IndexError：字符串索引超出范围？

Question

I keep getting an IndexError: string index out of range exception on the following code and I cannot figure out why.

I am supposed to solve the following problem recursively.

Finally, write transcribe( S ) . Here is its description:

In an incredible molecular feat called transcription, your cells create molecules of messenger RNA that mirror the sequence of nucleotides in your DNA. The RNA is then used to create proteins that do the work of the cell. Write a recursive function transcribe( S ) , which should take as input a string S , which will have DNA nucleotides (capital letter As, Cs, Gs, and Ts). There may be other characters, too, though they will be ignored by your transcribe function—these might be spaces or other characters that are not really DNA nucleotides.

Then, transcribe should return as output the messenger RNA that would be produced from that string S. The correct output simply uses replacement:

As in the input become Us in the output.
Cs in the input become Gs in the output.
Gs in the input become Cs in the output.
Ts in the input become As in the output.
any other input characters should disappear from the output altogether

def transcribe( S ):
  tran = '' * len(S)
  if S[0] == 'A':
    tran = tran + 'U' + transcribe( S[1:] )
    return tran
  elif S[0] == 'C':
    tran = tran + 'G' + transcribe( S[1:] )
    return tran
  elif S[0] == 'G':
    tran = tran + 'C' + transcribe( S[1:] )
    return tran
  elif S[0] == 'T':
    tran = tran + 'A' + transcribe( S[1:] )
    return tran
  else:
    return ""

print transcribe('GATTACA')

Answer 1

Trying to index with [0] into an empty string raises an IndexError .

You need to test if the string is empty first , as eventually S[1:] will pass an empty sting into transcribe() :

if not S:
    return ''

You should drop the tran string altogether; that is always an empty string. '' * <any number> is still '' .

Rather than test each possible letter in a separate if statement, use a dictionary to map one character to another:

mapping = {'A': 'U', 'C': 'G', 'G': 'C', 'T': 'A'}
def transcribe(S):
    if not S:
        return ''
    replacement = mapping.get(S[0], '')  # default is empty
    return replacement + transcribe(S[1:])

Answer 2

Here's something a bit closer to your original:

def transcribe( S ):
    if S == '':
        return ''
    elif S[0] == 'A':
        return 'U' + transcribe( S[1:] )
    elif S[0] == 'C':
        return 'G' + transcribe( S[1:] )
    elif S[0] == 'G':
        return 'C' + transcribe( S[1:] )
    elif S[0] == 'T':
        return 'A' + transcribe( S[1:] )
    else:
        return transcribe( S[1:] )

print transcribe('GATTACA')