循環遍歷 Python 中的列表項

Question

一個簡單的問題，我試圖遍歷這個列表的每個元素，列表本身形成為一個列表列表..

Price = [500, 300, 100, 200]
list_of_pairs = [[1, 1], [2, 2], [1, 1], [2, 2]]

p_11 = []
p_22 = []
p_33 = []

for i in list_of_pairs:
    if i[0] == 1 and i[1] == 1:
        p_11.append(Price[list_of_pairs.index(i)])

    elif i[0] == 2 and i[1] == 2:
        p_22.append(Price[list_of_pairs.index(i)])

    elif i[0] == 3 and i[1] == 3:
        p_33.append(Price[list_of_pairs.index(i)])


print(list_of_pairs)

print(p_11)
print(p_22)

問題是當我遍歷列表並嘗試通過它的第一個和第二個值對每個元素（子列表）進行分類時，它看起來像這樣：

#the output:
[[1, 1], [2, 2], [1, 1], [2, 2]]
[500, 500]
[300, 300]

#what I expected(wanted) for the output:
[[1, 1], [2, 2], [1, 1], [2, 2]]
[500, 100]
[300, 200]

Answer 1

您可以使用enumerate而不是list_of_pairs.index來獲取當前元素的索引：

for j, i in enumerate(list_of_pairs):
    if i[0] == 1 and i[1] == 1:
        p_11.append(Price[j])

    elif i[0] == 2 and i[1] == 2:
        p_22.append(Price[j])

    elif i[0] == 3 and i[1] == 3:
        p_33.append(Price[j])

然而，這並不是一個很好的解決方案，也許做同樣事情的更好方法可能是這樣的：

price = [500, 300, 100, 200]
list_of_pairs = [(1, 1), (2, 2), (1, 1), (2, 2)]

# Dictionary that stores the prices for all pairs
p = {}

# Iterate over all prices and pairs at the same time
for price, pair in zip(price, list_of_pairs):
    if pair not in p:
        p[pair] = [price]
    else:
        p[pair].append(price)

print(p[(1,1)]) # [500, 100]
print(p[(2,2)]) # [300, 200]

Answer 2

嘗試使用enumerate直接獲取索引，它更容易、更快：

Price = [500, 300, 100, 200]
list_of_pairs = [[1, 1], [2, 2], [1, 1], [2, 2]]

p_11 = []
p_22 = []
p_33 = []

for index,i in enumerate(list_of_pairs):
    if i[0] == 1 and i[1] == 1:
        p_11.append(Price[index])

    elif i[0] == 2 and i[1] == 2:
        p_22.append(Price[index])

    elif i[0] == 3 and i[1] == 3:
        p_33.append(Price[index])


print(list_of_pairs)

print(p_11)
print(p_22)

Answer 3

因為index返回list一個匹配的值的索引，因為你有重復，你得到錯誤的值，使用enumerate獲取索引：

Price = [500, 300, 100, 200]
list_of_pairs = [[1, 1], [2, 2], [1, 1], [2, 2]]

p_11 = []
p_22 = []
p_33 = []

for i, (a,b) in enumerate(list_of_pairs):
    if a == 1 and b == 1:
        p_11.append(Price[i])

    elif a == 2 and b == 2:
        p_22.append(Price[i])

    elif a == 3 and b == 3:
        p_33.append(Price[i])


print(list_of_pairs)
>>> [[1, 1], [2, 2], [1, 1], [2, 2]]

print(p_11)
>>> [500, 100]

print(p_22)
>>> [300, 200]