Got Something to Work and Not sure why its working

[JamesHelp]

So i made a mistake in my code, but it did what i wanted it to do. But strangely i don't see the logic of how its working, in fact i would think it should have ended a never ending loop.

So the program takes a string, in this case "PLAYFAIR" and it is supposed to get rid of any duplicate letters,so "PLAYFAIR" becomes "PLAYFIR". When i was writing the program I kept getting "PLYFAIR" which is not what i wanted. Eventually i ran the code with "j--" in the second loop by mistake and i got the result that i wanted. However i'm confused as to why this works? The test case in the loop tests if j is smaller then the length of the string, then j is negated after each loop, so this test case should always be true and go indefinitely shouldn't it?

#include <iostream>
using namespace std;
int main()
{
	string key = "PLAYFAIR";
	bool keep = true;
	string keep_this = "";
	for (int i = 0; i < key.length(); i++)
	{
		for (int j = i ; j < key.length(); j--)
		{
			if (key.substr(i, 1) == key.substr(j , 1) && j != i)
			{
				
				keep = false;
			}
		}
		if (keep)
		{
			keep_this = keep_this + key.substr(i, 1);
		}
		keep = true;
	}
	cout << endl << keep_this << endl;
}

[Peter87]

int is a signed integer type.

key.length() gives you an unsigned integer type.

When you compare a signed type with an unsigned type that is at least as big the signed type will automatically be converted into the unsigned type. This means that the expression j < key.length() will first convert j to unsigned before comparing it to key.length(). Unsigned types can't represent negative values so when j becomes negative it will be converted to a very large positive value which is much greater than key.length() so that is why the expression returns false and the loop ends.

[Peter87]

Compilers will often warn about mixing signed and unsigned types in comparisons because they are so error prone. If you use GCC/MinGW or Clang you should at least use the -Wall compiler flag which enables this and many other warnings (-Wextra also enables many useful warnings).

[againtry]

Problem solved by the look of it.
Note also that the <string> class has many powers:

#include <iostream>
#include <string>

int main()
{
    std::string key = "ABRACADABRA";
    std::string keep_this;
    
    char ch;
    for (int i = 0; i < key.length(); i++)
    {
        ch = key[i];
        
        if( keep_this.find(ch) == std::string::npos )
            keep_this += ch;
    }
    std::cout
    << key << '\n'
    << keep_this << '\n';
}

ABRACADABRA
ABRCD
Program ended with exit code: 0

http://www.cplusplus.com/reference/string/string/find/