Answer 1

After inserting, the str length will increases by 1 so you need to track that also in loop.

Answer 2

Manav Roy
How about this?

https://code.sololearn.com/cvO2jZ2Wsqaz/?ref=app

it is not very robust though. It fails on some negative numbers.
It is suggested to use <locale.h> for more specific separators, like in India.

Answer 3

Manav Roy Are you sure,?
You don't having camma inserted so is not it increase length?
See 1000 (len=4) after insert 1,000 (len=5, with camma added)...!