Reverse a string with a single loop

Question

I want to reverse a string by only using a single loop. How can I do it? I tried one simple way but doesn't get the output I wish.

#include<iostream>
using namespace std;

int main()
{
    int i;
    char a[10];

    cout << " Enter String :";
    cin >> a;

    for(a[i] = '\0'; i > 0; i--)
    {   
        cout << "Reverse string is" << a[i];
    }

    return 0;
}

`a[i]='\0'` is undefined behaviour. It should be `for(i=9; i>=0; i--)` — perivesta, Apr 08 '19 at 08:14
One of the possible algorithms is swapping string characters: first and last, second and penultimate and so on. — vahancho, Apr 08 '19 at 08:14
`a[i] = '\0'` is wrong in that place, just as `x*x = 4` will not magically set `x` to `2` — 463035818_is_not_an_ai, Apr 08 '19 at 08:20
@mch I was using `i=9` because the array has length 10. Really, this is a good example of why to use `std::string` since it has a `length()` function. — perivesta, Apr 08 '19 at 08:27
@dave yes, but in this case it should be `i=8`, `a[9]` is the nul-terminator. — mch, Apr 08 '19 at 08:36

Michiel uit het Broek · Answer 1 · 2019-08-12T08:04:56.580

Note that you do not initialize your iterator variable i and that you could declare the index within the for-statement as you do not need this variable outside the scope of the for-loop. Furthermore, you can also use a std::string instead of a char array, this also allows for strings longer than the ten characters as you had specified.

EDIT: I assume that you can use C++17 or newer. From C++17 they finally added the non-member version std::size (which I prefer over the member related versions since I read the book effective modern c++ from Scott Meyers). If, for some reason, you cannot use this function, then you can just replace it by the string member function length, i.e., a.length().

If you only want to print the reverse of the string you could use something like the following

int main()
{
    string a;

    cout << "Enter String: ";
    cin >> a;

    for (int idx = size(a) - 1; idx >= 0; --idx)
      cout << a[idx];
    cout << endl;
}

You may also use the built-in reverse function (see description here). This function actually reverses the string itself.

int main()
{
    std::string a;

    std::cout << "Enter String: ";
    std::cin >> a;

    std::reverse(std::begin(a), std::end(a));

    std::cout << a << endl;
}

Another option is to manually reverse the string (which doesn't make sense since std::reverse is part of the standard library, but as a learning exercise it might be helpful). Note that we only iterate to half the size of the string as otherwise each element is swapped twice, resulting in the original string.

int main()
{
  string a;

  cout << "Enter String: ";
  cin >> a;

  for (size_t idx = 0, n = size(a); idx < n / 2; ++idx)
    swap(a[idx], a[n - idx - 1]);
  cout << a << endl;
}

@mch did know that either, see here https://en.cppreference.com/w/cpp/iterator/size — 463035818_is_not_an_ai, Apr 08 '19 at 08:30
Uh, C++17, I learned something, have my +1, but it should be `size(a) - 1`, otherwise the first output would be the nul-terminator. — mch, Apr 08 '19 at 08:35
@mch `size` and `length` are synonyms I believe. Next, indeed the `-1` was missing, sometimes answering too fast results in unnecessary typos. — Michiel uit het Broek, Apr 08 '19 at 08:37

Reverse a string with a single loop

1 Answers1