\chapter{Gretl data types}
\label{chap:datatypes}

\section{Introduction}

\app{Gretl} offers the following data types:
%
\begin{center}
\begin{tabular}{lp{.8\textwidth}}
\texttt{scalar} & holds a single numerical value\\
\texttt{series} & holds $n$ numerical values, where $n$
 is the number of observations in the current dataset \\
\texttt{matrix} & holds a rectangular array of numerical
 values, of any dimensions\\
\texttt{list} & holds the ID numbers of a set of series \\
\texttt{string} & holds an array of characters\\
\texttt{bundle} & holds a variable number of objects of 
 various types
\end{tabular}
\end{center}

The ``numerical values'' mentioned above are all double-precision
floating point numbers.

In this chapter we give a run-down of the basic characteristics of
each of these types and also explain their ``life cycle'' (creation,
modification and destruction). The list and matrix types, whose uses
are relatively complex, are discussed at greater length in the
following two chapters. 

\section{Series}
\label{sec:Series}

We begin with the \texttt{series} type, which is the oldest and in a
sense the most basic type in gretl. When you open a data file in the
gretl GUI, what you see in the main window are the ID numbers, names
(and descriptions, if available) of the series read from the file. All
the series existing at any point in a gretl session are of the same
length, although some may have missing values. The variables that can
be added via the items under the \textsf{Add} menu in the main window
(logs, squares and so on) are also series.

For a gretl session to contain any series, a common series length must
be established. This is usually achieved by opening a data file, or
importing a series from a database, in which case the length is set by
the first import. But one can also use the \texttt{nulldata} command,
which takes as it single argument the desired length, a positive
integer.

Each series has these basic attributes: an ID number, a name, and of
course $n$ numerical values. In addition a series may have a
description (which is shown in the main window and is also accessible
via the \texttt{labels} command), a ``display name'' for use in
graphs, a record of the compaction method used in reducing the
variable's frequency (for time-series data only) and a flag marking
the variable as discrete. These attributes can be edited in the GUI by
choosing \textsf{Edit Attributes} (either under the \textsf{Variable}
menu or via right-click), or by means of the \texttt{setinfo} command.

In the context of most commands you are able to reference series by
name or by ID number as you wish. The main exception is the definition
or modification of variables via a formula; here you must use names
since ID numbers would get confused with numerical constants.

Note that series ID numbers are always consecutive, and the ID number
for a given series will change if you delete a lower-numbered series.
In some contexts, where gretl is liable to get confused by such
changes, deletion of low-numbered series is disallowed.  

\section{Scalars}
\label{sec:Scalars}

The scalar type is relatively simple: just a convenient named holder
for a single numerical value. Scalars have none of the additional
attributes pertaining to series, do not have public ID numbers, and
must be referenced by name. A common use of scalar variables is to
record information made available by gretl commands for further
processing, as in \texttt{scalar s2 = \$sigma\^{}2} to record the square of
the standard error of the regression following an estimation command
such as \texttt{ols}.

You can define and work with scalars in gretl without having any
dataset in place.

In the gretl GUI, scalar variables can be inspected and their values
edited via the \texttt{Scalars} item under the \texttt{View} menu in
the main window.

\section{Matrices}
\label{sec:Matrices}

Matrices in gretl work much as in other mathematical software (e.g.\
\textsf{MATLAB}, \textsf{Octave}). Like scalars they have no public ID
numbers and must be referenced by name, and they can be used without
any dataset in place. Matrix indexing is 1-based: the top-left element
of matrix \texttt{A} is \texttt{A[1,1]}.  Matrices are discussed at
length in chapter~\ref{chap:matrices}; advanced users of gretl will
want to study this chapter in detail.

Matrices have one optional attribute beyond their numerical content:
they may have column names attached, which are displayed when the
matrix is printed. See the \texttt{colnames} function for details.

In the gretl GUI, matrices can be inspected, analysed and edited via
the \texttt{Icon view} item under the \texttt{View} menu in the main
window: each currently defined matrix is represented by an icon.

\section{Lists}
\label{sec:Lists}

As with matrices, lists merit an explication of their own (see
chapter~\ref{chap-persist}).  Briefly, named lists can (and should!)
be used to make commands scripts less verbose and repetitious, and
more easily modifiable. Since lists are in fact lists of series ID
numbers they can be used only when a dataset is in place.

In the gretl GUI, named lists can be inspected and edited under the
\textsf{Data} menu in the main window, via the item \textsf{Define or
  edit list}.

\section{Strings}
\label{sec:Strings}

String variables may be used for labeling, or for constructing
commands. They are discussed in chapter~\ref{chap-persist}. They must
be referenced by name; they can be defined in the absence of a
dataset.

Such variables can be created and modified via the command-line in
the gretl console or via script; there is no means of editing them
via the gretl GUI.


\section{Bundles}
\label{sec:Bundles}

A \texttt{bundle} is a container or wrapper for various sorts of
objects --- specifically, scalars, series, matrices, strings and
bundles. (Yes, a bundle can contain other bundles). A bundle takes
the form of a hash table or associative array: each item placed in the
bundle is associated with a key string which can used to retrieve it
subsequently. We begin by explaining the mechanics of bundles then
offer some thoughts on what they are good for.

To use a bundle you must first ``declare'' it, as in

\begin{code}
bundle foo
\end{code}

To add an object to a bundle you assign to a compound left-hand value:
the name of the bundle followed by the key string in square
brackets. For example, the statement

\begin{code}
foo["matrix1"] = m
\end{code}

adds an object called \texttt{m} (presumably a matrix) to bundle
\texttt{foo} under the key \texttt{matrix1}. To get an item out of a
bundle, again use the name of the bundle followed by the bracketed
key, as in

\begin{code}
matrix bm = foo["matrix1"]
\end{code}

A bundle key may be given as a double-quoted string literal, as shown
above, or as the name of a pre-defined string variable. Key strings
have a maximum length of 15 characters and cannot contain spaces.

Note that the key identifying an object within a given bundle is
necessarily unique. If you reuse an existing key in a new assignment,
the effect is to replace the object which was previously stored under
the given key. It is not required that the type of the replacement
object is the same as that of the original.

Also note that when you add an object to a bundle, what in fact
happens is that the bundle acquires a copy of the object. The external
object retains its own identity and is unaffected if the bundled
object is replaced by another. Consider the following script fragment:

\begin{code}
bundle foo
matrix m = I(3)
foo["mykey"] = m
scalar x = 20
foo["mykey"] = x
\end{code}

After the above commands are completed bundle \texttt{foo} does not
contain a matrix under \texttt{mykey}, but the original matrix
\texttt{m} is still in good health.

To delete an object from a bundle use the \texttt{delete} command, as
in

\begin{code}
delete foo["mykey"]
\end{code}

This destroys the object associated with the key and removes the key
from the hash table.

Besides adding, accessing, replacing and deleting individual items,
the other operations that are supported for bundles are union,
printing and deletion. As regards union, if bundles \texttt{b1} and
\texttt{b2} are defined you can say

\begin{code}
bundle b3 = b1 + b2
\end{code}

to create a new bundle that is the union of the two others. The
algorithm is: create a new bundle that is a copy of \texttt{b1}, then
add any items from \texttt{b2} whose keys are not already present in
the new bundle. (This means that bundle union is not commutative if
the bundles have one or more key strings in common.)

If \texttt{b} is a bundle and you say \texttt{print b}, you get a
listing of the bundle's keys along with the types of the corresponding
objects, as in

\begin{code}
? print b
bundle b:
 x (scalar)
 mat (matrix)
 inside (bundle)
\end{code}

\subsection{What are bundles good for?}

Bundles are unlikely to be of interest in the context of standalone
\app{gretl} scripts, but they can be very useful in the context of
complex function packages where a good deal of information has to be
passed around between the component functions. Instead of using a
lengthy list of individual arguments, function $A$ can bundle up the
required data and pass it to functions $B$ and $C$, where relevant
information can be extracted via a mnemonic key.

In this context bundles should be passed in ``pointer'' form
(see chapter~\ref{chap:functions}) as illustrated in the following
trivial example, where a bundle is created at one level then filled
out by a separate function.

\begin{code}
# modification of bundle (pointer) by user function

function void fill_out_bundle (bundle *b)
  b["mat"] =  I(3)
  b["str"] = "foo"
  b["x"] = 32
end function

bundle my_bundle 
fill_out_bundle(&my_bundle)
\end{code}

The bundle type can also be used to advantage as the \textit{return
  value} from a packaged function, in cases where a package writer
wants to give the user the option of accessing various results. In the
\app{gretl} GUI, function packages that return a bundle are treated
specially: the output window that displays the printed results
acquires a menu showing the bundled items (their names and types),
from which the user can save items of interest. For example, a
function package that estimates a model might return a bundle 
containing a vector of parameter estimates, a residual series and a
covariance matrix for the parameter estimates, among other
possibilities.

As a refinement to support the use of bundles as a function return
type, the \texttt{setnote} function can be used to add a brief
explanatory note to a bundled item --- such notes will then be shown
in the GUI menu.  This function takes three arguments: the name of a
bundle, a key string, and the note. For example

\begin{code}
setnote(b, "vcv", "covariance matrix")
\end{code}

After this, the object under the key \texttt{vcv} in bundle \texttt{b}
will be shown as ``covariance matrix'' in a GUI menu.

\section{The life cycle of gretl objects}

\subsection{Creation}

The most basic way to create a new variable of any type is by
\textit{declaration}, where one states the type followed by the name
of the variable to create, as in

\begin{code}
scalar x
series y
matrix A
\end{code}

and so forth. In that case the object in question is given a default
initialization, as follows: a new scalar has value \texttt{NA}
(missing); a new series is filled with zeros; a new matrix is null
(zero rows and columns); a new string is empty; a new list has no
members, and a new bundle is empty.

Declaration can be supplemented by a definite initialization, as in

\begin{code}
scalar x = pi
series y = log(x)
matrix A = zeros(10,4)
\end{code}

With the exception of bundles (as noted above), new variables in
\app{gretl} do not \textit{have} to be declared by type.  The
traditional way of creating a new variable in \app{gretl} was via the
\app{genr} command (which is still supported), as in

\begin{code}
genr x = y/100
\end{code}

Here the type of \texttt{x} is left implicit and will be determined
automatically depending on the context: if \texttt{y} is a scalar, a
series or a matrix \texttt{x} will inherit \texttt{y}'s type
(otherwise an error will be generated, since division is applicable to
these types only). Moreover, the type of a new variable can be left
implicit \textit{without} use of \texttt{genr}:

\begin{code}
x = y/100
\end{code}

In ``modern'' \app{gretl} scripting we recommend that you state the
type of a new variable explicitly. This makes the intent clearer to a
reader of the script and also guards against errors that might
otherwise be difficult to understand (i.e.\ a certain variable turns
out to be of the wrong type for some subsequent calculation, but you
don't notice at first because you didn't say what type you needed). An
exception to this rule might reasonably be granted for clear and
simple cases where there's little possibility of confusion.

\subsection{Modification}

Typically, the values of variables of all types are modified
by assignment, using the \texttt{=} operator with the name of the
variable on the left and a suitable value or formula on the right:

\begin{code}
z = normal()
x = 100 * log(y) - log(y(-1))
M = qform(a, X)
\end{code}

By a ``suitable'' value we mean one that is conformable for the type
in question. A \app{gretl} variable acquires its type when it is first
created and this cannot be changed via assignment; for example, if you
have a matrix \texttt{A} and later want a string \texttt{A}, you will
have to delete the matrix first.

\tip{One point to watch out for in \app{gretl} scripting is type
  conflicts having to do with the names of series brought in from a
  data file. For example, in setting up a command loop (see
  chapter~\ref{chap:looping}) it is very common to call the loop index
  \texttt{i}. Now a loop index is a scalar (typically incremented each
  time round the loop). If you open a data file that happens to
  contain a series named \texttt{i} you will get a type error (``Types
  not conformable for operation'') when you try to use \texttt{i} as a
  loop index.}

Although the type of an existing variable cannot be changed on the
fly, \app{gretl} nonetheless tries to be as ``understanding'' as
possible. For example if \texttt{x} is a series and you say

\begin{code}
x = 100
\end{code} 

\app{gretl} will give the series a constant value of 100 rather than
complaining that you are trying to assign a scalar to a series. This
issue is particularly relevant for the matrix type --- see
chapter~\ref{chap:matrices} for details.

Besides using the regular assignment operator you also have the option
of using an ``inflected'' equals sign, as in the C programming
language. This is shorthand for the case where the new value of the
variable is a function of the old value. For example,

\begin{code}
x += 100 # in longhand: x = x + 100
x *= 100 # in longhand: x = x * 100
\end{code} 

For scalar variables you can use a more condensed shorthand for simple
increment or decrement by 1, namely trailing \texttt{++} or \verb|--|
respectively:

\begin{code}
x = 100
x--     # x now equals 99
x++     # x now equals 100
\end{code}

In the case of objects holding more than one value --- series,
matrices and bundles --- you can modify particular values within the
object using an expression within square brackets to identify the
elements to access. We have discussed this above for the bundle type
and chapter~\ref{chap:matrices} goes into details for matrices. As for
series, there are two ways to specify particular values for
modification: you can use a simple 1-based index, or if the dataset is
a time series or panel (or if it has marker strings that identify the
observations) you can use an appropriate observation string. Such
strings are displayed by \app{gretl} when you print data with the 
\verb|--byobs| flag. Examples:

\begin{code}
x[13]      = 100  # simple index: the 13th observation
x[1995:4]  = 100  # date: quarterly time series
x[2003:08] = 100  # date: monthly time series
x["AZ"]    = 100  # the observation with marker string "AZ"
x[3:15]    = 100  # panel: the 15th observation for the 3rd unit
\end{code}

Note that with quarterly or monthly time series there is no ambiguity
between a simple index number and a date, since dates always contain a
colon. With annual time-series data, however, such ambiguity exists
and it is resolved by the rule that a number in brackets is always
read as a simple index: \texttt{x[1905]} means the nineteen-hundred
and fifth observation, \textit{not} the observation for the year 1905.
You can specify a year by quotation, as in \verb|x["1905"]|.

\subsection{Destruction}

Objects of the types discussed above, \textit{with the important
  exception of named lists}, are all destroyed using the
\texttt{delete} command: \texttt{delete} \textsl{objectname}.

Lists are an exception for this reason: in the context of gretl
commands, a named list expands to the ID numbers of the member series,
so if you say

\begin{code}
delete L
\end{code} 

for \texttt{L} a list, the effect is to delete all the series in
\texttt{L}; the list itself is not destroyed, but ends up empty.  To
delete the list itself (without deleting the member series) you must
invert the command and use the \texttt{list} keyword:

\begin{code}
list L delete
\end{code} 
