在 C++ 中反向迭代字符串的字符

Question

I want to print the reverse of a string

#include <iostream>
#include <string>
using namespace std;
int main()
{
    string str = "no", str1 = "yes";
    for (int i = str.size() - 1; i >= 0; i--)
        cout << str[i];
    for (int j = str1.size(); j >= 0; j--) //without minus one
        cout << str1[j];
}

What is the difference between the two loops ? When it gives null pointer ?

Answer 1

See here :

So in C++11 this should be fine as it prints the null character, but in earlier versions, it could also be undefined behavior. Which means that anything could happen, including any kind of error, or even the expected behavior.

If it's working on your system but not on the system you're doing the exam on, it's probably because the exam system uses an old version where it's undefined behavior. Or it is undefined behavior on both systems but just manifests in different ways.

Answer 2

@Blaze already answered why accessing str[str.size()] might cause problems.

One way to avoid these problems when iterating array-like is to use iterator - in your case reverse_iterator would do the trick:

std::string::reverse_iterator itr = str1.rbegin();

while (itr != str1.rend())
   std::cout << *itr++;