Lists: Creating, Slicing, Methods, Sorting & Comprehensions

cities = ["Pune", "Delhi", "Kochi", "Delhi"]
mixed = [101, "Kavya", 97000.5, True, None]
empty = []
print(cities, len(cities), type(cities).__name__)
print(list("data"), list(range(5)), list((1, 2, 3)), [0] * 5)

nums = [10, 20, 30, 40, 50, 60]
print(nums[0], nums[-1], nums[1:4], nums[:2], nums[-2:], nums[::2], nums[::-1])
nums[0] = 99                   # change one item
nums[1:3] = [21, 31, 32]      # replace a slice (the length may change)
print(nums)
print(3 in nums, 99 in nums, nums.count(40), nums.index(40))

a = [1, 2, 3]
a.append(4); a.extend([5, 6]); a.insert(0, 0); a += [7]
print(a)
print(a.pop(), a.pop(0), a)          # removes 7, then 0
a.remove(3); del a[0]; print(a)
print([1, 2] + [3], [1, 2] * 2)       # concatenation creates new lists
print(a.append(9))                    # None! methods that change a list return None

scores = [72, 95, 61, 88, 95]
print(sorted(scores), sorted(scores, reverse=True), scores)         # sorted() returns a new list
scores.sort(); print(scores)                                         # .sort() changes the list in place
words = ["banana", "Apple", "cherry", "apple"]
print(sorted(words), sorted(words, key=str.lower), sorted(words, key=len))
staff = [("Asha", 29, 61000), ("Ravi", 34, 88000), ("Meera", 41, 72000)]
print(sorted(staff, key=lambda p: p[2], reverse=True))               # sort records by the 3rd field
print(list(reversed([1, 2, 3])), min(staff, key=lambda p: p[1])[0])

data = [4, 8, 15, 16, 23, 42]
print(len(data), sum(data), min(data), max(data), sum(data) / len(data))
print(any(x > 40 for x in data), all(x > 0 for x in data))
print(list(enumerate(["a", "b"], 1)), list(zip([1, 2, 3], "abc")))
a, b, *rest = data; print(a, b, rest)                                # unpacking with *
first, *middle, last = data; print(first, middle, last)

a = [1, 2, 3]
b = a                      # b is another name for the SAME list
b.append(4)
print(a, b, a is b)
c = a.copy()               # a real (shallow) copy; also list(a) or a[:]
c.append(99)
print(a, c, a is c)

import copy
grid = [[1, 2], [3, 4]]
shallow = grid.copy(); deep = copy.deepcopy(grid)
grid[0][0] = 100
print(shallow, deep)       # shallow copy sees the change in the inner list; deep copy does not

matrix = [[1, 2, 3], [4, 5, 6], [7, 8, 9]]
print(matrix[1][2], [row[0] for row in matrix], [sum(row) for row in matrix])
print([list(col) for col in zip(*matrix)])                # transpose
flat = [x for row in matrix for x in row]; print(flat)    # flatten

squares = [n ** 2 for n in range(1, 8)]
evens = [n for n in range(20) if n % 2 == 0]
upper = [c.upper() for c in ["pune", "delhi"]]
labels = ["High" if s >= 80 else "Low" for s in [95, 60, 82]]
pairs = [(i, j) for i in range(1, 4) for j in range(i, 4)]
print(squares, evens, upper, labels, pairs, sep="\n")

result = []
for n in range(20):
    if n % 2 == 0:
        result.append(n)
print(result == [n for n in range(20) if n % 2 == 0])

import pandas as pd
emp = pd.read_csv("employees_small.csv")
salaries = emp.salary.tolist(); names = emp.name.tolist(); depts = emp.department.tolist()
top3 = sorted(zip(salaries, names), reverse=True)[:3]
print("Top 3:", [(n, s) for s, n in top3])
fin = [s for s, d in zip(salaries, depts) if d == "Finance"]
print("Finance:", len(fin), "people, average", round(sum(fin) / len(fin)))
above_avg = [n for n, s in zip(names, salaries) if s > sum(salaries) / len(salaries)]
print(len(above_avg), "people earn above the average;", above_avg[:3], "…")

# Remove duplicates but keep the original order (set() would lose the order)
raw = ["Pune", "Delhi", "Pune", "Kochi", "Delhi", "Goa"]
seen, unique = set(), []
for c in raw:
    if c not in seen: seen.add(c); unique.append(c)
print(unique, "| shortcut:", list(dict.fromkeys(raw)))
# Chunk a list into groups of 4
ids = list(range(1, 11)); print([ids[i:i + 4] for i in range(0, len(ids), 4)])
# Moving average of 3
x = [10, 12, 9, 14, 15, 13]; print([round(sum(x[i:i + 3]) / 3, 1) for i in range(len(x) - 2)])