如何在 Python 的 for 循环中更改列表元素的位置

Question

I'm fairly new to programming and I'm really stuck in a problem.

Say I have the following list:

a = [2, "**", 2, "@"]

The list might contain more elements.

What I need to do is change positions between "**" and "@", so I would have

a = [2, "@", 2, "**"]

I've been trying to do it with a for loop, using element indexes to perform the change, but the list index is getting out of range.

How could I do it?

Here's my code:

for j in range (len(expression)):
    if expression[j] == "**":
        if expression[j+2] == "@":
            expression[j], expression[j+2] = expression[j+2], expression[j]
print(expression)

Answer 1

My comment in an answer (altho pretty simple tbf)

>>> expression = [2, "**", 2, "@", "**"]
>>> for j in range (len(expression)):
...     if expression[j] == "**":
...         if (
...             len(expression) > j +2 
...             and expression[j+2] == "@"
...         ):
...             expression[j], expression[j+2] = expression[j+2], expression[j]
... 
>>> print(expression)
[2, '@', 2, '**', '**']

Explanation : if the current value is ** you are attempting to access j+2 . However, your list might not have that index (for example what if ** is the last element?). To cater for this case I extend your if statement to first check for length and then check for j+2 values. If/when the first check/condition fails, the second condition is skipped (not checked) and thus the IndexError does not happen.

(updated the input list to show that even ** at the end of the list wont raise an error)

Answer 2

Try the below

# assuming "**" and "@" apper once in the list
a = [2, "**", 2, "@"]
idx_1 = a.index("**")
idx_2 = a.index("@")
# I need to change if and only if ** and @ are separated by another element
if abs(idx_1 - idx_2) > 1:
  del a[idx_1]
  a.insert(idx_1,"@")
  del a[idx_2]  
  a.insert(idx_2,"**")
print(a)

Answer 3

I thought about just getting the positions out of the list and then swapping them.

for idx , j in enumerate(a):
    if (j == "**"):
        pos1 = idx
    elif (j=="@"):
        pos2 = idx
    
a[pos1],a[pos2] = a[pos2],a[pos1]