Python, divide string into several substrings

Question

I have a string of RNA i.e:

AUGGCCAUA

I would like to generate all substrings by the following way:

#starting from 0 character
AUG, GCC, AUA
#starting from 1 character
UGG, CCA
#starting from 2 character
GGC, CAU

I wrote a code that solves the first sub-problem:

for i in range(0,len(rna)):
  if fmod(i,3)==0:
    print rna[i:i+3]

I have tried to change the starting position i.e.:

 for i in range(1,len(rna)):

But it produces me the incorrect results:

 GCC, UA #instead of UGG, CCA

Could you please give me a hint where is my mistake?

Answer 1

The problem with your code is that you are always extracting substring from the index which is divisible by 3. Instead, try this

a = 'AUGGCCAUA'
def getSubStrings(RNA, position):
    return [RNA[i:i+3] for i in range(position, len(RNA) - 2, 3)]

print getSubStrings(a, 0)
print getSubStrings(a, 1)
print getSubStrings(a, 2)

Output

['AUG', 'GCC', 'AUA']
['UGG', 'CCA']
['GGC', 'CAU']

Explanation

range(position, len(RNA) - 2, 3) will generate a list of numbers with common difference 3, starting from the position till the length of the list - 2. For example,

print range(1, 8, 3)

1 is the starting number, 8 is the last number, 3 is the common difference and it will give

[1, 4, 7]

These are our starting indices. And then we use list comprehension to generate the new list like this

[RNA[i:i+3] for i in range(position, len(RNA) - 2, 3)]