Some very basic python

Showing some very basic python, variables, arrays, math and plotting:

Variables and printing them



In [1]:

    
#  setting a variable
a = 1.23

#  although just writing the variable will show it's value, but this is not the recommended
#  way, because per cell only the last one will be printed and stored in the out[]
#  list that the notebook maintains

a
a+1









    Out[1]:





2.23

The right way to print is using the official print() function in python and this way you can also print out multiple lines in the out[]



In [2]:

    
print(a)
print(type(a))









    



1.23
<class 'float'>

Now you can see that each call to print() will cause output on a new line,and also note they are not in the Out[] list

overwriting the same variable , now as a string



In [5]:

    
a="1.23"
print(a,type(a))









    



('1.23', <type 'str'>)



In [4]:

    
# checking the value of the variable
a









    Out[4]:





'1.23'

Python versions

Python2 and Python3 are still being used today. So safeguard printing between python2 and python3 you will need a special import for old python2:



In [5]:

    
from __future__ import print_function

Now we can print(pi) in python2. The old style would be print pi



In [6]:

    
pi = 3.1415
print("pi=",pi)
print("pi=%15.10f" % pi)









    



pi= 3.1415
pi=   3.1415000000



In [7]:

    
# for reference, here is the old style of printing in python2 
# print "pi=",pi

Control stuctures

Most programming languages have a way to control the flow of the program. The common ones are

if/then/else
for-loop
while-loop

While you are looking at this code, note that white-space (indentation) controls the extent of the code, and unlike other language, there is no special symbol or end type statement. This is probably the single most confusing thing about those new to the python language.



In [10]:

    
n = 1

if n > 0:
    print("yes, n>0")
else:
    print("not")     

    
for i in [2,4,n,6]:
    print("i=",i)
print("oulala, after this for loop, i=",i)

n = 10
while n>0:
    #  n = n - 2
    print("whiling",n)
    n = n - 2

print("last n",n)









    



yes, n>0
('i=', 2)
('i=', 4)
('i=', 1)
('i=', 6)
('oulala, after this for loop, i=', 6)
('whiling', 10)
('whiling', 8)
('whiling', 6)
('whiling', 4)
('whiling', 2)
('last n', 0)

Python Data Structures

A list is one of four major data structures (lists, dictionaries, sets, tuples) that python uses. It is the most simple one, and has direct parallels to those in other languages such as Fortran, C/C++, Java etc.

Python Lists

Python uses special symbols to make up these collection, briefly they are:

list: [1,2,3]
dictionary: { "a":1 , "b":2 , "c": 3}
set: {1,2,3,"abc"}
tuple: (1,2,3)



In [ ]:



In [9]:

    
a1 = [1,2,3,4]
a2 = range(1,5)
print(a1)
print(a2)
a2 = ['a',1,'cccc']
print(a1)
print(a2)
a3 = range(12,20,2)
print(a3)









    



[1, 2, 3, 4]
[1, 2, 3, 4]
[1, 2, 3, 4]
['a', 1, 'cccc']
[12, 14, 16, 18]



In [10]:

    
a1=range(3)
a2=range(1,4)
print(a1,a2)









    



[0, 1, 2] [1, 2, 3]



In [11]:

    
a1+a2









    Out[11]:





[0, 1, 2, 1, 2, 3]

Math and Numeric Arrays



In [12]:

    
import math
import numpy as np



In [13]:

    
math.pi









    Out[13]:





3.141592653589793



In [14]:

    
np.pi









    Out[14]:





3.141592653589793



In [15]:

    
# %matplotlib inline
import matplotlib.pyplot as plt



In [16]:

    
a=np.arange(0,1,0.01)



In [17]:

    
b = a*a
c = np.sqrt(a)



In [18]:

    
plt.plot(a,b,'-bo',label='b')
plt.plot(a,c,'-ro',label='c')
plt.legend()









    Out[18]:





<matplotlib.legend.Legend at 0x7fcf86dc1810>



In [19]:

    
plt.plot(a,a+1)









    Out[19]:





[<matplotlib.lines.Line2D at 0x7fcf86dc1a10>]



In [20]:

    
plt.show()



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