780.20: 1094 Session 1
Handouts: 780 webpage and gameplan;
area.cpp/make_area and other .cpp/.py printouts; Recommended C++ options;
"Useful Unix Commands"; GSL intro
Your goals for today (note: the goals are always over-ambitious!):
- Get used to the GNU/Linux environment (using Linux directly
or via Cygwin on Windows)
- Download, unzip, compile and run some simple C++ programs,
then modify and repeat
- Try out Python
- Understand how underflow, overflow, and machine precision
limits can be determined "experimentally"
- Try out a makefile: using g++ compiler warnings
- Try out a program that uses a function from
the Gnu Scientific Library (GSL)
Please work in pairs with comparable computing and physics experience.
The instructors will bounce around 1094 and answer questions (don't be
shy to ask about anything!).
Fill out and hand in this sheet at the end of class.
GNU-Unix Environment with Cygwin (If you want to use Windows)
- Log on to your Physics Department Windows account.
(If you don't know your username and/or password, please let one of the
- 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).
- Start up a web browser. 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.
(You will also be able to unzip it from the Cygwin command line.)
- Double-click on the Cygwin icon (or Start -> All Programs -> Cygwin
-> Cygwin Bash Shell) to start a "Bash shell" window. Ignore
any error messages. At the prompt ($ or #, it may take up to a
minute the first time), type startxwin.exe
to start the X-windows system. Again, ignore messages and hit OK
if you get a pop-up warning. You should get a white-background
"xterm" window, which scrolls using the mouse wheel and can be
resized as desired. You can start additional xterm windows
by typing the command xterm & at the prompt.
- Take a look at the unix command summary handout.
Type pwd ("present working directory") to see where
you are. Change to the 780 directory with (if your working folder
is on the Z drive)
and then use cd to go to your working directory
you created above and then the session_01 subdirectory.
Use the tab
key for "name completion" (try it!).
GNU-Unix Environment (If you want to use Linux)
- If your computer is in Windows, logoff and "restart".
Just after the Dell screen, there will be a countdown --- hit any key to
At the Grub
prompt, use the arrows to highlight "Red Hat Enterprise Linux Client"
and hit return.
Your login account is your Department unix account (see instructor if
you need one).
- Right click on background for a menu with "Konsole". Start up
one or two terminal windows.
- Take a look at the unix command summary sheet.
Create a directory for 780 ("mkdir 780"), go to that directory
- Figure out how to
launch a web browser (e.g., Firefox has an "earth with mouse" icon
or type firefox & at a command line),
find the 780 homepage
download session01.zip into the 780 directory.
Unpack it with "unzip session01.zip", which should create the
session_01 subdirectory with some files in it.
Go to the session_01 subdirectory.
A Simple C++ Program (look at the printout first!)
Mark on the printout what you don't understand about the code
for the simple program area.cpp.
(If you are new to C++, you're not expected to understand
much; see the discussion in the background notes for some
Editor: use nedit or emacs or vi or ...
(I suggest using nedit if you don't
know any of these) to look at area.cpp.
E.g., type nedit area.cpp &. (The "&" runs the editor in the
"background" so the terminal is still available.)
Why use "area" and "radius" for variables rather than "A"
and "R", which are quicker to type?
- Compile and link area.cpp to create area (or area.exe on Windows) using
g++ -o area area.cpp
(the executable would be called a.out if you left out "-o area").
Run it by typing "area" (or "./area" if you get an error
message about not finding the file; why does this work?).
Add "<< endl" to the end of the output line (starting "cout ...")
and recompile and rerun. What did this do?
- To get used to error messages, modify the program to create
some errors and see what
the compiler says (try these then invent your own):
Each time, try compiling again. The compiler should 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 "not declared"
errors are reported as being at lines that are incorrect. Why?
- remove a semi-colon (leaving out a ; is the most common
error you will make!)
- comment out using namespace std; (using //)
- remove a } (or change it from "}" to ")" or "]")
- List two ways to verify that the program is
giving correct answers using known special cases and scaling
arguments (e.g., multiply radius by factors of 10):
Later (at the end today, if time) you may be asked to
carry out the "to do" list in the comments (with assistance, as needed).
Which, if any, of them do you know how to do now?
- Copy, rename, and modify area.cpp to make a program
volume.cpp, which calculates
the volume of a sphere. Test it. [Warning: Many people
get this wrong the first time. Test carefully!]
- Let's try Python!
For the experts:
Write a program ("script") to duplicate what area.cpp does.
For everyone else: Look at area0.py in the editor,
and run it (python area0.py is one way).
Compare with the C++ version.
Do they have the same pseudocode? List at least three differences
between the C++ and Python implementations.
Overflows, Underflows, and Machine Precision
Refer to the background notes on number representation as you do this
Look at the printout for the code "flows.cpp".
Where does the output go?
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 and compare to the notes)
- where underflow and overflow occur for single-precision
- where underflow and overflow occur for double-precision
- Now for the machine precision, which is calculated crudely in
[Note: This code has an intentional bug. Compare to
flows.cpp to track it down.]
Explain briefly what
the machine precision is and how
it is found (note the output file):
- the machine precision for single-precision
- the machine precision for double-precision
floating-point numbers. [Note: you have to change
the code for this to work correctly. Hint:
the correct answer is smaller than 1e-12.]
Using a Makefile
See the handout with the printout of the makefile "make_area".
Makefiles are useful tools to organize the code for computational
They contain instructions for how to build an executable (what files
have the code, what options to use in compiling, what libraries to link
Here we have only one source (.cpp) file, but the makefile
keeps track of all the g++ options.
- Look at make_area in an editor. We'll go through the details of
a makefile later; for now note the following:
- Comments start with "#" and continuation lines
are indicated by "\" (so "SRCS= \" and "area.cpp" is all on one line).
[Warning: There can't be any spaces after the "\".]
- The list of options defined by "WARNFLAGS" are options to g++
(see C++ options handout for details).
- Run the makefile using "make -f make_area". It will try to
compile area.cpp and then link to create area. Did it work?
- Make a copy of make_area ("cp make_area make_flows") and
edit it so that it compiles and links flows.cpp.
[Look for all places "area" appears in the file.
Ask an instructor about changing them all with the editor.]
Using the GSL Scientific Library
A link to the full 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.
- Examine the file "J0_test.cpp" in
an editor. 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 a graph on the page
to check you are calculating the right thing?
- Compile J0_test.cpp according to the directions in the file and
verify that the correct answer is given. Is it?
- Now calculate for x=3
(modify the code for this value or add code to read in any x).
- Verify your answer using MATLAB or Mathematica or Python.
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
Did you succeed?
- Discuss with your partner how a more general program could
be structured, and make at least two suggestions here.
Some More Python (if time permits)
- Create volume0.py.
- Look at and run the Python versions of the flows and precision
codes (included in session01.zip).
- Try running Python interactively. At the prompt, just
type python and return. You will get a
>>> prompt. Try import area1 and then
help(area1). Use quit() to exit.
- Tkinter demo: Look at hello_world_gui.py and run it. Try to
change the text and font.
Copying Your Session to Another Account
If you were logged into your partner's account, you should make
your own copy of everything you did.
the easiest way to get your own copy is to work on the share disk
and then copy the working directory to your own account
(after logging in). On Linux (or from Cygwin to a unix account),
use "secure copy" or scp:
- If you are currently in the directory "session_01", go up one
directory using "cd .." (the two dots mean the directory immediately
above). You should now be in the directory you originally created
- If your username is "myname", then you can transfer the entire
session_01 directory to your account with the command:
scp -pr session_01 myname@fox:
Answer "yes" if asked about connecting and then enter your password
The "p" and "r" are options to scp that preserve the date and
permissions of the file (this is the "p") and transfer all
subdiretories recursively (this is the "r"). Don't forget the @ or
the : at the end. This will create the directory "session_01" on
the top-level directory on the account "myname" (with the same name).
780.20: 1094 Session 1.
Last modified: 12:56 pm, January 03, 2012.