#include using namespace std; /* string s2 = "sololearn"; 0 1 2 3 4 5 6 7 8 (normal index) ------------------------------------- s | o | l | o | l | e | a | r | n | ------------------------------------- -9 -8 -7 -6 -5 -4 -3 -2 -1 (python index) What "_substr(st, -1)" does is that will return all the characters from the index -1 upto the end. for example, _substr(s2, -1) will give us "n". _substr(s2, -5) will give us "learn". Don't you think that's cool! :) */ int main(){ auto _substr = [](string st, int minus_idx){ int n = st.length(); return st.substr(((minus_idx%n)+n)%n); }; string s1 = "abcdefghijklmnopqrstuvwxyz"; cout << s1 << ":\n"; cout << _substr(s1, -1) << "\n"; cout << _substr(s1, -2) << "\n"; cout << _substr(s1, -3) << "\n"; cout << _substr(s1, -4) << "\n"; cout << _substr(s1, -5) << "\n"; cout << _substr(s1, -6) << "\n"; cout << _substr(s1, -7) << "\n"; cout << _substr(s1, -8) << "\n\n"; string s2 = "sololearn"; cout << s2 << ":\n"; cout << _substr(s2, -1) << "\n"; cout << _substr(s2, -5) << "\n"; cout << _substr(s2, -(s2.length())) << "\n\n"; //lets try doing it with the normal substr string s3 = "programming"; cout << s3 << ":\n"; try{ cout << s3.substr(-1); } catch(const exception& e) { cout << "does not work! -> "; cout << e.what() << "\n"; } } /* Explanation for why we cannot achieved this with normal substr in cpp? "If pos is greater than the string length, an out_of_range exception is thrown." substr(pos, len) where pos = position to start slicing the string from. len = how many characters to be slice. The default value of pos is 0 and from the documentation of cpp we know that the value of pos cannot be less than 0(that is cannot be negative) and cannot be greater than the length of the string itself. But since in this case we are trying to implement Python slicing feature with negative number and since we cannot do with normal substr that is why we have this line here, st.substr(((minus_idx%n)+n)%n) to be able to slice using negative number that is from the end of the string. */

negative_idx_substr.cpp