python 数据处理入门0

this file is derived from exercise 0

#1. Variables and data types

1.1 operator

Some relational operators:

Symbol	Task Performed
==	True, if both sides are equal
!=	True, if both sides are not equal
<	True, if the left side is smaller
>	True, if the left side is greater
<=	True, if the left side is smaller or equal to the right side
>=	True, if the left side is greater or equal to the right side

1.2 basic functions

• print()

x = "Hello"
y = "World"
print (x + y)
# "HelloWorld"

• type(variable)
• str(variable)
• len(variable)

2. Data Structures

2.1 List

List can contain datas with same or different types, List is mutable.

lol = [[1,2,3], ["a","b","c"], [1.2,2.3,4.5,6.7,8.9]]

• lol.append(value)
• x = lol.pop(), pop out the last element of lol
• lol.length()

2.2 Tuple

Tuples are just the same as lists, but are immutable.

t = (1,2,3)
t1 = ((1,2),(1,3),3)

2.3 Set

Sets are similar collections, but have no order and can contain each element only once.

s = set()
s.add(50)
s.add(20)
s.add(10)
s.add(20)
#s = {10, 20, 50} 

• traverse all elements of a list and extract them to a set

l = [2,3,4,5,6,2,3,4,5,2]
s = set()
for x in l:
    s.add(x)
    
list (set(l))
#[2, 3, 4, 5, 6]

2.4 Dictionary

Python’s built-in mapping type. They map keys, which can be any immutable type, to values, which can be any type.

• initialization 1st

presidents_inauguration = {
     }
presidents_inauguration ['Trump'] = 2017 
presidents_inauguration['Obama'] = 2009
presidents_inauguration['Bush'] = 2001
print(presidents_inauguration)
#{'Trump': 2017, 'Obama': 2009, 'Bush': 2001}

• initialization 2nd

d = {
     'Trump': 2017, 'Obama': 2009, 'Bush': 2001}

• keys, values

print (presidents_inauguration.keys())
#dict_keys(['Trump', 'Obama', 'Bush'])
print (presidents_inauguration.values())
#dict_values([2017, 2009, 2001])

• len(dic)

2.5 Control Statements

control flow in Python noticeable does not use ANY (,),[,],{,},…
Instead indendation determines what belongs to block of commands

• if-elif-else

x = 15
if x > 20:
    print ('This is a very big number!')
elif x > 10:
    print ('This is a big number!')
else:
    print ('this is a small number!')

• loops

first_names = ['John', 'Paul', 'George', 'Ringo']
for name in first_names:
    print("Hello " + name + "!")

• range function
  Contrary to many other programming languages there is no built-in for… counting loop.
  However, you can use the range function:

for x in range(10,25,5):
    print (x)
#10
#15
#20

• enumerate

# enumerate is a useful convenience functions:
for index, name in enumerate (first_names):
    print("Name "+ str(index) + ": " + name)
# Name 0: John
# Name 1: Paul
# Name 2: George
# Name 3: Ringo

• while loop

# while loops functions very similar to many popular languages:
x = 1
while x < 100:
    x = x * 2
    print(x)

2.6 Functions

• definition

def print_all_names(names):
    for x in names:
        print (x)

• lambda function

Lambda functions are a concise way of defining functions, e.g.:

f = lambda x: x*5 + 1
f(10)

2.7 Import

Python has a lot of built-in packages you can use, or you can download and install more packages from the internet.
Using such packages is easy:

import antigravity
import statistics
statistics.mean([3,5,7,9])
# 6

# you can also import just single functions from a package
from math import log
log (2.71 * 2.72)
# 1.9975805151995156

2.8 Exceptions

def to_int_safe (x):
    try:
        z = int (x)
    except:
        print ("We cannot convert everything like this!")
        z = -1
    return z

to_int_safe("5.5")
# We cannot convert everything like this!
# -1

2.9 File Handling

• open for reading

with open("filename", "r") as f: #r ==> open for reading
    for line in f:
        print (line.strip())

• open for writing

my_list = ["a", "bc", "de"]
with open("fienae.txt", "wb") as f: #r ==> open for writing
    for element in my_list:
        f.write (element + "\n")