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Contemporary Physics 2007 Q
Rec #1A : The Big Introduction
Welcome to Contemporary Physics, a class specifically designed to explore the physical nature of our universe. In lecture, Dr. Goldberg will teach you the theory and concepts. In recitation we will explore these ideas by programming models with the computer. Before we get started we need to understand some computing basics. This assignment is meant as a gentle introduction to Unix, and Python. Please ask questions at any step of the way.
1 Python : Introduction
Write the output of each step. You will turn in this paper for credit (put your name on it).
It is very important that you complete and understand each step of this assignment; these
skills will be expected of you in subsequent classes.
To use python on the command-line, type:
python
You should be at a prompt that looks like this: >>> While it has many other functions, python can be used as an over-grown calculator. Type
5+
and press enter. Notice it returns exactly what you expect (write down the result next to each line). Lets try some more:
28*
Nothing to complicated here. Python does have the nice capability of handling arbitrarily large integers, for example lets try 2^80.
2**
The ** operator is short for exponentiation. Lets save some of these results. You can define a variable by:
a=
Here a is now defined to be the value of 20. Set b = 50. Anything that you can do with numbers, you can do with variables. For example try:
a-b
There is an important difference between integer and decimal division, try this example:
5/
5/2.
Every time you exit python, you will lose all your saved variables. However, you can write a python program that acts as a single session. Exit python by pressing Ctrl+D at the same time. Using Emacs open a new file called rec1.py.
emacs rec1.py &
The & allows you to use both the command-line and emacs at the same time. Lets create a simple program. Type the following into emacs and save the file. Go back into the command line and type:
python rec1.py
to run the program. Your first python program (write its output below):
a= b= print a,b print a>b, a<b, a==b
The print command tells python to output the result to the screen. >, < and == are comparisons for “greater than”, “less than” and “equals to” respectively. Take note that = means set equal to, and == means check to see if they are equal (a subtle difference, but one that comes up for beginning programmers). Go back to the emacs windows (rec1.py) and change the code and rerun it:
a= b= #print a,b #print a>b, a<b, a==b
K = [1,7,8,10] P = range(5) print K print K[0], K[1], K[3] print print P
Note that the previous lines were not printed out. The # tells python that the line is a comment, i.e. it will be ignored by the machine. K and P are lists. Lists are collections of objects, a handy tool that we will use many times. To access an element of a list, we can index it by K[n], where n is the sequential element we want to grab. Notice that the first element is labeled n = 0 not n = 1. The command range creates a special list, one that is sequential, ordered from 0...n. Lets add some more to our code by introducing loops. Append the following lines to the bottom of the code and rerun it again:
J = [ ] for g in K: print g J.append( g**2 ) print J
If the code does not run, make sure that you have correctly indented the lines following the f or statement. The first line creates an empty list, we will be adding something to it later. The f or command is a loop. The variable g at each iteration of the loop is sent to the next value of K. The append command adds the value of g^2 sequentially to J. If you want you can plot this result (sketch the plot below).
from pylab import * plot(K,J) show()
The f rom line tells python to use all the utilities in pylab. It only needs to be included once in the code, even if you have multiple plots. The plot(X, Y ) command plots X vs. Y (or in our case K vs J). The show command displays the output to the screen. Notice there are several buttons on the plotting screen. The one that is disk-shaped allows you to save your graph to a file. Easy enough. Let’s move on to the real assignment.
