IndexError: string index out of range - what is the reason?

Question

Python

Good day!
I'm a beginner, I'm trying to write a program that will find duplicate letters in a string, count their number in a row, output something like this: s = 'aaaabbсaa' is converted to 'a4b2с1a2'.
But at the output I get the error IndexError: string index out of range. The error occurs after using the for in loop.
Please help, I am attaching the code below.

genome = 'aaaabbcaa'
a = 1
s = 1
g = ''
for i in genome:
    if genome[a-1]==genome[a]:
        a += 1
        s += 1
    else:
        g += (genome[a-1]+str(s))
        a += 1
        s += 1
print(g)
----------------------------------------------------------------------------
IndexError                                Traceback (most recent call last)
<ipython-input-176-241c9dc2be5a> in <module>
      4 g = ''
      5 for i in genome:
----> 6     if genome[a-1]==genome[a]:
      7         a += 1
      8         s += 1

IndexError: string index out of range

Answer 1

HelloDarknessMyOldFried , I would suggest

following code

genome = 'aaaabbcaa'
cnt = 0
res = ''
for i in range(len(genome)):
  newSymb = i == 0 or genome[i] != genome[i - 1]
  if newSymb:
      if cnt > 0: g += str(cnt)
      res += genome[i]
      cnt = 1
  else:
      cnt += 1
res += str(cnt)

print(res)

Answer 2

Alternatively, you can also consider my version:

spoiler

genome = 'aaaabbcaa'
gn = list(genome)
arr = []
text = ''
ln = len(gn)
for i in range(ln):
  a = 0
  for j in range(ln-1):
    if gn[i] == gn[j+1]:
      a +=1	
  text = gn[i] + str(a)
  arr.append(text)

arr = ''.join(arr)
print (arr)

The only thing left to do is remove the duplicate elements and do something with the last 'aa'. Good luck!