string compare problem

Code:

int main() {
	string s1 = "", song = "blablabla.mp3", outputsong( song, 0, song.length()-4 );
	int j = 0, i = 0;
	while(song[j++]!='.');
	while(song[j]) {
		s1[i++] = song[j++];
	}

	if(s1=="mp3") {                             //Please, check this line!!!!
		outputsong += ".ogg";
	}
	else if(s1=="ogg") {
		outputsong += ".mp3";
	}
	else {
	}

        cout << song << endl << outputsong << endl;

	return 0;
}

What make me confuse is the output of the program is:
blablabla.mp3
blablabla
What I expected from this program is:
blablabla.mp3
blablabla.ogg

When I check this program with gdb:
After this statement executed:

Code:

while(song[j]) {
		s1[i++] = song[j++];
}

(gdb) p s1
$18 = {static npos = 4294967295,
_M_dataplus = {<allocator<char>> = {<No data fields>},
_M_p = 0x804a4bc "mp3"}, static _S_empty_rep_storage = {0, 0, 1, 0}}

But why if(s1=="mp3") statement does not give true condition????

place a the line:


cout << s1 << endl;

between the end of the second loop, and the line:

if(s1 == "mp3")

to see what s1 is. I suspect s1 may be .mp3 and not just mp3 so the if statement is never identified. I also see no way for s1 to ever be ogg based on this code since the value of s1 is dependent on song and the value of song never changes in the code.

Surprising!!!!!
The output is:

blablabla.mp3
blablabla

So it means s1 = nothing. How come?
Here's my code again:

Code:

int main() {
	string song = "blablabla.mp3", outputsong( song, 0, song.length()-4 );
	string s1;
	int j = 0, i = 0, type;
	while(song[j++]!='.');
while(song[j]) {
s1[i++] = song[j++];
}

	cout << s1 << endl;

	if(s1=="mp3") {
		outputsong += ".ogg";
	}
	else if(s1=="ogg") {
		outputsong += ".mp3";
	}
	else {
	}

        cout << song << endl << outputsong << endl;

	return 0;
}

I found solution already with this code: string s1( song, song.length()-3, 3 );

But I am just curios. How come?

Originally Posted by gandalf_bar

Surprising!!!!!
The output is:

blablabla.mp3
blablabla

So it means s1 = nothing. How come?
Here's my code again:


I found solution already with this code: string s1( song, song.length()-3, 3 );

But I am just curios. How come?

This won't work with c++ strings:

Code:

while(song[j]) {
  s1[i++] = song[j++];
}

Use

Code:

s1 += song[j++];

Dave

Thank you!!!! It worked.