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### 3.2 Row Order

You can sort the rows of each table according to the values in the table. Normally you will want to sort on a numeric column, but other values may be sortable too, for instance a String column will sort alphabetically. Some kinds of columns (e.g. array ones) don't have any well-defined order, and it is not possible to select these for sorting on.

At any time, each table has a current row order, and this affects the Apparent Table. You can always see what it is by looking under the Sort Order item in the Control Window when that table is selected; by default it is blank, which means the rows have the same order as that of the table they were loaded in from. The little arrow (/) indicates whether the sense of the sort is up or down. You can change the sort order by selecting a column name or entering an algebraic expression in this selector, and change the sense by clicking on the arrow. To sort on multiple columns, use the Add/Remove Selector () buttons to change the number of selectors; selectors to the left are more significant, and ones to the right are used in case of a tie in earlier values. The sort order can also be changed by using menu items in the Columns Window or right-clicking popup menus in the Data Window.

Selecting values to sort by calculates the new row order by performing a sort on the cell values there and then. If the table data change somehow (e.g. because you edit cells in the table) then it is possible for the sort order to become out of date. If that happens you can resort by making a new selection and then changing it back again.

The current row order affects the Apparent Table, and hence determines the order of rows in tables which are exported in any way (e.g. written out) from TOPCAT. You can always see the rows in their currently sorted order in the Data Window.

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TOPCAT - Tool for OPerations on Catalogues And Tables
Starlink User Note253
TOPCAT web page: http://www.starlink.ac.uk/topcat/
Author email: m.b.taylor@bristol.ac.uk
Mailing list: topcat-user@jiscmail.ac.uk