### Lecture #1: iPython notebooks



In [1]:

#  This is also how to print text to the screen
print "Hello world!"




File "<ipython-input-1-58c57bc55686>", line 2
print "Hello world!"
^
SyntaxError: Missing parentheses in call to 'print'




In [2]:

#  This is how to print text to the screen using Python v3.x
print("Hello world!")




Hello world!



### Simple formulas



In [4]:




Out[4]:

6




In [5]:

4-2        #  Subtraction




Out[5]:

2




In [7]:

4/3        #  Division




Out[7]:

1.3333333333333333




In [8]:

4//3       # Integer Division in old style, cutting of decimals




Out[8]:

1




In [9]:

4*2        #  Multiplication




Out[9]:

8




In [10]:

4**2       #  Exponents (this reads 4 squared)




Out[10]:

16




In [11]:

4%2        #  Modulo (the answer is the remainder)




Out[11]:

0




In [12]:

5%2        #  Modulo (the answer is the remainder)




Out[12]:

1



### Variable assignments and printing



In [13]:

x = 3      #  x is now a variable




In [14]:

print(x)   #  Print the variable x to the screen




3




In [16]:

x          #  In Jupyter notebook this prints to the screen




Out[16]:

3




In [17]:

x**2
x = x**2
#  What does this print?




In [19]:

print(x)
#  What is the value of x?




9



### Strings



In [20]:

name = 'ASTR'            #  You can use single or double quotes




In [22]:

name + '2600\'s'            #  Adding strings
name + "2600's"




Out[22]:

"ASTR2600's"




In [24]:

name = name + '2600   '

#  What is the value of name?
print(name)
print(name * 2)




ASTR2600
ASTR2600   ASTR2600




In [25]:

print(name ** 2)           #  Multiplying strings

#  What is printed?




---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-25-279592643bfb> in <module>()
----> 1 print(name ** 2)           #  Multiplying strings
2
3 #  What is printed?

TypeError: unsupported operand type(s) for ** or pow(): 'str' and 'int'



### "Markdown," i.e., fancy typesetting (just for fun!)

$\sqrt{2}$

$\Gamma = 1.4$

$\frac{1}{2} = 0.5$

### Raw Text

The formatting we used in markdown is known as LaTeX formatting. For more information, a good place to start is: http://en.wikibooks.org/wiki/LaTeX/Mathematics or, just Google it!

$\int e^x dx = e^x$

### Notebook Recap

• Use to execute cells, or select "Run" from the "Cell" drop-down menu.

• Use raw text to write short answers to questions that appear in homework.

• You don't need to know how to do markdown, but you're welcome to if you want.

### Path recap

Remember: Everything on Linux is a file. A directory is just a special file that can contain other files.

There are 2 different kind of paths: absolute and relative paths.

Imagine your friend asks you how to get somewhere. You will instinctively ask where your friend is, so that you can give her instructions relative to her current location! That would be a relative path to where she wants to be.

Absolute paths are always correct and complete (hence absolute), and always start with a /, the "root" symbol. As / is the lowest point on a storage device, the path that is attached to it is always unique.

Relative paths are very flexible, they just take one instruction after the other, separated by the /. So, for example to go from /home/student to /usr/bin, the relative path would be ../../usr/bin.

Note how it does NOT start with a / because from /home/student to go up one level is just .., not /... (Even more so, /.. does never make sense, can you see why?).

The cd command just knows it has stack path instructions one by one, separated by the / so to execute the above relative path in one go is totally okay (and don't forget to use , it will help you to maneuver paths immensely!). So here the command would be cd ../../usr/bin, if launched from /home/student.

### So, what's in a path?

Because everything in Linux is a file, everything in Linux has a path. This also means that there is not a very certain way to know what kind of object is at the end of a path. There is no strict rule for extensions like .txt or .doc. These are just conventions, but a Linux/Unix system does not need them to work.

So, at the end of a path can be a directory/folder, or it can be a real file. But that doesn't mean you know what that file is or can do.

For example, for the path /home/student/test2, it could be any of these things:

• a folder
• an executable progam
• a text file with data or configuration values read by a program
• a script to be executed by an interpreter like Bash or Python

You have several options to find out what something is:

• just look at it with the less command. If it's a huge bunch of binary, less will ask you if you really want to look at binary gibberish.
• use the file command. It was written to exactly reduce the confusion what kind of thing could be at the end of a path. Use it like so:

file /home/student/test2



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