在迭代列表时执行操作

Question

I am iterating over a list

xxx
yyy
**start word**
xxx
yyy
zzz
**stop word** 
break

I need to append to another list all the data between start and stop words, how do I do that?

The stop word appears couple times in the list. So the appending should be stopped when loop finds first stop word on it's way.

For example:

list = [1,2,3 ... 1000]
new_list = []
for i in list:
    # Once i = 5 I need to start appending i values to new_list until i = 25.

Answer 1

You can maintain a boolean to indicate when to start appending and when to stop appending. For this, you could write your code something like -

old_list = ['axz','bbbdd','ccc','start','Hello World','Bye','end','ezy','foo']
another_list=[]

append_to_list = False     # Boolean to indicate if we should append current element
start_word = 'start'
end_word = 'end'
for element in old_list:
    if element == end_word :
        append_to_list = False
    if append_to_list :    # Appending to list if the Boolean is set
        another_list.append(element)
    if element == start_word :
        append_to_list = True


print(another_list)
    

Output:

['Hello World', 'Bye']

Here, start and end are the start and stop words, you could modify them as per your start and stop words of the program.

---

Another possible solution would be to fetch the index of your start and stop words and just store the elements between those indexes into your another_list as follows -

old_list = ['axz','bbbdd','ccc','start','Hello World','Bye','end','ezy','foo']

start_idx = old_list .index("start")
stop_idx = old_list .index("end")

another_list = old_list[start_idx+1:stop_idx]

print(another_list)
    

Output:

['Hello World', 'Bye']

Hope this helps !

Answer 2

Would be great to get more information, but from what you've provided, you could use the index of your "start" and "stop" words to append to a new list:

list1 = ["xxx", "yyy", "start_word", "xxx", "yyy", "zzz", "end_word"]

a = list1.index("start_word")
b = list1.index("end_word")

list2 = []
list2.append(list1[a:b])

print(list2)

Output:

[['start_word', 'xxx', 'yyy', 'zzz']]