cout字符串不会在屏幕上打印任何内容

Question

Its a simple string manipulation question.Shift each chara in the string by 2 positions to the right.

I used a second string to copy the characters from first string to the desired position in the second string. But cout ain't printing the second string. If I try to print the string like in the comment it work. BUt why cout<<dups won't work?

#include<iostream>
 #include<string>
 using namespace std;
 int main()
 {
     string s;
     string dups;
     cin>>s;
     int k;
     cin>>k;
     for(int i=0;s[i];i++)
     {
         int idk = (i+k)%6;
         dups[idk] = s[i];
     }
      dups[6] = '\0';
      cout<<dups;


     /*for(int i=0;dups[i];i++)
        cout<<dups[i]; */
 }

input: hacker 2 output: erhack

first cout ain't printing anything. if i do like in the comments it works.

Answer 1

You have undeined behavior everywhere because you default initialized your string(zero size) and access out of bounds:

string dups;

One fix is

string s;
cin>>s;
string dups = s;

Answer 2

if i do like in the comments it works.

You may have successfully written to some memory location when you did this:

string dups;              // dups.size() == 0 after this
...
    dups[idk] = s[i];     // You write to memory you don't "own".

And you may be able to retrieve the same data when extracting it like this:

for(int i=0;dups[i];i++)  
    cout<<dups[i];        // dups.size() is still 0

But it's just appearing to work. The memory you've written to may be overwritten by just about anything so you can't expect it to give you the correct result. The behaviour of the program is undefined. Anything can happen.

You can fix it by resizing dups when you know how many characters it needs to contain:

cin>>s;
dups.resize(s.size());