefine the variables macro to analyze different variables:
DEFINE !vars() v4 v5 !ENDDEFINE.
!commands
@end example

The @code{!commands} macro would be easier to use if it took the
variables to analyze as an argument rather than through another macro.
The following section shows how to do that.

@node Macro Arguments
@subsection Macro Arguments

Macros may take any number of arguments, which are specified within
the parentheses in the DEFINE command.  Arguments come in two
varieties based on how their values are specified when the macro is
called:

@itemize @bullet
@item
A @dfn{positional} argument has a required value that follows the
macro's name.  Use the @code{!POSITIONAL} keyword to declare a
positional argument.

References to a positional argument in a macro body are numbered:
@code{!1} is the first positional argument, @code{!2} the second, and
so on.  In addition, @code{!*} expands to all of the positional
argument values, separated by a space.

The following example uses a positional argument:

@example
DEFINE !analyze(!POSITIONAL !CMDEND)
DESCRIPTIVES !1.
FREQUENCIES /VARIABLES=!1.
!ENDDEFINE.

!analyze v1 v2 v3.
!analyze v4 v5.
@end example

@item
A @dfn{keyword} argument has a name.  In the macro call, its value is
specified with the syntax @code{@var{name}=@var{value}}.  Because of
the names, keyword argument values may take any order in a macro call.
If one is omitted, then a default value is used: either the value
specified in @code{!DEFAULT(@var{value})}, or an empty value
otherwise.

In declaration and calls, a keyword argument's name may not begin with
@samp{!}, but references to it in the macro body do start with a
leading @samp{!}.

The following example uses a keyword argument that defaults to ALL if
the argument is not assigned a value:

@example
DEFINE !analyze_kw(vars=!DEFAULT(ALL) !CMDEND)
DESCRIPTIVES !vars.
FREQUENCIES /VARIABLES=!vars.
!ENDDEFINE.

!analyze_kw vars=v1 v2 v3.  /* Analyze specified variables.
!analyze_kw.                /* Analyze all variables.  
@end example
@end itemize

If a macro has both positional and keyword arguments, then the
positional arguments must come first in the DEFINE command, and their
values also come first in macro calls.

Each argument declaration specifies the form of its value:

@table @code
@item !TOKENS(@var{count})
Exactly @var{count} tokens, e.g.@: @code{!TOKENS(1)} for a single
token.  Each identifier, number, quoted string, operator, or
punctuator is a token.  @xref{Tokens}, for a complete definition.

The following variant of @code{!analyze_kw} accepts only a single
variable name (or @code{ALL}) as its argument:

@example
DEFINE !analyze_one_var(!POSITIONAL !TOKENS(1))
DESCRIPTIVES !1.
FREQUENCIES /VARIABLES=!1.
!ENDDEFINE.

!analyze_one_var v1.
@end example

@item !CHAREND('@var{token}')
Any number of tokens up to @var{token}, which should be an operator or
punctuator token such as @samp{/} or @samp{+}.  The @var{token} does
not become part of the value.

With the following variant of @code{!analyze_kw}, the variables must
be following by @samp{/}:

@example
DEFINE !analyze_parens(vars=!CHARNED('/'))
DESCRIPTIVES !vars.
FREQUENCIES /VARIABLES=!vars.
!ENDDEFINE.

!analyze_parens vars=v1 v2 v3/.
@end example

@item !ENCLOSE('@var{start}','@var{end}')
Any number of tokens enclosed between @var{start} and @var{end}, which
should each be operator or punctuator tokens.  For example, use
@code{!ENCLOSE('(',')')} for a value enclosed within parentheses.
(Such a value could never have right parentheses inside it, even
paired with left parentheses.)  The start and end tokens are not part
of the value.

With the following variant of @code{!analyze_kw}, the variables must
be specified within parentheses:

@example
DEFINE !analyze_parens(vars=!ENCLOSE('(',')'))
DESCRIPTIVES !vars.
FREQUENCIES /VARIABLES=!vars.
!ENDDEFINE.

!analyze_parens vars=(v1 v2 v3).
@end example

@item !CMDEND
Any number of tokens up to the end of the command.  This should be
used only for the last positional parameter, since it consumes all of
the tokens in the command calling the macro.

The following variant of @code{!analyze_kw} takes all the variable
names up to the end of the command as its argument:

@example
DEFINE !analyze_kw(vars=!CMDEND)
DESCRIPTIVES !vars.
FREQUENCIES /VARIABLES=!vars.
!ENDDEFINE.

!analyze_kw vars=v1 v2 v3.
@end example
@end table

By default, when an argument's value contains a macro call, the call
is expanded each time the argument appears in the macro's body.  The
@code{!NOEXPAND} keyword in an argument declaration suppresses this
expansion.  @xref{Controlling Macro Expansion}, for details.

@node Controlling Macro Expansion
@subsection Controlling Macro Expansion

Multiple factors control whether macro calls are expanded in different
situations.  At the highest level, @code{SET MEXPAND} controls whether
macro calls are expanded.  By default, it is enabled.  @xref{SET
MEXPAND}, for details.

A macro body may contain macro calls.  By default, these are expanded.
If a macro body contains @code{!OFFEXPAND} or @code{!ONEXPAND}
directives, then @code{!OFFEXPAND} disables expansion of macro calls
until the following @code{!ONEXPAND}.

A macro argument's value may contain a macro call.  By default, these
macro calls are expanded.  If the argument was declared with the
@code{!NOEXPAND} keyword, they are not expanded.

The argument to a macro function is a special context that does not
expand macro calls.  For example, if @code{!vars} is the name of a
macro, then @code{!LENGTH(!vars)} expands to 5, as does
@code{!LENGTH(!1)} if positional argument 1 has value @code{!vars}.
In these cases, use the @code{!EVAL} macro function to expand macros,
e.g.@: @code{!LENGTH(!EVAL(!vars))} or @code{!LENGTH(!EVAL(!1))}.
@xref{Macro Functions}, for details.

These rules apply to macro calls.  Uses of macro functions and macro
arguments within a macro body are always expanded.

@node Macro Functions
@subsection Macro Functions

Macro bodies may manipulate syntax using macro functions.  Macro
functions accept tokens as arguments and expand to sequences of
characters.

The arguments to macro functions have a restricted form.  They may
only be a single token (such as an identifier or a string), a macro
argument, or a call to a macro function.  Thus, @code{x}, @code{5.0},
@code{x}, @code{!1}, @code{"5 + 6"}, and @code{!CONCAT(x,y)} are valid
macro arguments, but @code{x y} and @code{5 + 6} are not.

Macro functions expand to sequences of characters.  When these
character strings are processed further as character strings, e.g.@:
with @code{!LENGTH}, any character string is valid.  When they are
interpreted as PSPP syntax, e.g.@: when the expansion becomes part of
a command, they need to be valid for that purpose.  For example,
@code{!UNQUOTE("It's")} will yield an error if the expansion
@code{It's} becomes part of a PSPP command, because it contains
unbalanced single quotes, but @code{!LENGTH(!UNQUOTE("It's"))} expands
to 4.

The following macro functions are available.  Each function's
documentation includes examples in the form @code{@var{call}
@expansion{} @var{expansion}}.

@deffn {Macro Function} !BLANKS (count)
Expands to @var{count} unquoted spaces, where @var{count} is a
nonnegative integer.  Outside quotes, any positive number of spaces
are equivalent; for a quoted string of spaces, use
@code{!QUOTE(!BLANKS(@var{count}))}.

In the examples below, @samp{_} stands in for a space to make the
results visible.

@c Keep these examples in sync with the test for !BLANKS in
@c tests/language/control/define.at:
@example
!BLANKS(0)                  @expansion{} @r{empty}
!BLANKS(1)                  @expansion{} _
!BLANKS(2)                  @expansion{} __
!QUOTE(!BLANKS(5))          @expansion{} '_____'
@end example
@end deffn

@deffn {Macro Function} !CONCAT (arg@dots{})
Expands to the concatenation of all of the arguments.  Before
concatenation, each quoted string argument is unquoted, as if
@code{!UNQUOTE} were applied.

@c Keep these examples in sync with the test for !CONCAT in
@c tests/language/control/define.at:
@example
!CONCAT(x, y)                @expansion{} xy
!CONCAT('x', 'y')            @expansion{} xy
!CONCAT(12, 34)              @expansion{} 1234
!CONCAT(!NULL, 123)          @expansion{} 123
@end example
@end deffn

@deffn {Macro Function} !EVAL (arg)
Expands macro calls in @var{arg}.  This is especially useful if
@var{arg} is the name of a macro or a macro argument that expands to
one, because arguments to macro functions are not expanded by default.

The following examples assume that @code{!vars} is a macro that
expands to @code{a b c}:

@example
!vars                        @expansion{} a b c
!QUOTE(!vars)                @expansion{} '!vars'
!EVAL(!vars)                 @expansion{} a b c
!QUOTE(!EVAL(!vars))         @expansion{} 'a b c'
@end example

These examples additionally assume that argument @code{!1} has value
@code{!vars}:

@example
!1                           @expansion{} a b c
!QUOTE(!1)                   @expansion{} '!vars'
!EVAL(!1)                    @expansion{} a b c
!QUOTE(!EVAL(!1))            @expansion{} 'a b c'
@end example
@end deffn

@deffn {Macro Function} !HEAD (arg)
@deffnx {Macro Function} !TAIL (arg)
@code{!HEAD} expands to just the first token in an unquoted version of
@var{arg}, and @code{!TAIL} to all the tokens after the first.

@example
!HEAD('a b c')               @expansion{} a
!HEAD('a')                   @expansion{} a
!HEAD(!NULL)                 @expansion{} @r{empty}
!HEAD('')                    @expansion{} @r{empty}

!TAIL('a b c')               @expansion{} b c
!TAIL('a')                   @expansion{} @r{empty}
!TAIL(!NULL)                 @expansion{} @r{empty}
!TAIL('')                    @expansion{} @r{empty}
@end example
@end deffn

@deffn {Macro Function} !INDEX (haystack, needle)
Looks for @var{needle} in @var{haystack}.  If it is present, expands
to the 1-based index of its first occurrence; if not, expands to 0.

@example
!INDEX(banana, an)           @expansion{} 2
!INDEX(banana, nan)          @expansion{} 3
!INDEX(banana, apple)        @expansion{} 0
!INDEX("banana", nan)        @expansion{} 4
!INDEX("banana", "nan")      @expansion{} 0
!INDEX(!UNQUOTE("banana"), !UNQUOTE("nan")) @expansion{} 3
@end example
@end deffn

@deffn {Macro Function} !LENGTH (arg)
Expands to a number token representing the number of characters in
@var{arg}.

@example
!LENGTH(123)                 @expansion{} 3
!LENGTH(123.00)              @expansion{} 6
!LENGTH( 123 )               @expansion{} 3
!LENGTH("123")               @expansion{} 5
!LENGTH(xyzzy)               @expansion{} 5
!LENGTH("xyzzy")             @expansion{} 7
!LENGTH("xy""zzy")           @expansion{} 9
!LENGTH(!UNQUOTE("xyzzy"))   @expansion{} 5
!LENGTH(!UNQUOTE("xy""zzy")) @expansion{} 6
!LENGTH(!1)                  @expansion{} 5 @r{if @t{!1} is @t{a b c}}
!LENGTH(!1)                  @expansion{} 0 @r{if @t{!1} is empty}
!LENGTH(!NULL)               @expansion{} 0
@end example
@end deffn

@deffn {Macro Function} !NULL
Expands to an empty character sequence.

@example
!NULL                        @expansion{} @r{empty}
!QUOTE(!NULL)                @expansion{} ''
@end example
@end deffn

@deffn {Macro Function} !QUOTE (arg)
@deffnx {Macro Function} !UNQUOTE (arg)
The @code{!QUOTE} function expands to its argument surrounded by
apostrophes, doubling any apostrophes inside the argument to make sure
that it is valid PSPP syntax for a string.  If the argument was
already a quoted string, @code{!QUOTE} expands to it unchanged.

Given a quoted string argument, the @code{!UNQUOTED} function expands
to the string's contents, with the quotes removed and any doubled
quote marks reduced to singletons.  If the argument was not a quoted
string, @code{!UNQUOTE} expands to the argument unchanged.

@example
!QUOTE(123.0)                @expansion{} '123.0'
!QUOTE( 123 )                @expansion{} '123'
!QUOTE('a b c')              @expansion{} 'a b c'
!QUOTE("a b c")              @expansion{} "a b c"
!QUOTE(!1)                   @expansion{} 'a ''b'' c' @r{if @t{!1} is @t{a 'b' c}}

!UNQUOTE(123.0)              @expansion{} 123.0
!UNQUOTE( 123 )              @expansion{} 123
!UNQUOTE('a b c')            @expansion{} a b c
!UNQUOTE("a b c")            @expansion{} a b c
!UNQUOTE(!1)                 @expansion{} a 'b' c @r{if @t{!1} is @t{a 'b' c}}

!QUOTE(!UNQUOTE(123.0))      @expansion{} '123.0'
!QUOTE(!UNQUOTE( 123 ))      @expansion{} '123'
!QUOTE(!UNQUOTE('a b c'))    @expansion{} 'a b c'
!QUOTE(!UNQUOTE("a b c"))    @expansion{} 'a b c'
!QUOTE(!UNQUOTE(!1))         @expansion{} 'a ''b'' c' @r{if @t{!1} is @t{a 'b' c}}
@end example
@end deffn

@deffn {Macro Function} !SUBSTR (arg, start[, count])
Expands to a substring of @var{arg} starting from 1-based position
@var{start}.  If @var{count} is given, it limits the number of
characters in the expansion; if it is omitted, then the expansion
extends to the end of @var{arg}.

@example
!SUBSTR(banana, 3)           @expansion{} nana
!SUBSTR(banana, 3, 3)        @expansion{} nan
!SUBSTR("banana", 3)         @expansion{} @r{error (@code{anana"} is not a valid token)}
!SUBSTR(!UNQUOTE("banana"), 3) @expansion{} nana
!SUBSTR("banana", 3, 3)      @expansion{} ana

!SUBSTR(banana, 3, 0)        @expansion{} @r{empty}
!SUBSTR(banana, 3, 10)       @expansion{} nana
!SUBSTR(banana, 10, 3)       @expansion{} @r{empty}
@end example
@end deffn

@deffn {Macro Function} !UPCASE (arg)
Expands to an unquoted version of @var{arg} with all letters converted
to uppercase.

@example
!UPCASE(freckle)             @expansion{} FRECKLE
!UPCASE('freckle')           @expansion{} FRECKLE
!UPCASE('a b c')             @expansion{} A B C
!UPCASE('A B C')             @expansion{} A B C
@end example
@end deffn

@node Macro Conditional Expansion
@subsection Macro Conditional Expansion

The @code{!IF} construct may be used inside a macro body to allow for
conditional expansion.  It takes the following forms:

@example
!IF (condition) !THEN true-expansion !IFEND
!IF (condition) !THEN true-expansion !ELSE false-expansion !IFEND
@end example

When the @var{condition} evaluates to true, @var{true-expansion} is
expanded.  When it evaluates to false, @var{false-expansion} is
expanded, if it is present.  The unquoted value @samp{0} is considered
false, and all other values are considered true.

Within @var{condition}, macros, macro arguments, and macro functions
are expanded.  After expansion, the condition is evaluated as a macro
expression that may use only the following operators, in descending
order of operator precedence:

@example
()
!EQ !NE !GT !LT !GE !LE = ~= <> > < >= <=
!NOT ~
!AND &
!OR |
@end example

All of these operators yield a Boolean result, either @samp{0} for
false or @samp{1} for true.

If an operand is a quoted string, then the operator considers the
contents of the quoted string; otherwise, it must be a single token.
Thus, @code{1 = '1'} is true, and @code{'a b' = a b} is in error
because the right-hand operand is two tokens.

Comparisons in macro expressions are always string comparisons.  This
can be surprising when the operands are numbers: e.g.@: @code{1 < 1.0}
and @code{10 < 2} both evaluate to @samp{1} (true).

Macro expression comparisons are case sensitive, so that @code{a = A}
evaluates to @samp{0} (false).

@node Macro Settings
@subsection Macro Settings

Some macro behavior is controlled through the SET command
(@pxref{SET}).  This section describes these settings.

Any SET command that changes these settings within a macro body only
takes effect following the macro.  This is because PSPP expands a
macro's entire body at once, so that the SET command inside the body
only executes afterwards.

The MEXPAND setting (@pxref{SET MEXPAND}) controls whether macros will
be expanded at all.  By default, macro expansion is on.  To avoid
expansion of macros called within a macro body, use @code{!OFFEXPAND}
and @code{!ONEXPAND} (@pxref{Controlling Macro Expansion}).

When MPRINT (@pxref{SET MPRINT}) is turned on, PSPP logs an expansion
of each macro in the input.  This feature can be useful for debugging
macro definitions.

MNEST (@pxref{SET MNEST}) limits the depth of expansion of macro
calls, that is, the nesting level of macro expansion.  The default is
50.  This is mainly useful to avoid infinite expansion in the case of
a macro that calls itself.

MITERATE

PRESERVE...RESTORE

SET MEXPAND, etc. doesn't work inside macro bodies.

@node Macro Notes
@subsection Extra Notes

@code{!*} expands to all the positional arguments.

Macros in comments.

Macros in titles.

@node DO IF
@section DO IF
@vindex DO IF

@display
DO IF condition.
        @dots{}
[ELSE IF condition.
        @dots{}
]@dots{}
[ELSE.
        @dots{}]
END IF.
@end display

@cmd{DO IF} allows one of several sets of transformations to be
executed, depending on user-specified conditions.

If the specified boolean expression evaluates as true, then the block
of code following @cmd{DO IF} is executed.  If it evaluates as
missing, then
none of the code blocks is executed.  If it is false, then
the boolean expression on the first @cmd{ELSE IF}, if present, is tested in
turn, with the same rules applied.  If all expressions evaluate to
false, then the @cmd{ELSE} code block is executed, if it is present.

When @cmd{DO IF} or @cmd{ELSE IF} is specified following @cmd{TEMPORARY}
(@pxref{TEMPORARY}), the @cmd{LAG} function may not be used
(@pxref{LAG}).

@node DO REPEAT
@section DO REPEAT
@vindex DO REPEAT

@display
DO REPEAT dummy_name=expansion@dots{}.
        @dots{}
END REPEAT [PRINT].

expansion takes one of the following forms:
        var_list
        num_or_range@dots{}
        'string'@dots{}
        ALL

num_or_range takes one of the following forms:
        number
        num1 TO num2
@end display

@cmd{DO REPEAT} repeats a block of code, textually substituting
different variables, numbers, or strings into the block with each
repetition.

Specify a dummy variable name followed by an equals sign (@samp{=})
and the list of replacements.  Replacements can be a list of existing
or new variables, numbers, strings, or @code{ALL} to specify all
existing variables.  When numbers are specified, runs of increasing
integers may be indicated as @code{@var{num1} TO @var{num2}}, so that
@samp{1 TO 5} is short for @samp{1 2 3 4 5}.

Multiple dummy variables can be specified.  Each
variable must have the same number of replacements.

The code within @cmd{DO REPEAT} is repeated as many times as there are
replacements for each variable.  The first time, the first value for
each dummy variable is substituted; the second time, the second value
for each dummy variable is substituted; and so on.

Dummy variable substitutions work like macros.  They take place
anywhere in a line that the dummy variable name occurs.  This includes
command and subcommand names, so command and subcommand names that
appear in the code block should not be used as dummy variable
identifiers.  Dummy variable substitutions do not occur inside quoted
strings, comments, unquoted strings (such as the text on the
@cmd{TITLE} or @cmd{DOCUMENT} command), or inside @cmd{BEGIN
DATA}@dots{}@cmd{END DATA}.

Substitution occurs only on whole words, so that, for example, a dummy
variable PRINT would not be substituted into the word PRINTOUT.

New variable names used as replacements are not automatically created
as variables, but only if used in the code block in a context that
would create them, @i{e.g.}@: on a @cmd{NUMERIC} or @cmd{STRING} command
or on the left side of a @cmd{COMPUTE} assignment.

Any command may appear within @subcmd{DO REPEAT}, including nested @subcmd{DO REPEAT}
commands.  If @cmd{INCLUDE} or @cmd{INSERT} appears within @subcmd{DO REPEAT},
the substitutions do not apply to the included file.

If @subcmd{PRINT} is specified on @cmd{END REPEAT}, the commands after
substitutions are made should be printed to the listing file, prefixed
by a plus sign (@samp{+}).  This feature is not yet implemented.

@node LOOP
@section LOOP
@vindex LOOP

@display
LOOP [@var{index_var}=@var{start} TO @var{end} [BY @var{incr}]] [IF @var{condition}].
        @dots{}
END LOOP [IF @var{condition}].
@end display

@cmd{LOOP} iterates a group of commands.  A number of
termination options are offered.

Specify index_var to make that variable count from one value to
another by a particular increment.  @var{index_var} must be a pre-existing
numeric variable.  @var{start}, @var{end}, and @var{incr} are numeric expressions
(@pxref{Expressions}.)

During the first iteration, @var{index_var} is set to the value of @var{start}.
During each successive iteration, @var{index_var} is increased by the value of
@var{incr}.  If @var{end} > @var{start}, then the loop terminates
when @var{index_var} > @var{end};
otherwise it terminates when @var{index_var} < @var{end}.  If @var{incr} is not specified
then it defaults to +1 or -1 as appropriate.

If @var{end} > @var{start} and @var{incr} < 0, or if @var{end} < @var{start} and
 @var{incr} > 0, then the
loop is never executed.  @var{index_var} is nevertheless set to the value of
start.

Modifying @var{index_var} within the loop is allowed, but it has no effect on
the value of @var{index_var} in the next iteration.

Specify a boolean expression for the condition on @cmd{LOOP} to
cause the loop to be executed only if the condition is true.  If the
condition is false or missing before the loop contents are executed the
first time, the loop contents are not executed at all.

If index and condition clauses are both present on @cmd{LOOP}, the
index variable is always set before the condition is evaluated.  Thus,
a condition that makes use of the index variable will always see the
index value to be used in the next execution of the body.

Specify a boolean expression for the condition on @cmd{END LOOP} to cause
the loop to terminate if the condition is true after the enclosed
code block is executed.  The condition is evaluated at the end of the
loop, not at the beginning, so that the body of a loop with only a
condition on @cmd{END LOOP} will always execute at least once.

If the index clause is not
present, then the loop is executed at most @var{max_loops} (@pxref{SET}) times
(but possibly fewer, if a condition clause evaluates to false or if
@cmd{BREAK} executes).
The default value of @var{max_loops} is 40.

@cmd{BREAK} also terminates @cmd{LOOP} execution (@pxref{BREAK}).

Loop index variables are by default reset to system-missing from one
case to another, not left, unless a scratch variable is used as index.
When loops are nested, this is usually undesired behavior, which can
be corrected with @cmd{LEAVE} (@pxref{LEAVE}) or by using a scratch
variable as the loop index.

When @cmd{LOOP} or @cmd{END LOOP} is specified following @cmd{TEMPORARY}
(@pxref{TEMPORARY}), the @cmd{LAG} function may not be used
(@pxref{LAG}).