Reverse index in a list

Question

For the list k1=[31.0, 72, 105.0, 581.5, 0, 0, 0], I would like to add a constant for example 100 to the first non-zero element in the reverse list.
this is what I want: newk1=[0, 0, 0, 681.5, 105, 72, 31]
As a beginner in Python I could not figure it out. Could you please help me. That is my code:

k1=[31.0, 72, 105.0, 581.5, 0, 0, 0]
Inverselist=[]


for i in range(len(etack1)):
    Inverselist.append(etack1[-(i+1)])
    print("Inverselist", Inverselist)
newk1=Inverselist
run_once = 0
while run_once < 1:      
    for j in  range(len(newk1)): 
        if newk1[j-1]>0: 
            newk1[j-1]=newk1[j-1]+100 
            run_once = 1 
            break
print("Newk1", newk1 )

Asked By: rezzz

||

Source

Answer 1

You can try this:

k1=[31.0, 72, 105.0, 581.5, 0, 0, 0]
new_list = []
flag = False
for i in k1[::-1]:
    if i > 0 and not flag:
        new_list.append(i+100)
        flag = True
    else:
        new_list.append(i)

Output:

[0, 0, 0, 681.5, 105.0, 72, 31.0]

Answered By: Ajax1234

Answer 2

I think you’re overthinking this:

First, reverse the list:

inverselist = k1[::-1]

Then, replace the first nonzero element:

for i, item in enumerate(inverselist):
    if item:
        inverselist[i] += 100
        break

Answered By: Tim Pietzcker

Answer 3

If you want to reverse, you can just do it by slicing. As below,

>>> a = [1,2,3]
>>> reverse_a = a[::-1]
>>> reverse_a
[3, 2, 1]

Once you go through the list, you just need to check when the first element is a non-zero element

k1=[31.0, 72, 105.0, 581.5, 0, 0, 0]
newk1= k1[::-1]
for i in range(len(newk1)):
    if newk1[i] != 0:
        newk1[i] += 100
        break
print("Newk1", newk1 ) #prints Newk1 [0, 0, 0, 681.5, 205.0, 172, 131.0]

Answered By: yash

Answer 4

Just a silly way. Modifies the list instead of creating a new one.

k1.reverse()
k1[list(map(bool, k1)).index(1)] += 100

Answered By: Stefan Pochmann

Answer 5

Here’s a solution using reversed instead of slicing with [::-1]:

items = [31.0, 72, 105.0, 581.5, 0, 0, 0]

inverse_items = []
found_non_zero = False
for item in reversed(items):
    if not found_non_zero and item:
        found_non_zero = True
        item += 100
    inverse_items.append(item)

Answered By: user8651755

Answer 6

list1 = [31.0, 72, 105.0, 581.5, 0, 0, 0]
last_idx = len(list1)-1
print(last_idx)
for i in range(len(list1)):
    list1.insert(i,list1.pop(last_idx))  
print(list1)

Output :

[0, 0, 0, 581.5, 105.0, 72, 31.0]

Answered By: vijay ananth

Reverse index in a list

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