Apple iWork '09 - Manual (Page 24)

To view error and warning messages:
Click the icon.
m
A message window summarizes each error and warning condition associated with
"the cell.
To have Numbers issue a warning when a cell referenced in a formula is empty, choose
Numbers > Preferences and in the General pane select “Show warnings when formulas
reference empty cells.” This option is not available in Keynote or Pages.
Removing Formulas
If you no longer want to use a formula that’s associated with a cell, you can quickly
remove the formula.
To remove a formula from a cell:
1 Select the cell.
2 Press the Delete key.
In Numbers, if you need to review formulas in a spreadsheet before deciding what to
delete, choose View > Show Formula List.
Referring to Cells in Formulas
All tables have reference tabs. These are the row numbers and column headings. In
Numbers, the reference tabs are visible anytime the table has focus; for example, a cell
in the table is currently selected. In Keynote and Pages, reference tabs appear only when
a formula within a table cell is selected. In Numbers, the reference tabs look like this:
The reference tabs are the gray box at the top of each column or at the left of each
row containing the column letters (for example, “A”) or row numbers (for example, “3”).
The look of the reference tabs in Keynote and Pages is similar to the look in Numbers.
You use cell references to identify cells whose values you want to use in formulas.
In Numbers, the cells can be in the same table as the formula cell, or they can be in
another table on the same or a different sheet.
24 Chapter 1 Using Formulas in Tables