@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{}

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 variables
(which may be existing variables or new variables or some combination),
numbers, or strings.  When new variable names are
specified, @cmd{DO REPEAT} creates them as numeric 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 as a token,
including command and subcommand names.  For this reason,
words commonly used in command and subcommand names should not be used
as dummy variable identifiers.

If 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 [index_var=start TO end [BY incr]] [IF condition].
        @dots{}
END LOOP [IF 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.  index_var must be a pre-existing
numeric variable.  start, end, and incr are numeric expressions
(@pxref{Expressions}.)  

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

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

Modifying index_var within the loop is allowed, but it has no effect on
the value of 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 MXLOOPS (@pxref{SET}) times.

@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}).
@setfilename ignored
