python: Add list elements based on original indexing (while updating list)

Question

I have a sequence like this:

ABCDEFGHIJKL

I would like to insert the strings:

'-(c0)-' after elements 1 and 3
'-(c1)-' after elements 2 and 4
'-(c2)-' after elements 5 and 6

This is the kind of code I was writing:

list_seq = list('ABCDEFGHIJKL')
new_list_seq = list('ABCDEFGHIJKL')
start_end = [(1,3), (2,4), (5,6)]
for index,i in enumerate(start_end):
    pair_name = '-(c' + str(index) + ')-'
    start_index = int(i[0])  
    end_index = int(i[1]) 
    new_list_seq.insert(start_index, pair_name)
    new_list_seq.insert(end_index, pair_name)
print ''.join(new_list_seq)

The output I would want is:

AB-(c0)-C-(c1)-D-(c0)-E-(c1)-F-(c2)-G-(c2)-HIJKL

(where c0 was inserted after 1 and 3 positions, c1 was inserted after 2 and 4, and c2 was inserted after 5 and 6).

But the output I get is:

A-(c0)--(c1)-B-(c1)--(c2)--(c2)--(c0)-CDEFGHIJKL

I think possibly the problem is that as I incorporate an item into the string, the indices change, so then all subsequent inclusions after the first one are not in the right positions?

Could anyone explain how to do this properly?

You could also just insert your elements the other way round from last to first. then you would not change the target index of the next item to be inserted. — SpghttCd
– SpghttCd, Commented Aug 22, 2018 at 12:12

SpghttCd · Accepted Answer · 2018-08-22 12:50:52Z

1

Based on @r.user.05apr's very good idea to simply step through the whole input string char by char, I'd like to add a possibility to generalize this for an arbitrary long start_end-list:

s = 'ABCDEFGHIJKL'
res = list()
for nr, sub in enumerate(s):
    res.append(sub)
    try:
        i = [nr in x for x in start_end].index(True)
        res.append('-(c' + str(i) + ')-')
    except:
        pass
res = ''.join(res)        
print(res)    
# AB-(c0)-C-(c1)-D-(c0)-E-(c1)-F-(c2)-G-(c2)-HIJK

edited Aug 22, 2018 at 12:50

answered Aug 22, 2018 at 12:38

SpghttCd

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r.user.05apr · Accepted Answer · 2018-08-22 12:18:06Z

1

Hope it helps:

s = 'ABCDEFGHIJKL'
res = list()
for nr, sub in enumerate(s):
    res.append(sub)
    if nr in (1, 3):
        res.append('-(c0)-')
    elif nr in (2, 4):
        res.append('-(c1)-')
    elif nr in (5, 6):
        res.append('-(c2)-')
res = ''.join(res)        
print(res)    
# AB-(c0)-C-(c1)-D-(c0)-E-(c1)-F-(c2)-G-(c2)-HIJKL

answered Aug 22, 2018 at 12:18

r.user.05apr

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