Assembling a Selection Set

Many of the editing commands start by displaying the Select Objects: prompt. The Select Objects: prompt signals the beginning of the process in which you can assemble a selection set by selecting the desired objects using a variety of methods. Most of the selection options covered in the following list are invoked by typing a letter or two at a Select Objects: prompt. The typical selection process is open-ended, which means that you can invoke any of the options listed, as many times as you want and in any order that you want. When an object is selected, it is highlighted on the screen as a visual confirmation. To finish selecting objects for any given command, you can press the Enter key or spacebar, or just right-click on the input device. Then you will pass the selection on to the current command that is active.

The typical selection process provides you with the following options, which are discussed in detail in the sections that follow:

• Picking the objects directly

• Using an implied window, window, and crossing window

• Selecting the last object

• Selecting all objects

• Using a fence

• Using a window and crossing polygon

• Selecting the previously selected objects

• Using the multiple option

• Undoing the last selection option

• Removing and adding objects to the selection

• Using object cycling

Picking Objects Directly

When you’re prompted to select objects, the normal pointer is replaced with a box cursor, which is referred to as the “pickbox.” To select an object directly, position the pickbox over the object and select it. The size of the pickbox is controlled through the Selection tab in the Options dialog box (see Figure 10.1).

Using Implied Windowing,Window, and Crossing Window

The basic method for selecting large groups of objects is to create a rectangular window around the objects. The window boundary method selects only items that are totally enclosed within the window. In the case of the crossing window method, objects enclosed or crossed by the boundary window itself are selected. Initializing a type of window is best done with implied windowing.

Implied Windowing

When you position the pickbox over an empty portion of the drawing and pick a point, the system assumes you want to place the first corner of a rectangular selection window at that point. You determine its size by moving the cursor to the opposite corner and then picking a second point (the opposing diagonal corner point relative to the initial point). When the window is defined from left to right, all objects completely enclosed in the window are selected. On the left of Figure 10.2, using the window method would select just the circle and vertical line.

When the window is defined right to left, it is referred to as a “crossing window.” In a crossing window, all objects that are completely enclosed in the window or that merely cross the boundaries of the crossing window are selected. A crossing window is drawn with a dotted line, whereas a window is drawn with a continuous line. As you can see on the right side of Figure 10.2, using a crossing window selects the circle and all the lines. If you disable the Implied Windowing setting in the Selection tab of the Options dialog box, this feature is disabled.

Explicitly Using the Window or Crossing Window Method

Window type selection methods include the typical window and crossing window. They are accessed by typing W or C at the Select Objects: prompt. When you use the Window option, you explicitly define a window with which to select objects. Unlike an implied window, the first point selected does not have to be located in an empty portion of the drawing because you establish it via point selection. Furthermore, it does not matter whether the window is defined from left to right or right to left. Using an explicit window or crossing method is superior to using an implied window when you deal with a crowded drawing and encounter difficulties finding an empty area of the drawing in which to anchor the first point of the implied window.

Selection Modes Available for Selecting Objects

To display the Selection tab in the Options dialog box, choose Options from the Tools menu and click the Selection tab. In this dialog box, you find the settings for controlling various aspects of the selection process (see Figure 10.3). The following sections discuss these controls in more detail.

Understanding Noun/Verb Selection

The first option in the Object Selection Settings dialog box is Noun/Verb Selection, which is enabled by default. With this setting enabled, you have the option to select the objects to be manipulated prior to invoking a command. When the Noun/Verb option is disabled, you must invoke a command first and then select the objects to be modified. This option affects only those commands that begin with the Select Objects: prompt (such as the ERASE and LIST commands).

Most of the controls from Release 14’s Object Selection Settings dialog box have been incorporated into the Options dialog box’s Selection tab.

Replacing and Adding Selected Objects with the Use Shift to Add Option

The Use Shift to Add selection mode is disabled by default. Therefore, when you’re selecting additional objects, the objects are automatically added to the current selection set. If you enable the Use Shift to Add option, any selection of objects during the same edit routine that occurs after the initial object(s) have been selected replaces those objects rather than adding to them. To add more objects to the current selection set, you must hold down the Shift key as you select the additional objects.

Using Press and Drag

By default, the Press and Drag option on the Selection tab of the Options dialog box is disabled. Consequently, when you define a rectangular window (whether it is an implied window, window, or crossing window), you do so by picking the first corner of the window and then picking another point as the location of the opposing corner point. When Press and Drag is enabled, you first pick the initial corner point and then hold and press the pick button as you drag to the opposing corner of the rectangular selection window. You establish the location of the second corner point by releasing the pick button. Enabling this option makes the process of selecting objects with a rectangular window in AutoCAD similar to the process of using a window to select objects in Windows operating systems.

Other Useful Selection Options

In some cases a typical selection method will not suffice. Fortunately, AutoCAD provides several more methods of selecting objects. The following list describes these secondary selection methods. For each one, the character in parentheses can be entered to invoke the respective option.

• Last (L). The Last option automatically selects the last object drawn that is visible in the display. This option can select an unexpected object, so you should use it only when you are working in a contained area.

• ALL (ALL). The All option selects all objects not residing on a locked or frozen layer. This option selects objects even if they are not visible in the current view.

• Fence (F). This option enables you to define a series of crossing line segments, which is referred to as a “fence.” All objects the fence intersects or crosses are selected. The fence path is drawn as dotted lines (as shown in Figure 10.4). In Figure 10.4, only the lines and not the circle would be selected.

  Figure 10.4. Using Fence to quickly select objects in a confined area to define a path.

  

• Wpolygon (WP). The Wpolygon (Window Polygon) option is similar to the Window option, except that you can define an irregular polygon-shaped window as opposed to a rectangular window. Figure 10.5 illustrates using the Window Polygon option to select just the circle and a single line. You can define the window polygon with as many points as necessary. The Wpolygon option automatically draws the closing segment back to the beginning point.

  Figure 10.5. The Window Polygon option allows for precise selection.

  

• Cpolygon (CP). The Cpolygon (Crossing Polygon) option is similar to the Crossing option, except that you define an irregular polygon-shaped crossing window instead of a rectangular window. Figure 10.6 illustrates using the Cpolygon option to select the circle and two of the three lines.

  Figure 10.6. The Crossing Polygon option allows crossing selection by a polygonal area.

  

• Previous (P). Use this option to reselect the objects in the previous selection set, which was created during the most recent editing command.

• Group (G). The Group option enables you to specify the name of a predefined group. Groups are discussed in Chapter 11.

• Undo (U). When you enter U at the Select Objects: prompt, AutoCAD undoes the last selection option performed. Choosing the Undo tool cancels the current command when it’s chosen at the Select Objects: prompt.

In addition to the options listed previously, you can use several other options, including BOX, Auto, and Single. Programmers commonly use these options for AutoLISP and ARX-based programs or menu and toolbar macros.

The following exercise illustrates some alternative selection methods available in AutoCAD 2002.

Exercise 10.1 Using a Variety of Selection Options to Edit Objects

1.	Open the drawing CHAP10-1.dwg from the accompanying CD-ROM.
2.	Choose Named Views from the View pull-down menu. Then select the view named OFFICE-D, click the Set Current button in the View Control dialog box, then click OK (see Figure 10.8). Figure 10.8. The top right area of the drawing used for the select objects exercise.
3.	Use the MOVE command to relocate items in the room. Start the MOVE command and type ALL to select everything in the drawing. Notice that all objects in the drawing are now highlighted. Although this is a useful selection method, for our purposes we don’t want to move everything in the drawing.
4.	To undo the previous selection, type U and press Enter.
5.	Now use the Crossing Polygon option to select all the chair and desk objects in the center of the room. Type CP at the Select objects: prompt and pick the approximate centers of the four chairs. Select the points in a circular pattern at , , , and , but do not cross over the center with the polygonal selection box.
6.	You should now have a highlighted group of objects including the four chairs and the desks in the center. Let’s explore another method to select the chairs. Press the Esc key once to cancel the command.
7.	Press Enter to start the MOVE command again. At the Select objects: prompt, type F to use the Fence selection method. Select through again.
8.	Now you have the chairs and a similar selection of desk objects. However, we don’t actually need to move anything, so press Esc to cancel the command. As you can see, there are many methods of selecting objects for editing. Feel free to explore the other methods such as Previous, Last, and Window Polygon. You can leave this drawing open for the following exercise.

Refining the Selection Set

It is often impossible to get your initial selection set to include exactly the objects needed. To better control the object selection set, you can use a few tools AutoCAD offers for refining your selection.

By default, when you’re selecting objects, you work in the Add mode so that any objects selected are automatically added to the current selection set. How to remove objects from a selection set, the use of the Shift key, and object cycling are discussed in this section.

Removing Objects from the Selection Set

The Remove option switches you from the default Add mode to the Remove mode. In this mode, all objects selected are removed from the selection set. Undo is often used to remove the objects just selected from the selection set, but the Remove option enables you to select the specific objects you want to remove from the selection set. You remain in the Remove mode until you either end the selection process (by pressing the Enter key or the spacebar) or invoke the Add option by pressing A.

Holding Down the Shift Key

Instead of using the Remove option to deselect objects, you can hold down the Shift key and click on objects already in the selection set (already highlighted). This removes those objects from the current selection set. You can hold down the Shift key in combination with any of the aforementioned methods of selecting objects. When you release the Shift key, you are immediately placed back into the current mode.

Using Object Cycling

In a dense drawing, it can be difficult to select an object directly without inadvertently picking another nearby object. In such cases, you can zoom in on the area, making it easier to select the required object. You also can use object cycling to speed this process.

To use object cycling, start an edit command such as MOVE and position the pickbox over a dense area in the drawing so your cursor is on or touching more than one object. While holding down the Ctrl key, pick that location, then release the Ctrl key. This starts object selection cycling, so only a single object occupying the area of the pickbox is highlighted. If more than one object was found in the area of the pickbox, <Cycle on> appears at the prompt line. The next time you left-click (without holding down the Ctrl key), another object occupying the area of the pickbox is highlighted. If the highlighted object is not the required object, you can pick again anywhere in the drawing area, and the next object found in the area of the original pick location is highlighted. Every time you left-click, the next object found in the area of the original pick location is highlighted, cycling among all objects in that original pickbox area. When the desired object is highlighted, you can end the cycling by pressing the Enter key or the spacebar, or by right-clicking.