PyBasic1.Rmd

---
title: "Python Basic 1"
author: "Raghu Sidharthan"
output: 
  html_document:
    highlight: pygments
---

**Basic Python Commands**
```{python,eval=FALSE}
type(x) #to get the data type of a variable
len(x)  #gives the length of the list/array 
#Profile functions 
import cProfile
cProfile.run('cdfmvna(aa,rr,ss)',sort=1)

# function definition
def square(x):
    return x**2
    
#Get and set working directory
import os
os.getcwd()
os.chdir('C:\\Users\\sidharthanr\\Temp')
os.listdir('C:\\Users\\sidharthanr\\Temp') # lists all files

import sys
sys.exit("Error message")  #To stop the program execution progrmatically
sys.path.append('C:\\Temp') #after this modules in this directory can be imported
print("\n".join(sys.path)) # prints all the folders in the system path
#Get Time  for time consumed computations
import time
ts = time.time()

hrs = raw_input("Enter Name:") # to enter data from command prompt

del inpMatPD # Delete an object
__val = gc.collect() #Force garbage collection as we are deleting large objects

#Read last N lines from a file
from collections import deque
with open('data.csv', 'r') as f:
    q = deque(f, 2)  # reads last two lines

```
**Python commands with evaluation**
```{python}   
print(list(xrange(4))) #xrange used for iterators
print('4'.zfill(3)) #Padding Zeros in a string
print(5%2) # modulus operator 

#Multiple list counters - like a Cartesian product
print([prefix+'_'+elt for elt in ['aa','bb', 'cc'] for prefix in ('p1','p2') ])

```

```{python}   
#Equivalent of R match function (with zero indexing)
match = lambda a, b: [ b.index(x) if x in b else None for x in a ]
print(match([2,1],[1,3,1,3]))


```
```{python}   
#Subset a list using another Boolean list
from itertools import compress
list_a = [1, 2, 4, 6]
fil = [True, False, True, False]
print(list(compress(list_a, fil)))

```

**Unpacking a list of lists**

```{python}   
LofL = [['x01', 'x02'], ['x03', 'x04'], ['x05', 'x06']] # This is a list of list

flat_list = [item for sublist in LofL for item in sublist]
print(flat_list)
# This is equivalent to below
flat_list=[]
for sublist in LofL:
    for item in sublist:
        flat_list.append(item)
		

```

**String Operations**
```{python}   
s = "  \t a string example\t  "
print(s.strip()) 
print(s.rstrip()) #Whitespace on the right side:
print(s.lstrip()) #Whitespace on the left side:
print(s.strip(' \t\n\r')) #strip arbitrary characters to any of these functions like this:
str = "this is string example....wow!!! this is really string";
print(str.replace("is", "was"))


```
**Time and Date**
```{python}   
import time
from datetime import datetime
import pytz

print(datetime.fromtimestamp(1284286794)) #Convert seconds from epoch to datetime object
print(datetime.now()) # Current time
print(datetime.now(pytz.timezone('UTC')).astimezone(pytz.timezone('US/Eastern')))
```

**Itertools Pacakge**
```{python}   
import itertools
print list(itertools.permutations([1,2,3], 2))
print list(itertools.combinations([1,2,3], 2))
```

```{python}   


```