Does calculating a value in condition in C++ for loop in advance make the loop run much faster ?

my  goal is to get the right half of a string I enter

Example:
Input: abcd
Output:dc
Input:abcde
Output:ed

Here's Code One. It works fine

#include <iostream> #include <string> int main(void) {         std::string str;         std::cin >> str;         std::string::size_type nstr = str.size();         std::string str2;         for (std::string::size_type i = nstr - 1; i != ((nstr % 2 == 0) ? (nstr / 2 - 1) : (nstr / 2)); --i)                 str2.push_back(str[i]);         std::cout << str2 << std::endl;         return 0; }

Now I have modified the above code and get Code Two which also works fine.

#include <iostream> #include <string> int main(void) {         std::string str;         std::cin >> str;         std::string::size_type nstr = str.size();         std::string str2;         std::string::size_type nstr2 = (nstr % 2 == 0) ? (nstr / 2 - 1) : (nstr / 2);         for (std::string::size_type i = nstr - 1; i != nstr2; --i)                 str2.push_back(str[i]);         std::cout << str2 << std::endl;         return 0; }

I guess the Code Two works faster than Code One because the nstr2 is calculated in advance before the loop.

Otherwise, (nstr % 2 == 0) ? (nstr / 2 - 1) : (nstr / 2) lc needs to be calculated in each loop.

Question： Is my guess correct ?  And is it necessary in C++ pragramming?


https://stackoverflow.com/questions/3221577/what-is-a-loop-invariant