القوائم والصفوف والتقطيع

قوائم Python: مصفوفات ديناميكية

nums = [3, 1, 4, 1, 5]
print(nums[0])   # 3
print(nums[-1])  # 5  (last element)
nums.append(9)
print(len(nums)) # 6

عمليات القوائم الشائعة

stack = []
stack.append(1)
stack.append(2)
stack.append(3)
print(stack.pop())   # 3  O(1)
print(stack)         # [1, 2]

# insert at index 0 is O(n)
stack.insert(0, 0)
print(stack)         # [0, 1, 2]

شرح صيغة التقطيع

a = [0, 1, 2, 3, 4, 5]
print(a[1:4])    # [1, 2, 3]
print(a[:3])     # [0, 1, 2]
print(a[3:])     # [3, 4, 5]
print(a[::2])    # [0, 2, 4]  every other
print(a[::-1])   # [5, 4, 3, 2, 1, 0]  reversed

ينشئ التقطيع نسخًا سطحية

original = [1, 2, 3]
copy = original[:]  # shallow copy
copy[0] = 99
print(original)  # [1, 2, 3]  unchanged

# Nested list pitfall
nested = [[1, 2], [3, 4]]
shallow = nested[:]
shallow[0][0] = 99
print(nested)    # [[99, 2], [3, 4]]  changed!

Tuples: تسلسلات غير قابلة للتغيير

point = (3, 7)
print(point[0])   # 3

# Use as dict key
grid = {}
grid[(0, 0)] = 'start'
grid[(2, 3)] = 'end'
print(grid[(0, 0)])  # start

# Unpacking
x, y = point
print(x, y)  # 3 7

متى نستخدم Tuple بدلًا من List

import sys
a_list  = [1, 2, 3, 4, 5]
a_tuple = (1, 2, 3, 4, 5)
print(sys.getsizeof(a_list))   # e.g. 104 bytes
print(sys.getsizeof(a_tuple))  # e.g. 80 bytes

# Tuple returned from function
def min_max(nums):
    return min(nums), max(nums)

lo, hi = min_max([3, 1, 4, 1, 5])
print(lo, hi)  # 1 5

أخطاء الواحد الزائد في التقطيع

lst = [1, 2, 3, 4, 5, 6]
mid = len(lst) // 2    # 3
left  = lst[:mid]      # [1, 2, 3]
right = lst[mid:]      # [4, 5, 6]
print(left, right)

# How many elements?
print(len(lst[2:5]))   # 3  (indices 2,3,4)

الفهارس السالبة في المقابلات

s = 'abcde'
print(s[-1])    # 'e'
print(s[-3:])   # 'cde'
print(s[:-2])   # 'abc'  (all except last 2)

nums = [10, 20, 30, 40]
print(nums[-2])   # 30
nums[-1] = 99
print(nums)       # [10, 20, 30, 99]

فرز القوائم ومقارنتها

words = ['banana', 'fig', 'apple', 'kiwi']
words.sort(key=len)
print(words)  # ['fig', 'kiwi', 'apple', 'banana']

nums = [3, 1, 4, 1, 5]
print(sorted(nums, reverse=True))  # [5, 4, 3, 1, 1]
print(nums)  # unchanged: [3, 1, 4, 1, 5]

تعديل القوائم أثناء التكرار

# WRONG — skips elements
nums = [1, 2, 3, 4, 5]
for n in nums:
    if n % 2 == 0:
        nums.remove(n)  # skips 4!
print(nums)  # [1, 3, 5]... but 4 got skipped? Actually removes 2,4

# CORRECT — build new list
nums = [1, 2, 3, 4, 5]
nums = [n for n in nums if n % 2 != 0]
print(nums)  # [1, 3, 5]

تفكيك Tuples في الحلقات

nums = [10, 20, 30]
for i, v in enumerate(nums):
    print(i, v)
# 0 10 / 1 20 / 2 30

a = [1, 2, 3]
b = ['x', 'y', 'z']
for x, y in zip(a, b):
    print(x, y)

first, *rest = [1, 2, 3, 4]
print(first, rest)  # 1 [2, 3, 4]

تحقق سريع

مراجعة الدرس