Linux终端中C ++的奇怪输出

Question

I've recently started learning programming using the C++ language. I wrote a simple program that is supposed to reverse a string which I compile in the Terminal using gcc/g++.

#include <iostream>
using namespace std;

string reverse_string(string str)
{
    string newstring = ""; 
    int index = -1;
    while (str.length() != newstring.length())
    {
        newstring.append(1, str[index]);
        index -= 1;
    }
    return newstring;
}

int main()
{
    string x;
    cout << "Type something: "; cin >> x;
    string s = reverse_string(x);
    cout << s << endl;
    return 0;
}

I've rewritten it multiple times but I always get the same output:

Type something: banana
��

Has anyone had a problem like this or know how to fix it?

Answer 1

Your code initializes index to -1, and then uses str[index] but a negative index has no rational meaning in C++. Try instead initializing it like so:

index = str.length() - 1;

Answer 2

I can see several issues with your code. Firstly, you are initializing index to -1 , and then decrementing it. Maybe you meant auto index = str.length()-1; ?

I recommend you look at std::reverse , which will do the job you're after.

Your main function then becomes:

#include <algorithm>
#include <iostream>
#include <string>

using namespace std;

int main()
{
    string x;
    cout << "Type something: ";
    cin >> x;
    reverse(x.begin(), x.end());
    cout << x << endl;
    return 0;
}

If you really want to write your own reverse function, I recommend iterators over array indices. See std::reverse_iterator for another approach.

Note , the above will simply reverse the order of bytes within the string. Whilst this is fine for ASCII, it will not work for multi-byte encodings, such as UTF-8.

Answer 3

You should use a memory debugger like valgrind. It's a good practice to scan your binary with it, and will make you save so much time.