Differences between revisions 6 and 8 (spanning 2 versions)

SPSS Looping

SPSS offers a couple options for looping, but overall is less flexible as compared to other languages.

Contents

SPSS Looping

Arbitrary Lists

The DO REPEAT command can loop over arbitrary, fixed-length lists.

do repeat X='A' 'B' 'C' 'D' 'E'.
  if ltrim(rtrim(foo))=X bar=1.
end repeat.

Integer Ranges

The DO REPEAT command can loop over an integer range.

do repeat Y=1 to 5.
  if foo=X bar=1.
end repeat.

The LOOP command can be used similarly.

loop #y=1 to 5 by 1.
  if foo=#y bar=1.
end loop.

Variable Lists

The DO REPEAT command can loop over variables. If the variables are not pre-existing, a range is extrapolated from the variable names.

do repeat Z=new1 to new5.
  compute Z=foo.
end repeat.

A similar effect can be acheived by using the LOOP command with a vector.

vector new(4).
loop #z = 1 to 4.
  compute new(#z) = (foo=#z).
end loop.

Conditional Looping

The LOOP command can loop a variable number of times, based on either a 'while' or a 'break' condition. Specify the IF option on the LOOP command for the former behaviuor, or on the END LOOP command for the latter.

loop.
  compute foo = replace(foo,'  ',' ').
end loop if char.index(foo,'  ') = 0.

loop if char.index(foo,'  ') > 0.
  compute foo = replace(foo,'  ',' ').
end loop.

Indefinite Looping

The LOOP command can also be made loop indefinitely, if no IF option is specified.

loop.
  compute foo = replace(foo,'  ',' ').
end loop.

Macros

Advanced looping can be handled through macro programming. See here for examples.

CategoryRicottone

-  ⇤ ← Revision 6 as of 2023-01-13 21:06:30 → 
  Size: 2530
  Editor: DominicRicottone
  Comment:
+   ← Revision 8 as of 2023-06-12 15:43:48 → ⇥
  Size: 1971
  Editor: DominicRicottone
  Comment:
-Deletions are marked like this.
+Additions are marked like this.
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-SPSS offers a couple options for looping, but overall is less flexible as compared to other languages.
+SPSS offers a couple options for '''looping''', but overall is less flexible as compared to other languages.
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-== Do Repeat ==
+== Arbitrary Lists ==
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-The '''`DO REPEAT ... END REPEAT`''' command loops over...

 * arbitrary sequences
 * variable ranges (either existing or new)
 * integer ranges

This command is ''only valid for data transformation commands'' (i.e. `COMPUTE`).

Multiple sequences/ranges can be iterated over in parallel as long as each has the same number of elements.
+The [[SPSS/DoRepeat|DO REPEAT]] command can loop over arbitrary, fixed-length lists.
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+Line 16:
-do repeat X=A B C D E / Y=new1 to new5 / Z=1 to 5.
  if (original=Z) Y=ltrim(rtrim(X)).
+do repeat X='A' 'B' 'C' 'D' 'E'.
  if ltrim(rtrim(foo))=X bar=1.
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+Line 20:
-To print the evaluated loop syntax, use `END REPEAT PRINT`.

Because only transformation commands are valid inside `DO REPEAT`, it isn't possible to do nested looping with it.
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+Line 25:
-== Loop ==
+== Integer Ranges ==
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+Line 27:
-The '''`LOOP ... END LOOP`''' command can arbitrarily loop.
+The [[SPSS/DoRepeat|DO REPEAT]] command can loop over an integer range.

{{{
do repeat Y=1 to 5.
  if foo=X bar=1.
end repeat.
}}}

The [[SPSS/Loop|LOOP]] command can be used similarly.

{{{
loop #y=1 to 5 by 1.
  if foo=#y bar=1.
end loop.
}}}

----
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+Line 47:
-=== Indefinite Looping ===
+== Variable Lists ==
-Line 45:
+Line 49:
-The `LOOP` command on its own will loop indefinitely (but actually until a maximum depth is reached). A warning will be emitted.
+The [[SPSS/DoRepeat|DO REPEAT]] command can loop over variables. If the variables are not pre-existing, a range is extrapolated from the variable names.

{{{
do repeat Z=new1 to new5.
  compute Z=foo.
end repeat.
}}}

A similar effect can be acheived by using the [[SPSS/Loop|LOOP]] command with a [[SPSS/Vectors|vector]].

{{{
vector new(4).
loop #z = 1 to 4.
  compute new(#z) = (foo=#z).
end loop.
}}}

----



== Conditional Looping ==

The [[SPSS/Loop|LOOP]] command can loop a variable number of times, based on either a 'while' or a 'break' condition. Specify the `IF` option on the `LOOP` command for the former behaviuor, or on the `END LOOP` command for the latter.
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+Line 76:
-  compute sentence = replace(sentence,'  ',' ').
+  compute foo = replace(foo,'  ',' ').
end loop if char.index(foo,'  ') = 0.

loop if char.index(foo,'  ') > 0.
  compute foo = replace(foo,'  ',' ').
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+Line 84:
+----
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+Line 87:
-===  While Loop ===
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+Line 88:
-There are two options for a conditional while loop.
+== Indefinite Looping ==
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+Line 90:
-First, use the `if` subcommand on `END LOOP`. Note that this will still run once, as the condition won't be checked until then.
+The [[SPSS/Loop|LOOP]] command can also be made loop indefinitely, if no `IF` option is specified.
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+Line 94:
-  compute sentence = replace(sentence,'  ',' ').
end loop if char.index(sentence,'  ') = 0.
}}}

Second, use the `if` subcommand on `LOOP`.

{{{
loop if char.index(sentence,'  ') > 0.
  compute sentence = replace(sentence,'  ',' ').
end loop.
}}}



=== Integer Range ===

An explicit number of repetitions can be specified.

{{{
loop X=1 to 3.
  compute sentence = replace(sentence,'  ',' ').
end loop.
}}}

The variable `X` is created by the `LOOP` command and it will persist in the dataset. In the above example it will have a final value of 4. To avoid persistence, consider using a [[SPSS/ScratchVariables|scratch variable]] (i.e. `#i`).

The full specification is `LOOP <VARIABLE> = <START> TO <END> BY <STEP>`.



=== Variable Range ===

The integer range method can be combined with [[SPSS/Vectors|vectors]] to achieve looping over variables.

{{{
vector new(4).
loop #i = 1 to 4.
  compute new(#i) = (original=#i).
+  compute foo = replace(foo,'  ',' ').

Diff for "SPSS/Looping"