为什么它插入不止一次？

Question

Trying to insert " _ " before uppercase letters but it inserts more than once.

Instead of ["h", "e", "l", "l", "o", " _ ", "W", "o", "l", "r", "d"] it prints ['h', 'e', 'l', 'l', 'o', ' _ ', ' _ ', ' _ ', ' _ ', 'W', 'o', 'l', 'r', 'd']

new = ["h", "e", "l", "l", "o", "W", "o", "r", "l", "d"]

for i in range(len(new)):
    print(i)
    if new[i].isupper():
        new.insert(i, "_")

print(new)

Answer 1

Because after you insert '_' at index i , the uppercase letter ( W ) is moved to the right and thus gets the index i+1 . Now the for loop iteration completes, and i gets incremented and becomes i+1 , but guess what is at index i+1 ? (Hint: W ) So, this is a cycle that only exhausts when the for loop iterates len(new) times. Below is what the execution might look like:

• (i=0): new[i] is 'h'
• (i=1): new[i] is 'e'
• (i=2): new[i] is 'l'
• (i=3): new[i] is 'l'
• (i=4): new[i] is 'o'
• (i=5): new[i] is 'W' --> insert '_' --> 'W' moves to new[i+1]
• (i=6): new[i] is 'W' --> insert '_' --> and this pattern repeats

A simple solution would be to run backwards. As now, insert '_' will push 'W' to the right, while we are moving to the left; therefore, we won't end up in the cycle like before.

new = ["h", "e", "l", "l", "o", "W", "o", "r", "l", "d"]
start = len(new) - 1 # last element
end = -1 # one prior to the first element
         # remember, range end is non-inclusive,
         # so it goes till index 0
step = -1 # decrement to move backwards
for i in range(start, end, step):
    if new[i].isupper():
        new.insert(i, "_")
print(new)

Answer 2

You can create a new list:

new = ["h", "e", "l", "l", "o", "W", "o", "r", "l", "d"]

result = []
for i in range(len(new)):
    if new[i].isupper():
        result.append("_")
    result.append(new[i])

print(result)

Answer 3

In your code, each call to new.insert(i, "_") changes new so that the uppercase letter that was previously at index i is now at index i + 1 , since you have just inserted the character _ at index i and pushed he uppercase letter (and every item that comes after it in the list new ) one position higher.

One way to fix this would be to loop through new in reverse order using python's built-in functionreversed() :

new = ["h", "e", "l", "l", "o", "W", "o", "r", "l", "d"]

for i in reversed(range(len(new))):
    #print(i)
    if new[i].isupper():
        new.insert(i, "_")

print(new)

Now, after examining the character at index i , detecting that it's uppercase, and inserting _ at i , the next loop iteration examines the character at position i - 1 , which was unaffected by the prior iteration's call to new.insert(i, "_") .

Output:

['h', 'e', 'l', 'l', 'o', '_', 'W', 'o', 'r', 'l', 'd']