Question from coding bat practice

Question

My question:
why is the
result += str[:i+1]
instead of
result += str[:i]

from my understanding
#when i = 0 result = str[0:0] = C
#when i = 1 result = C + str[0:1] = C + CO
#when i =2 result = C + CO + str [0:2] = C + CO + COD
#when i = 3 result = C + CO + COD + str[0:3]
result= CCOCODCODE
https://code.sololearn.com/c3Djr94rOEk3/?ref=app

Answer 1

range(x) generates numbers from 0 up to x-1 (x is not included)
When you slice the string, str[:i] takes the indexes from 0 up to i-1 (i is not included and remember index starts with 0)

Answer 2

C: 0
O: 1
D: 2
E: 3
range(x) = 0, 1, 2, 3
result = result + str(:i) means i is not included
in order to include i , str(:i+1)