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This paper is a general presentation of the Xxl project and of its latest version, describing the choices that drove the design of Xxl and its main charactertistics.
Characteristics of Xxl

Xxl has most of the characteristics of a classic graphical spreadsheet program. It handles computing sheets containing information stored in cells. Each sheet is edged with scrollbars. The number of sheets is not limited except by the size of the physical memory.

Computing sheets are structured in rows and columns. Rows are denoted by numbers and columns by letters. A cell is the intersection between a row and a column and is denoted, as in Lotus 1-2-3, by a column number and a row number (e.g., A1, AB23, ZZA2345). A cell range is denoted with the first and last cell, separated by a colon (e.g. A1:A3, A1:C1, A1:B8). Note that the last example denotes a rectangle. Users can interactively and without limits increase the number of rows and columns of a sheet. Every sheet is headed with a typing area for entering the value of the current cell. Figure 1 shows an example of an Xxl spreadsheet.

The information stored in cells is made up of data (character strings, numeric values or booleans) or formulas which enable the computing of data. Integer arithmetic is of infinite precision. Formulas are mathematical expressions preceded by an equal sign. They involve arithmetic and classical relation operators, as well as a set of predefined functions. The functions deal with arithmetic (sum, prod, max, etc.), statistics (avg), logic (if, not, when), time (date, time) or character string handling (concat, len). Notice that all visible cell values are automatically computed anew.

Formulas can also contain internal or external references to cells in two modes: absolute or relative. A dollar sign in front of a row or column name denotes an absolute reference (e.g., \$B2, D\$13, \$A\$1). Absolute references do not change in move operations (addition or suppression of rows or columns, cut-and-paste operations). Without a dollar sign, the reference is a relative one (e.g., B2, D13, A1). It can be changed by a move operation. The cell C3 of Figure 1 contains the formula =A3-B3, which represents the difference between the values of the A3 and B3 cells.

The preceding references are internal ones, since they refer to cells in the same computing sheet. By contrast, external references refer to cells in other computing sheets and must be preceded by the name of the specified computing sheet. This name is that of the UNIX file containing the computing sheet.

Xxl offers several modes for displaying the cells. First, the size can be modified by enlarging or shrinking rows or columns. Second, Xxl offers several conventional modes for displaying the cell contents: several fonts in various sizes, bolding and slanting; several justification modes (left, right, centered), texts on several lines; several number representations (fixed, scientific, financial, percentage) or currencies (franc, dollar, euro).

One of the aims of Xxl is to provide a simple usage model by means of a graphical interface. All features of the spreadsheet can be accessed through a control panel, consisting of a bar with buttons, menus and a message area, all controllable by the mouse (see Figure 2). The control panel acts on the current sheet, which can be selected by the “sheets” menu or by a button in the upper left part of each sheet.

Figure 2. The control panel

The main functions to which the control panel provides access are the following:

• Selecting the current sheet: when several computing sheets are open at the same time, all commands of the control panel apply to the current sheet.

• Opening, closing and saving computing sheets: the storage format is the STk code that describes the computing sheet. Thus, loading a sheet is simply interpreting the program that describes it.

• Printing and previewing the computing sheets: these functions use LaTeX.

• Creating series: this function enables creation of a sequence of integer or textual values, with any step size.

• Cutting and pasting rows or columns

• Sorting rows or columns: this can be done by increasing or decreasing key values.

• Writing computing sheets in several formats: LaTeX, text, csv, HTML.

The mouse is also used for two important functions: selecting references and cut-and-paste operations. Entering cell references in a formula can be done automatically by selecting the needed cells with the mouse. This is especially useful for references to rows or to external references, where keyboard input is error-prone. Copying and pasting cells within a computing sheet or between two different sheets is done according to two modes: whether one wants to copy (or move) values with or without formulas. Xxl also allows cutting and pasting from a computing sheet to another X application and vice versa.

Finally, Xxl offers on-line help with dialog boxes and balloon contextual help on the buttons of the control panel. On-line help, however, is somewhat scarce, but it can be considered sufficient if the spreadsheet usage is intuitive enough.

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