For this article, I'm returning to portable science software on Android. In a previous article, I looked at a program called xcas/giac. This program is an open-source engine that is used to handle symbolic manipulation of mathematical equations. Because it is open source, it has been ported to several different platforms. Because Android's core is really Linux, a port to the Android platform has been made, and it's available on the Google Play store. Installation is as easy as a quick search on the store and clicking install.

When you first start Xcas Pad, it asks you to enable the keyboard included with the application. It takes you to the Language and Input section of the settings so you can activate the keyboard. When you finish, click the back button and go back to Xcas Pad's main screen. The main screen has four tabs along the top where you can access the main worksheet, a help screen, a plotter screen and a session pane. At the bottom of the xcas pane is an entry line where you can enter the individual xcas commands. The output from each command is displayed in the main portion of the xcas pane. As an example, you can find the derivative of the equation x³–x with the command:

diff(x^3-x)

Figure 1. The main screen is a worksheet where you can start working right away.

Once you enter the equation, you either can tap the done button on the keyboard, or if you tap the back button, you also can tap the enter button at the far right side of the entry line. You then will see a pretty-printed version of your command entry and the results line immediately below it (Figure 2).

Figure 2. The commands and results are displayed in pretty print on the main panel.

If you tap on the entered command in the display pane, it will be copied and pasted into the entry line, ready for you to edit. You also can tap on the result line to get it copied and pasted into the entry line so you can use it in the next step of your calculation. This is very useful, especially when you are doing discovery-level work.

Xcas is a very large system, however, with many different commands. Part of the problem is trying to find exactly the command you need to use. Tapping on the Help tab at the top of the screen brings up the help pages available within Xcas Pad. At the top, you can enter a search string to narrow the list a bit. You can find a description of the above example command by searching for the string "diff". Tapping the entry for "diff" pulls up a help page with a short description, a list of examples, related commands and the arguments for the command.

Figure 3. Help information is available for the commands within Xcas Pad.

At the very top of the help page is a list of command names that are aliases and equivalent to each other. Tapping on one of them enters that command name into the entry line of the main worksheet tab.

The list of examples is especially useful. You can tap on one of the examples available, and it will be copied into the entry line of the main worksheet tab. This is a good way to get a starting point for some calculation that you need to do, leaving you with just having to do some edits before you are doing useful work.

One thing to remember is that Xcas Pad is like most other symbolic mathematical programs that are used in that the commands are run sequentially. This means if you want to rerun an earlier command, it will be rerun again based on the current state of the engine. This might be different from one run to another based on what you have been doing between the two runs. For example, you may have rewritten a function that is used within the command in question.

You also can do plotting within Xcas Pad. Doing a search for "plot" in the help page will bring up a rather large number of available commands. Scrolling down to the command plot and tapping it will give you a list of simple examples for basic plots to see what the plots can look like. Entering the following example gives the plot shown in Figure 4.

plot(x-1/6*x^3,x)

Figure 4. Executing a plot command pops open the plotter tab.

Plots also are pretty-printed and look fairly nice. If you just accept the defaults, you will get bare axes with no labels. Unfortunately, there is not as much customizability available as there is with the desktop versions, so you are kind of stuck with just creating and looking at basic plots. This still can be extremely useful when you are trying to figure out what a particular equation is doing.

Let's say you've been working on some problem for the last hour and want to save your work. These worksheets are called sessions within Xcas Pad. In order to save your current worksheet, you need to tap the option button in the top right corner to get a pull-down menu. From this menu, tapping the entry Save Session brings up a new window where you can enter a filename to use.

Figure 5. The save session window allows you to enter a filename on your Android device.

There is a check box where you can select whether you also want to save off the results along with the commands from your worksheet. You can reload these saved sessions from the session tab. When you click on it, it gives you a file and directory listing for the default "home" directory on your Android. Any sessions you saved from Xcas Pad will be in the xcaspad sub-directory. You also can copy over other xcas-saved files from work you may have done on your desktop.

When you select one of these session-saved files, you only have the option of running it as a script. This is one major deficiency right now. If you want to make changes to a file before running it, you need to open the session file in a text editor first and make your edits there. Then you can open and run it within Xcas Pad. There are many very good text editors available on Android, so that shouldn't be a blocking problem.

Now you have another tool to help you get some heavy-duty science done on the go. With just your phone or tablet, you can work on your next big idea wherever you like. And if you use a file sharing service, such as Dropbox or Google Drive, you simply can pick up your work from the office wherever you have a few minutes to spare.