= SAS Data Step =

In SAS, data is manipulated in '''data steps'''.

<<TableOfContents>>

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== Data Model ==

The data model for SAS is:

 * Read a row of data
 * Process statements sequentially
 * On `run`, write data into the output table

{{{
data LIBREF.TABLE;
  STATEMENTS;
run;
}}}

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== Tables ==

The table named in a data step is always the ''output table''. By default it is also the ''input table'' and data is manipulated in place.

To set a different table as the input, use a `set` statement.

{{{
data LIBREF.OUTPUT;
  set LIBREF.INPUT;
  STATEMENTS;
run;
}}}

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== Data Input ==

See [[SAS/ReadingData|here]].

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== Transforming Data ==



=== Creating Variables ===

{{{
data LIBREF.OUTPUT;
  set LIBREF.INPUT;

  /* declare numeric variables directly with numeric literals */
  foo = 1;

  /* declare string variables by specifying the length first, then with string literals */
  length hello $5 world $5;
  hello = 'Hello';
  world = 'World';

  /* the length of a string variable cannot be changed again within the same data step */
  length greeting $12;
  greeting = CAT(hello, world);        /* 'Hello World'  */
  greeting = CATS(hello, world);       /* 'HelloWorld'   */
  greeting = CATX(', ', hello, world); /* 'Hello, World' */
  greeting = hello||world;             /* 'HelloWorld'   */
run;
}}}

Any valid [[SAS/Expressions|expression]] can be used to create a new variable.



=== Coercing Data Types ===

{{{
data LIBREF.NEWTABLE;
  set LIBREF.OLDTABLE;

  /* coerce string to numeric */
  numeric_zip_code = input(string_zip_code, 5.);

  /* left-align text */
  length TEXT_VAR $999.;
  TEXT_VAR = put(TEXT_VAR, $999. -L);

  /* parse timestamps like '01Jan1999' */
  format timestamp DATE9.;
  month = month(timestamp);

  /* round numerics to integers */
  format test_score 3.;
run;
}}}

The `input` function generates a variable `_ERROR_` by default, flagging cases that could not be formatted. To suppress this variable's creation, use `input(ALNUM_ID, ?? 8.)`.

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== Joining Data ==

See [[SAS/JoiningData|here]].

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== Filtering Data ==

=== Subset Cases ===

To subset the cases of a data table, use `where` statements.

{{{
data LIBREF.NEWTABLE;
  set LIBREF.OLDTABLE;
  where EXPR;
run;
}}}

Note that `where` can only be used with variables that exist before the data step.

`if` statements can be used similarly and also bypass that last restriction. The downsides of this approach are that the operation is more computationally expensive, and there isn't a clear analogy to the `where` statements on procedures.

{{{
data LIBREF.NEWTABLE;
  set LIBREF.OLDTABLE;
  NEWVAR=1;
  if NEWVAR=1;
run;
}}}

Alternatively, toggle the output data table with `if` statements.

{{{
data LIBREF.NEWTABLE1;
  set LIBREF.OLDTABLE;
  if EXPR then output LIBREF.NEWTABLE1; else output LIBREF.NEWTABLE2;
run;
}}}



=== Subset Variables ===

To subset the variables of a data table, use `keep` and `drop` statements.

{{{
data LIBREF.NEWTABLE;
  set LIBREF.OLDTABLE;
  keep VARLIST1;
  drop VARLIST2;
run;
}}}



=== De-duplication ===

This is actually best done using the `SORT` procedure. See [[SAS/Sort|here]] for details.



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