python_basic_for_algorithms.py

'''
common pieces of code across all the algorithms in python.
'''

# print
# =====
# concat arrays quickly in strings
print("my car is ",  ["a", "b", "c", "d"], "and is in ", ["a", "b", "c", "d"]) # my car is  ['a', 'b', 'c', 'd'] and is in  ['a', 'b', 'c', 'd']

# increment number
# ================
# in python you cannot do a++
a = 5
a += 1
print(a) # 6

# infinity
# ========
print(float("inf")) # inf

# swap values
# ===========
a = ["a", "b", "c", "d"]
a[1], a[2] = a[2], a[1] # ['a', 'c', 'b', 'd']
print(a)

# sort() vs sorted()
# ==================
# sort() update the original array, it return always None
a = [8, 5, 6, 2, 4, 1]
a.sort()
print(a) # [1, 2, 4, 5, 6, 8]
# sorted() instead return a new one sorted
b = [8, 5, 6, 2, 4, 1]
print(sorted(b)) # [1, 2, 4, 5, 6, 8]
print(b) # [8, 5, 6, 2, 4, 1]

# how can we map() in an array?
# =============================
a = [8, 5, 6, 2, 4, 1]
arr = [x-1 for x in a]
print(arr) # [7, 4, 5, 1, 3, 0]

b = map(lambda x: x+1, a)
print(list(b)) # [9, 6, 7, 3, 5, 2]


# range
# You cannot create a reverted range: range(5, 1), wrong
# =====
print(list(range(5)))  	  # [0, 1, 2, 3, 4]
print(list(range(1, 5)))  # [1, 2, 3, 4]
print(list(reversed(range(1, 5))))  # [4, 3, 2, 1]


# for + len + range VS for + enumerate
# ====================================
myrange = range(4)   # -> range never includes the number you pass in the returned list and it starts on zero!!
print(myrange) # [0, 1, 2, 3]

myrange = range(3,6)
print(myrange) # [3, 4, 5]

# you will see this loop a lot. Trick: is much easier to read in the second way (with enumerate)
for idx in range(len(["a", "b", "c", "d", "e"])):
	print(idx) # 0 1 2 3 4

for idx, _ in enumerate(["a", "b", "c", "d", "e"]):
	print(idx) # 0 1 2 3 4

# array slices
# ============
a = ["a", "b", "c", "d", "e"]
print(a[-1]) # e
# ending at (and not including)...
print(a[:0]) # []
print(a[:1]) # a
print(a[:3]) # ['a', 'b', 'c']
print(a[:-1]) # ['a', 'b', 'c', 'd']  # this and the next are the same
print(a[0:-1]) # ['a', 'b', 'c', 'd']

# starting at (and not including)...
print(a[0:]) # ["a", "b", "c", "d", "e"]
print(a[1:]) # ["b", "c", "d", "e"]
print(a[2:]) # c d e

# between, slice...
print(a[0:0]) # []
print(a[0:-0]) # []
print(a[1:3]) # b cs
print(a[1:-1]) # b c d

# between jumping...
print(a[1:-1:2]) # b d

print(a[-3]) # c
# invert an array
print(a[::-1]) # ['e', 'd', 'c', 'b', 'a']



# init matrixes
# =============
print([0]*5) 		# [0, 0, 0, 0, 0]
print([3] + [0]*4)  # [3, 0, 0, 0, 0]

print([ 0 for j in range(5)]) # [0, 0, 0, 0, 0]
print([[ 0 for x in range(3)] for y in range(5)])
# [
# 	[0, 0, 0],
# 	[0, 0, 0],
# 	[0, 0, 0],
# 	[0, 0, 0],
# 	[0, 0, 0]
# ]

# queue
# =====
a = []
a.append("a")
a.append("b")
a.append("c")
print(a.pop(0)) # a, a == ["b", "c"]

# stack
# =====
a = []
a.append("a")
a.append("b")
a.append("c")
print(a.pop()) # c
print(a) # ['a', 'b']


# filter a list of items
# =====================
names=["ismail","ali","ahmet","elif","ecrin"]
filtered_names = filter(lambda item: 'i' in item , names)
print(filtered_names) # ["ali", "elif", "ecrin"]

# custom sort of a list
def multi(num):
     return num*2

a=[1,4,70,2,9,6]
a.sort(key=multi)
print(a) #[1, 2, 4, 6, 9, 70]

# round numbers
import math
print(7/2) # 3.5
print(math.ceil(7/2)) # 4

# move items in a list
b = ["a", "b", "c"]
idx = b.index("b") # 1
b.pop(idx) # b
print(b) # [a, c]
b.insert(0, "b")
print(b) # [b, a, c]

# reverse string or list
a="abcdef"
print(a[::-1]) #'fedcba'
b = ["a", "b", "c"]
print(b[::-1]) # ['c', 'b', 'a']




# a classic configuration of "for loops" to investigate backward elements:
nums = [3, 2, 8, 9, 10]
for i, _ in enumerate([3, 2, 8, 9, 10]):
	print("---")
	print(nums[:i])
	for j, val in enumerate(nums[:i]):
		print(val)
# output:
# ---
# []
# ---
# [3]
# 3
# ---
# [3, 2]
# 3
# 2
# ---
# [3, 2, 8]
# 3
# 2
# 8
# ---
# [3, 2, 8, 9]
# 3
# 2
# 8
# 9