Abnormal behaviour of code in PALIN3

general
1

I was practicing a question PALIN3, here is the link-> PALIN3 Problem - CodeChef

I coded my logic and here it is-> CodeChef: Practical coding for everyone

Now when i run this code for the sample test case provided on the Codechef IDE, it sometimes displays 4 as output and sometimes 5 without any changes in the code.

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So something is wrong with my code or IDE??

For anyone who is going to test my code, i executed it contiously 5-6 times and observed this.

And since i have got a WA, it would be really helpful if you can tell my mistake. its a bit different from editorial but i think it should work.

2 
ll sum[s.length()]={0};
//More ccode
ll p[n]={0};
ll q[n]={0};

How did this even compile?! xD

I feel you are suffering from runtime error leading to undefined behaviour. Try checking any line like-

c=sum[start+len-1]-sum[start-1];

which may lead to accessing index -1 etc.

1 Like
3

Yeah. Once again the evil of the by-default-enabled VLA extension of g++. Please don’t do stuff like this folks. :frowning:

And you’re correct with

c=sum[start+len-1]-sum[start-1];

being problematic. For the example input the first time this line is run it tries to access sum[-1] twice - which is very much undefined behavior.

1 Like
4

@vijju123 i understood the problem thanks!!

But i wanted to ask why were you expecting compilation error,i mean i always declare arrays like that…is it a problem??

5

If that N is a constant, its fine. But if its a variable, then such initialization isnt allowed on some versions afaik. Some further say that variable sized arrays cannot be declared.

Usually I feel its a good practice to intialize them with a for loop if declaring like that.

Thanks for backup @schleppel :slight_smile:

1 Like