780.20: 1094 Session 1

Handouts: Background notes for Session 1; .cpp printouts; Recommended C++ options; GSL intro

Your goals for today (note: the goals are always over-ambitious!):

• Get used to the wxDev-C++ environment (even if you plan to use Linux later)
• Download, unzip, compile and run some simple C++ programs, then modify and repeat
• Understand how underflow, overflow, and machine precision limits can be determined "experimentally"
• Set up Dev-C++ compiler warnings and library
• Try out a program that uses a function from the Gnu Scientific Library (GSL)
• Use MATLAB or Mathematica to verify a result

Please work in pairs with comparable computing experience. The instructors will bounce around 1094 and answer questions (don't be shy to ask about anything!). Hand in this sheet at the end of class.

Dev-C++

The first step is to log on, download and unzip some C++ "source code", and then start up Dev-C++.

1. Log on with your Physics Department Windows account. (If you don't know your username and/or password, please let one of the instructors know.)
2. We recommend that you use a shared disk set up for Physics 780.20. To do so, click on the Start menu and right-click on My Computer. Select "Map Network Drive" and type \\phypcs\physics 780 for the Folder. Select a drive letter (Z: is a good choice). Now you can create a working directory ("folder") on this drive with your name (or make up a name for your group). Be sure to copy the folder to your personal space(s) (on the U: disk) at the end.
3. Start up a web browser ("Firefox" is recommended but you can use "Internet Explorer" as well). Go to the 780.20 home page (http://www.physics.ohio-state.edu/~ntg/780/) and find the "1094 Sessions" section. Download session01.zip and unzip it into your 780 folder (double clicking on session01.zip in an Explorer window, then "Extract all files" is one way to do this) or just open the .zip file with Winzip.
4. Start up Dev-C++ by looking under Start->All Programs->wx-devcpp. A Dev-C++ window should open up. Answer "ok" to all of the questions (set up may take a couple of minutes). Now we're ready to roll.
5. Load the program area.cpp by one of these three methods:
   1. From the menus: File->Open Project or File
   2. Shortcut: Ctrl+O (hold down the "Ctrl" and "O" keys together)
   3. Icons: Click the 2nd icon on the 1st row of icons (point at it with your mouse to get a momentary title)
   There are (at least) three ways to do anything in Dev-C++, analogous to these. Use the window that pops up to track down the area.cpp file and press "open".
6. The program should have appeared in the big window. Questions about the code? (If you are new to C++, you're not expected to understand everything; see the discussion in the background notes for some details.) Why use "area" and "radius" for variables rather than "A" and "R", which are quicker to type?
   
   
   
   
7. Let's compile, link, and run it. Use the 4th icon on the 2nd line ("Compile & Run") or F9 or Execute->Compile & Run. You should get a little status box that says it is compiling and then a pop-up window. Enter a radius and it disappears before you can do anything! To make the window stay open, add the line system("PAUSE"); before the return 0; line. Then try again.
8. Modify the program to create some errors and see what Dev-C++ does to get used to error messages. Some possibilities (try these then invent your own):
   1. remove a semi-colon
   2. comment out using namespace std; (using //)
   3. remove a } (or change it from "}" to ")" or "]")
   Each time, "compile and run" again. If there is an error, a section at the bottom of the Dev-C++ window should pop up and give an error message with the line number it thinks is a problem and an explanation (which is often not helpful!). Fix the error and make sure the program still works. Note that "undeclared" errors are reported as being at lines that are incorrect. Why?
9. List two ways to verify that the program is running correctly (i.e., giving correct answers):
   
   
   
   Later (not in class today!) you may be asked to carry out the "to do" list in the comments (with assistance, as needed). Which do you know how to do now?
   
   
   
   
10. Copy, rename, and modify area.cpp to make a program volume.cpp, which calculates the volume of a sphere. Test it. [Warning: Most people get this wrong the first time. Test carefully!]
11. Start up MATLAB. For the experts: Write a program ("script") to duplicate what area.cpp does.
    For everyone else: Load area.m, look at it in the editor, and run it. Think about the comparison with the C++ version; which is better to use? (There is also a Mathematica area.nb notebook to look at.)

Overflows, Underflows, and Machine Precision

Refer to the background notes on number representation as you do this section.

1. For this section, we need to turn off the default Dev-C++ optimization. Go to Tools->Compiler Options->Settings->Optimization->Further optimizations and set "Best Optimization" to "No".
2. If you're know C++ already and you're really fast, you can try writing your own codes. Otherwise, use "flows.cpp". (Look at the code before running it! Where does the output go? You can use ConTEXT (recommended) or Notepad to look at it.) A single-precision number in C++ is a "float" and a double-precision number in C++ is a "double". You are to empirically determine (put your answers in the blank spaces)
   1. where under- and overflow occur for single-precision floating-point numbers;
      
      
      
      
   2. where under- and overflow occur for double-precision floating-point numbers.
      
      
      
      
3. Now for the machine precision. Once again, code your own version or use "precision.cpp". [Note: This code has an intentional bug. Compare to flows.cpp to track it down.] Determine empirically
   1. the machine precision for single-precision floating-point numbers;
      
      
      
      
   2. the machine precision for double-precision floating-point numbers. [Note: you have to change the code for this to work correctly.]
      
      
      
      
   Explain in a couple of sentences how the code finds the machine precision (note that there is an output file):
   
   
   

Setting g++ Compiler Options

See the handout with the list of recommended C++ options. We'll set some using the Dev-C++ menus.

1. In Dev-C++, go to Tools->Environment Options->Files & Directories->User's Default Directories. Change this to your 780 directory (use icon at right).
2. Go to Tools->Compiler Options->Compiler. [Also look under the Programs tab.] Add the following to "calling compiler":
   -Werror -Wall -W -Wshadow -fno-common
   Add the following to "linker" (for using GSL):
   -lgsl -lgslcblas

Using the GSL Scientific Library

A link to the GSL documentation can be found on the 780.20 web page. Here we look at our first example program, which has been adapted from the documentation.

1. We'll first create a Dev-C++ "Project", which is a collection of files and instructions associated with a program. Create a New Project (first icon on second row or File->New->Project). Select "Console Application", "C++ Project", name it "J0_test", and save it in your 780/session_01 folder. The project files will be shown in the window at the upper left. Note that main.cpp was created.
2. Load the real main file J0_test.cpp" by Project->Add to Project. Then remove main.cpp by Project->Remove from Project (don't save the changes).
3. This test program calculates the cylindrical Bessel function J₀(x) at x=5 using the GSL library function "gsl_sf_bessel_J0" (you would find out the name of the function by looking at the web page with the GSL reference manual). Look up "Bessel function Wikipedia" using Google. How can you use this to check you are calculating the right thing?
   
   
4. Compile and run J0_test.cpp and verify that the correct answer (given in the file) is printed. Look at the "Compile Log" tab at the bottom. This is what you would have typed to compile and link!
5. Now calculate for x=3 (modify the code for this value or add code to read in any x). Answer:
   
   
6. Verify your answer using MATLAB or Mathematica. If MATLAB, under the Help menu, select "MATLAB Help", choose "Search Results", and search for "Bessel Functions". If Mathematica, under the Help menu, start the Help Browser, choose "Master Index", and look under Bessel function).
   Did you succeed?
   
   

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