The settings affecting the current configuration can be viewed and modified by pressing the gears button (leftmost button in the bottom tool bar). This results in a pop-over containing the settings for the current configuration. Note that you may need to scroll down to see all of the settings (especially if the iPad has been rotated to landscape orientation).
To see other stored configurations, swipe your finger left or right across the field. Note that MCMC Robot is installed with only one configuration (named “MCMC Robot”), so swiping will have no effect unless you define other configurations.
To define a new configuration, press the Info button, then press Configurations to see the list of existing configurations. Pressing the + button at the bottom of the Configurations screen creates a new configuration named “Untitled” which can then be modified.
Below is a key to the settings available:
- The name of the configuration (this is also displayed at the top center part of the top navigation bar).
- The number of steps taken by the robot (or robots) each time the play button is pressed.
- The number of chains (i.e. robots) that are wandering around at the same time. If greater than 1, then the first chain will be the cold chain and the remaining chains will be heated (i.e. they will explore a landscape that is gentler than the cold chain). The heating is done by raising the target distribution (hill heights) to a power less than 1.0 but greater than 0.0.
- Determines which chain (i.e. robot) to show. All shows the path taken by all active robots, but an individual chain can be shown by clicking 1, 2, 3 or 4. The chains that can be shown depend on the value of the chains setting (above).
- Dot radius
- This determines the size of the dots used to show where a robot was standing. The minimum dot size is 1, and the maximum is 5.
- Ignore topography
- If ON, any hills that are defined are ignored and the robot wanders randomly as if the landscape was devoid of topography. The app is designed so that the robot never leaves the visible field. If it encounters the edge of the field, it bounces off like a billiard ball back into the field. Ordinarily, if there are hills defined, the robot rarely encounters the edge of the field. If OFF, the robot behaves normally and does not ignore hills that have been defined.
- Show trajectory
- If OFF, only the positions where the robot was standing are shown. If ON, the places where the robot stopped are connected by lines.
- Show guides
- If ON, dotted lines are shown illustrating soft boundaries useful when defining new hills. If OFF, the dotted guide lines are hidden. These guides are useful only if you are defining landscapes that should be entirely visible when the iPad is rotated to other orientations. The fact that the status bar, navigation bar and tool bar change positions to accommodate different orientations makes it tricky to define hills that will not be chopped off after rotation.
- Show fails
- It is sometimes useful to see the proposed steps that were rejected. Ordinarily, when this setting is set to OFF, only successful proposals are shown. If this setting is ON, however, failed steps are shown in a different color. This can be useful in demonstrating the inefficiency resulting from setting a step size that is too large.
- Tapping this entry opens a new view that allows you to change the color of various aspects of the current configuration (e.g. background color, contour color, color of each chain, failed step color, etc.)
- Starting X
- The robot will begin walking from a point (x,y), and this setting determines the x-coordinate of this starting position. The units are in pixels, and the range is set so that the slider only chooses valid values.
- Starting Y
- The robot will begin walking from a point (x,y), and this setting determines the y-coordinate of this starting position. The units are in pixels, and the range is set so that the slider only chooses valid values.
- Mean step length
- This is the average step length in pixels. This setting determines the mean of a gamma distribution from which step sizes are drawn randomly.
- Std. dev. step length
- This is the standard deviation of the step length in pixels. This setting determines the standard deviation of a gamma distribution from which step sizes are drawn randomly.
- Random seed
- This is the seed used to initialize the pseudorandom number generator used for drawing all random deviates. If set to 0, a default value will be chosen. Setting this to a positive whole number will cause the random number sequence to be repeatable (that is, if you later set the seed to the same value, the robot will take exactly the same trajectory through the landscape).
- Number of hills
- This is simply a count of the number of hills currently defined for this configuration. There is no way to change the value, it is informational only. To change the number of hills, see the other steps in this tutorial concerned with adding and deleting hills.
- If ON, a beep will be emitted each time the robot goes on a foray. If OFF, no sounds will be emitted by the MCMC Robot app. The sounds is useful if you want others to be able to hear a beep every time the play button is pressed, and thus has some use when doing presentations.