HackerRank C++ Solution - Simple Text Editor

I JustWriteTheCode of the solution to the "Simple Text Editor" problem present on HackerRank (1 Week Preparation Kit - Day 6).
Programming Language: C++.
Problem:
Implement a simple text editor. The editor initially contains an empty string, S. Perform Q operations of the following 4 types:
1. append(W) - Append string W to the end of S.
2. delete(k) - Delete the last k characters of S.
3. print(k) - Print the k-th character of S.
4. undo() - Undo the last (not previously undone) operation of type 1 or 2, reverting S to the state it was in prior to that operation.

[@harryl2757]

why does for loop condition is (....; i