README (Denny-mac.local's conflicted copy 2020-05-14).org

1 Python CheatSheet

• PDF Link: cheatsheet-python-A4.pdf, Category: languages
• Blog URL: https://cheatsheet.dennyzhang.com/cheatsheet-python-A4
• Related posts: Golang CheatSheet, Ruby CheatSheet, #denny-cheatsheets

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1.1 Python Compact Coding

Name	Comment
if … return	if k == 0: return False
if… continue	if index == icol: continue
return if.. else	return val if i>0 else 0
multiple assignment	l, r = 2, 3
assign with check of none	a = b if b else 1
assignments	l[1]=l[0]=0
swap values	left, right = right, left
list Comprehensions	[x*x for x in range(1, 1001)]
list Comprehensions	l = [2, 3, 5]; [2*x for x in l if x>2]
use zip	for a, b in zip(nums, nums[3:])
build a list	dp = [1] + [0]*3
sum a subarray	sum(nums[0:k])
sort list in descending order	sorted(nums, reverse=True)
dictionary with defaults	m = collections.defaultdict(lambda: 1)
loop with single statement	while p.left: p = p.left
print multiple values	print(x, y)
get both index and item	for i, ch in enumerate(["a", "b", "c"]): print(i, ch)
mod negative	(-2)%5

1.2 Python Common Algorithms

Name	Comment
bfs	code/tree-bfs.py
trie tree	code/tree-trie.py
python Semaphore	Leetcode: Print Zero Even Odd

1.3 List

Name	Comment
return all but last	list[:-1]
The second last item	list[-2] or list[~1]
map	map(lambda x: str(x), [1, 2, 3])
create fixed size array	l = [None] * 5
insert elements to head	array.insert(0,var)
delete element by index	del a[1]
list as stack	item = l.pop()
sort in descending	l = sorted([8, 2, 5], reverse=True)
sort by attribute	l=sorted([(‘ebb’,12),(‘abc’,14)], key=lambda x: x[1])
in-place sort	l.sort()
generate a-z	map(chr, range(ord('a'), ord('z')+1))
map/reduce	functools.reduce((lambda x, y: "%s %s" % (x, y)), l)
replace ith to jth	list[i:j] = otherlist
combine two list	list1 + list2
get sum	sum(list)
unique list	set(["Blah", "foo", "foo", 1, 1, 2, 3])
Insert to sorted list	bisect.insort(l, 3)
Reverse a list	l[::-1]

1.4 String

Name	Comment
reverse string	‘hello world’[::-1]
array to string	’ ‘.join([‘a’, ‘b’])
split string to array	“hello, python”.split(“,”)
string to array	list('abc')
format to 2 digits	print "%02d" % (13)
capitalize string	‘hello world’.capitalize()
upper/lower string	‘aBc’.upper(), ‘aBc’.lower()
count substring	‘2-5g-3-J’.count(‘-‘)
pad leading zero	‘101’.zfill(10)
string remove tailing ‘0’	‘0023’.rstrip(‘0’)
string remove leading ‘0’	‘0023’.lstrip(‘0’)
check if string represent integer	‘123’.isdigit()
check if string alphabetic	‘aBc’.isalpha()
Check if string alphanumeric	‘a1b’.isalnum()
find location of substring	‘abc’.find(‘d’)= (returns -1)
find location of substring	‘abc’.index(‘d’)= (raise exception)
replace string	‘ab cd’.replace(’',”)
padd whitespace to the left	‘a’.ljust(10,=’=’)
padd whitespace to the right	‘a’.rjust(10,=’=’)

1.5 Integer

Name	Comment
max, min	sys.maxsize, -sys.maxsize-1
min, max	min(2, 3), max(5, 6, 2)
generate range	for num in range(10,20)
get ascii	ord('a'), chr(97)
print integer in binary	“{0:b}”.format(10)

1.6 Dict & Set

Name	Comment
dict get first element	m[m.keys()[0]]
intersection	list(set(l1).intersection(set(l2)))
list to set	set(list1)
remove from set	s.remove(2)
remove the first from set	s.pop()
sort dict by values	sorted(dict1, key=dict1.get)
deep copy dict	import copy; m2=copy.deepcopy(m1)

1.7 Bit Operator

Name	Comment
mod	x % 2
shift left	x << 1; a << 2
shift righ	x >> 2
and	x & y
complement	~x
xor	x ^ y
power	2 ** 3
bool complement	not x
binary format	bin(5) (get 101)
count 1 inside binary	bin(5).count('1')

1.8 File

Name	Comment
Append file	open("/tmp/test.txt", "ab").write("\ntest:")
Write file	open("/tmp/test.txt", "wab").write("\ntest:")
Read files	f.readlines()
Check file	os.path.exists("/tmp/test.txt")

1.9 Math

Name	Comment
sqrt	import math; math.sqrt(5)
power	import math; math.pow(2, 3)
random	random.randint(1, 10) 1 and 10 included
eval string	eval("2-11*2")

1.10 Networking

Name	Comment
Start a simple HTTP server	python -m SimpleHTTPServer <port_number>

1.11 Queue/heapq

Name	Comment
Initialize min heap	heapq.heapify(q)
heappush a tuple	q=[]; heapq.heappush(q, (5, 'ab'))
pop	print (heapq.heappop(q))
first item	q[0]
print heapq	print list(q)
create a queue	from collections import deque; queue = deque([1,5,8,9])
append queue	queue.append(7)
pop queue from head	element = queue.popleft()

Review: Heap Problems

Link: BINARY HEAP AND HEAPQ IN PYTHON

1.11.1 minheap & maxheap

import heapq

# initializing list
li = [5, 7, 9, 1, 3]

# using heapify to convert list into heap
heapq.heapify(li) # a minheap
heapq._heapify_max(li) # for a maxheap!

# printing created heap
print (list(li))

# using heappush() to push elements into heap
# pushes 4
heapq.heappush(li,4)

# printing modified heap
print (list(li))

# using heappop() to pop smallest element
print (heapq.heappop(li))

print (list(li))

1.12 Code snippets

• Initialize Linkedlist from array

def initListNodeFromArray(self, nums):
    head = ListNode(None)
    prev, p = head, head
    for num in nums:
        pre = p
        p.val = num
        q = ListNode(None)
        p.next = q
        p = p.next
    pre.next = None
    return head

• Print linkedlist

def printListNode(self, head):
    print("printListnode")
    while head:
        print("%d" % (head.val))
        head = head.next

• Print Trie Tree in level order

def printTrieTreeLevelOrder(self, node):
    print("printTrieTreeLevelOrder")
    if node.is_word:
        print("Node is a word")
    queue = []
    queue.append(node)
    while len(queue) != 0:
        s = ''
        for i in range(len(queue)):
            node = queue[0]
            del queue[0]
            for child_key in node.children:
                s = '%s %s' % (s, child_key)
                queue.append(node.children[child_key])
        if s != '':
            print 'print level children: %s' % (s)

• python sort with customized cmp function: -1 first

nums = [3, 2, 6]
def myCompare(v1, v2):
    return -1
sorted_nums = sorted(nums, cmp=myCompare)
print nums # [3, 2, 6]
print sorted_nums # [6, 3, 2]

• Initialize m*n matrix

col_count, row_count = 3, 2
matrix = [[None for j in range(col_count)] for i in range(row_count)]
print matrix

1.13 More Resources

License: Code is licensed under MIT License.