
@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.
* 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 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}.

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, 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 are printed to the listing file, prefixed by a plus sign
(@samp{+}).

@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 neither the index clause nor either condition clause is
present, then the loop is executed @var{max_loops} (@pxref{SET}) times.
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}).
