how does this palindrome exam works?

[ylxin1993]

h

	
char string[80];
cout << "Enter a stirng : " ;
cin.getline(string, 79);
int len;
for(len = 0; string[len] != '\0'; len++);

int i, j, flag = 1;
for(i = 0, j=0; i<len/2; i++, j--) { // I don't know this part
	if(string[i] != string[j]) {
		flag = 0;
		break;
	}
}

at the beginning, the for loop exam the string[0] != string[0], and next string[1] != string[-1], I don't understand what this meaning ,why in the beginning exam the two same char, and next exam the negetive index???

[pogrady]

because you are reversing the string. So one variable moves backwards while the other one moves forward.

[ylxin1993]

but I think the for loop should like this:

for(i = 0, j=len-1; i<len/2; i++, j--) 
	if(string[i] != string[j]) 

but use j = 0; also can do the exam, I don't understand the 0 means, I think j = len-1; is more easier to understand

[Edwards]

Perhaps to you, but when someone unfamiliar looks at the code and sees your for loop they will automatically know, oh J = 0 (they can pretty much skim past it). But if you put len-1, they will have to go back up and read what len is, why J is being put at len-1 and how len is used...

This is just me looking at it. It appears easier to read when J=0 from an outsider and not having to re-look at the same code twice...

Side Note: Most people, not all, do like to make things easier to read for everyone to understand, especially on bigger projects which multiple people are working. This way it's uniformly the same to everyone and not just easily read by one person.

[ResidentBiscuit]

Wouldn't this cause negative indices? Which is undefined behavior? Not to mention just weird, I agree with OP that j = len - 1 makes more sense.

[ylxin1993]

Hi, Edwards.
Though it's easier to read the j = 0; but what does j = 0; mean? in the for loop, first compare the string[0] != string[0]; and next loop, the index will be negative, what does this mean? I don't know what the negative index mean. Can you tell me?

Hi, ResidentBiscuit. I also think j = len -1 makes more sense, but I saw the video in YouTube, he use the j = 0, I can't understand, but when I use j = 0; to exam it, it works.(also j = len -1 works). I really wonder why

[guestgulkan]

Code as posted:

char string[80];
cout << "Enter a stirng : " ;
cin.getline(string, 79);
int len;
for(len = 0; string[len] != '\0'; len++);

int i, j, flag = 1;
for(i = 0, j=0; i<len/2; i++, j--) { // I don't know this part
	if(string[i] != string[j]) {
		flag = 0;
		break;
	}
}

NO - THAT CODE IS WRONG !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

The youtube video also clearly shows the consequences of inadequate
testing - that person should be defrocked..

[ResidentBiscuit]

After researching, you can use negative indices but it's completely implementation defined and just shouldn't be done.

[LB]

ResidentBiscuit wrote:

After researching, you can use negative indices but it's completely implementation defined and just shouldn't be done.

This, I did not know...I knew it worked in Python, but in C++? Really? Where does it come from? And It may be unhelpful with pointers...

[ylxin1993]

hi, guestgulkan

The youtube video also clearly shows the consequences of inadequate
testing - that person should be defrocked..

but why it works? I test it in my computer, it works, it can ouput "is palindrome" and "not palindrome".
I think it shouldn't work out

[LB]

Did you read ResidentBiscuit's post? You must be using a compiler that supports negative indexes as indexing from the end of the array.

[cire]

It is legal to index an array with a negative index, but you should remember that indexing an array is always equivalent to *(array+index). If that causes you to access memory that is out of the bounds of the array, it results in undefined behavior as it does in the code in the original post.

but why it works?

It doesn't. The appearance of working and working are different things.

[LB]

@cire I think some nonstandard compilers actually use negative indexes in the same way Python does.

[guestgulkan]

In the video, the guy is shown using VC ++ 2010 express.

[ylxin1993]

guestgulkan, you have a amazing memory, yeah, the guy using VS 2010 Express, and I use the VS 2010 Ultimate, can VS 2010 work with negative index?
I write a code in VS like this:

string s = "xxxxx";
	if(s[1] != s[-1])
	cout << "yes" << endl;

and the output is :

yes

can the VS compiler compare the negative index and positive index?

[LB]

Now, hold on a minute -
char string[80];
and
string s = "xxxxx";
are completely different. std::string will have its own behavior for the [] operator because it overloads it. The character array string will have the compiler's behavior.

[cire]

No, the character array will have the behavior described by the standard. Which is to say memory out of bounds will be accessed and the behavior is undefined.

[LB]

cire wrote:

the behavior described by the standard

cire wrote:

the behavior is undefined

I call undefined behavior compiler behavior, sorry for my ignorance and your misunderstanding :p