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Basic data structure and Algorithms in Python

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1. Unpacking Variables

colors = ["red","green","blue"]
a,b,c = colors
print(a)
# red

2. Unpacking Group of Variables

fruits = ["orange","apple","kiwi","strawberry","grape"]
a, *b, c = fruits
print(b)
# ["apple","kiwi","strawberry"]

3. Finding the Largest/Smallest N Items

from heapq import nlargest, nsmallest
numbers = [1,2,3,4,5,6,7,8,9]
three_largest = nlargest(3,numbers)
print(three_largest)
# [7,8,9]
three_smallest = nsmallest(3,numbers)
print(three_smallest)
# [1,2,3]

4. The defaultdict

d={}
print(d["a"])
# Error

from collections import defaultdict
d = defaultdict(list)
print(d["a"])
# []

5. Keeping the Dictionaries Order

from collections import OrderedDict
d = OrderedDict()
d["a"] = 1
d["b"] = 2
d["c"] = 3
d["d"] = 4
for key in d:
    print(key,d[key])
# a 1
# b 2
# c 3
# d 4

6. Calculate with Dictionaries

prices = {"orange":2.99,"apple":3.99,"kiwi":1.99,"melon":5.99}
min_price = min(zip(prices.keys(),prices.values()))
print(min_price)
# 1.99 "kiwi"
max_price = min(zip(prices.keys(),prices.values())
print(max_price)
# 5.99 "melon"

7. Finding Commonalities in Two Dictionaries

foo = {"x":1,"y":2,"z":3}
bar = {"x":2,"y":3,"a":4}
print(foo.keys() & bar.keys())
# {"x","y"}
print(foo.keys() - b.keys())
# {"z"}

8. Store slice in Variables

fruits = ["orange","apple","kiwi","strawberry","grape"]
a = slice(1,4)
sliced_fruits = fruits(a)
print(sliced_fruits)
# ["apple","kiwi","strawberry"]

9. Count the Items Appearance in a List

from collections import Counter
numbers = [1,2,5,2,1,2,3,5,6]
numbers_count = Counter(numbers)
print(numbers_count)
# Counter({2: 3, 1: 2, 5: 2, 3: 1, 6: 1})
top_three = numbers_count.most_common(3)
print(top_three)
# [(2, 3), (1, 2), (5, 2)]

10. Extracting a Subset of a Dictionary

prices = {"orange":2.99,"apple":3.99,"kiwi":1.99,"melon":5.99}
extracted = { key:value for key,value in prices.items() if value > 3 }
print(extracted)
# {'apple': 3.99, 'melon': 5.99}

11. Named Tuple Object

from collections import namedtuple
Fruit = namedtuple("Fruit",["price","color"])
banana = Fruit("2.99","yellow")
print(banana)
# Fruit(price='2.99', color='yellow')
print(banana.price)
# 2.99

12. Combining Multiple Mappings

from collections import ChainMap
a = {"x":1,"y":2,"z":3}
b = {"x":2,"y":3,"j":4}
c = ChainMap(a,b)
print(c["x"])
# 1
print(c["y"])
# 2
print(c["j"])
# 4
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