Walkthrough: List Basics

例题: 列表入门

≈ 8 min · python-lists · Open in WolfCode

Five tiny operations every later lesson reuses. Index, slice, mutate, copy, len.

The five functions

Python · runnable

def first_element(t):     return t[0]
def last_element(t):      return t[-1]
def first_three(t):       return t[0:3]
def length(t):            return len(t)
def with_first_replaced(t, new_value):
    copy = t[:]
    copy[0] = new_value
    return copy

numbers = [42, 13, 7, 99, 100]
print("First :", first_element(numbers))
print("Last  :", last_element(numbers))
print("First3:", first_three(numbers))
print("Length:", length(numbers))
print("Replaced:", with_first_replaced(numbers, 1000))
print("Original:", numbers, "(unchanged)")

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Negative indexing

Python · runnable

t = [42, 13, 7, 99, 100]
#  idx:  0   1   2   3    4
#  neg: -5  -4  -3  -2   -1
print(t[0], t[-1])           # 42  100
print(t[-2], t[-3])          # 99  7

# Out of range raises IndexError:
try:
    t[5]
except IndexError as e:
    print("error:", e)

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