C++ array of fixed size getting garbage values

Question

I was trying to rotate a string circularly by 1 shift. Here's the simple code:

#include <bits/stdc++.h>
using namespace std;

int main() {
  string s = "1010";
  char a[s.length()];
  for (int i = 0; i < s.length() - 1; i++) {
    a[i] = s[i+1];
  }
  a[s.length()-1] = s[0];
  std::cout << a << '\n';
  std::cout << strlen(a) << '\n';
}

When I run this code, I get the following output:

aditya@aditya-Inspiron-3558:~/miscCodes$ ./a.out 
0101�
6

How's my array's length is changing? And due to garbage values?

Answer 1

Variable Length Arrays (VLA) are not part of Standard C++. Read more in Why aren't variable-length arrays part of the C++ standard?

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But let's say, that you use a compiler extension that allows them, then the problem is that you use a C function here:

std::cout << strlen(a) << '\n';

which expects a C string, which means a NULL-terminated string.

That means that you should make your array big enough to hold the NULL termination character, like this:

char a[s.length() + 1];

since string::length() will return 4 for the string "1010". That means that the C string should be this: "1010\0", i.e. the actual string, appended with the NULL terminating character. As a result, you would need an array of size 5 to store that string.

An easy fix would be:

char a[s.length() + 1] = {0};

which will NULL initialize every cell of the array. Then you will overwrite every cell of it with characters, except from the last cell, specially reserved for the NULL terminator.

Another approach would be to only assign the NULL terminator to the last cell of your string, like a[s.length()] = '\0';. Notice how s.length() now is the index to the last element of your array.

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Standard C string functions (like strlen()) depend on the NULL termination character to mark the end of the string. In the absence of that important character, they do not know when to stop, thus accessing memory beyond from the point they are meant to visit.

That invokes Undefined Behavior (UB), which, in your computer, is accessing memory with garbage values.

Answer 2

First, your code is not valid C++, as you are using Variable Length Arrays. Use an std::vector instead:

std::vector<char> a(s.length() + 1, 0); // Adding +1 to add space for the 0-terminated string

for (int i = 0; i < a.size() - 2; i++) { // Because of the null terminator and the first offset
  a[i] = s[i+1];
}
a[a.size()-2] = s[0];

Then after output using data:

std::cout << a.data() << '\n';
std::cout << a.size() << '\n'; // Will give you +1 because we give the size of the container and not thew size of the string

Answer 3

You've already gotten excellent answers so this is just an add-on.

#include <bits/stdc++.h> is not a standard header file and it will make you lazy since it includes all that you probably need and a lot more. Include only the headers you need, especially if you are going to do using namespace std;, which is also a bad idea.

Apart from that, to solve the problem of rotating elements in your string, take a look at std::rotate. It can rotate not only strings, but std::vectors etc. Example doing a left rotate one step:

#include <iostream>
#include <algorithm> // std::rotate

int main() {
    std::string s = "1234";
    std::string cpy = s;

    //          first elem   new first elem   last elem
    std::rotate(cpy.begin(), cpy.begin() + 1, cpy.end());
    std::cout << s.size() << " " << s << "\n";
    std::cout << cpy.size() << " " << cpy << "\n";
}

Output

4 1234
4 2341

Answer 4

Instead of this char a[s.length()];

Do this char a[s.length() + 1]