CODEWITHSUNDEEP
python
Yaekiou
Yaekiou

Reputation: 17

How to update the value in for loop?

I don't know how to update the value. I want next value is only greater 1 than previous value.

In this code 4 will change to 3, great. Since 4 changed to 3 so I expect 5 will change to 4 ...

my_list = [1, 2, 4, 5, 6, 7, 1, 2, 3, 4, 5]

for i, curr_item in enumerate(my_list):
    prev_item = my_list[i - 1] if i >= 1 else 0

    if curr_item - prev_item > 1:
        print(curr_item)  # -> 4

        curr_item = prev_item + 1

I got this

[1, 2, 3, 5, 6, 7, 1, 2, 3, 4, 5]

expected

[1, 2, 3, 4, 5, 6, 1, 2, 3, 4, 5]

Upvotes: 0

Views: 1036

Answers (2)

umläute
umläute

Reputation: 31304

you are setting the curr_item (and i) with the for clause in each iteration. you are free to do whatever you want with those variables, but at the beginning of the next iteration they will be set again.

i'm just re-writing your code to make this a bit clearer:

for x, y in enumerate(my_list):
    i = x
    curr_item  = y

    prev_item = my_list[i - 1] if i >= 1 else 0
    if curr_item - prev_item > 1:
        curr_item = prev_item + 1

so whatever you do to curr_item in your loop body will be lost in the next iteration.

Upvotes: 0

j1-lee
j1-lee

Reputation: 13939

You need to update my_list[i] (not curr_item):

my_list = [1, 2, 4, 5, 6, 7, 1, 2, 3, 4, 5]

for i, curr_item in enumerate(my_list):
    prev_item = my_list[i - 1] if i >= 1 else 0

    if curr_item - prev_item > 1:
        print(curr_item)  # -> 4

        my_list[i] = prev_item + 1

print(my_list) # [1, 2, 3, 4, 5, 6, 1, 2, 3, 4, 5]

Writing curr_item = prev_item + 1 assigns prev_item + 1 to the name curr_item; here, curr_item is not something like a pointer in C.

If you want a non-destructive way, the following makes a new list:

output = []
prev = 0
for x in my_list:
    if x - prev > 1:
        prev = prev + 1
    else:
        prev = x
    output.append(prev)
print(output) # [1, 2, 3, 4, 5, 6, 1, 2, 3, 4, 5]

Alternatively, using list comprehension and walrus operator:

prev = 0
output = [prev := (prev + 1 if x - prev > 1 else x) for x in my_list]
print(output) # [1, 2, 3, 4, 5, 6, 1, 2, 3, 4, 5]

Upvotes: 2

Related Questions