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zero-impact

Tue Sep 15, 2009 9:26 pm


Weird Program Crash

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Hi everybody.

I was working on a DWITE question from last year 
#include 
#include 
#include 

using namespace std;

int main (void) {
	ifstream fin ("coins.txt");
	for (int abc = 0; abc < 5; abc++) {
		int sum,N; 							
		
		fin >> sum >> N;	
		
		vector  state(sum + 1, 99999);

		
									
		vector  coin (N); 	
		
		for (int i=0; i < N; i++)	
		fin >> coin 

The test case is attached.


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DemonWasp

Wed Sep 16, 2009 8:49 am


RE:Weird Program Crash

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Without compiling, running or debugging your program (I don't have a C++ compiler available at the moment; I'm posting from work while waiting on another machine), I'd guess that the issue has to do with invalid access to your vector. Try replacing the state[foo] accesses with state.at ( foo ) and then try running it.


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zero-impact

Wed Sep 16, 2009 11:55 am


RE:Weird Program Crash

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Thanks DemonWasp, I figured thats what it was but couldn't find it for the life of me.

What happened was that I didn't take into account that int t = j + coin [i]; could be larger than the target value, which is also the size of my vector. DOH!


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DemonWasp

Wed Sep 16, 2009 2:24 pm


RE:Weird Program Crash

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I'm a little surprised that I got it in one.

Regardless, you should probably be using at() whenever possible. It does just the same thing as [], except that it does bounds-checking. Since correctness is more important than programming for speed, I'd use at() until you're dead certain that it will never exceed its bounds AND that it needs to be faster.


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zero-impact

Wed Sep 16, 2009 3:13 pm


RE:Weird Program Crash

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Thanks for the tip, but thats not actually how I found it. When I replaced it all with at() vista just gave me a message saying the program closed in an unusual fashion or something along those lines. I ended up just doing another careful read through to find it. Oh how I wish linux had better power saving performance on my laptop.


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DemonWasp

Wed Sep 16, 2009 3:26 pm


RE:Weird Program Crash

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Ah, right. The reason it terminated abnormally is because vector::at() will throw an exception if you index it outside the array bounds; if you put a try-catch around your access blocks, you can catch that exception and do something useful with the information (generally logging or similar).


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zero-impact

Wed Sep 16, 2009 7:18 pm


RE:Weird Program Crash

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ooooook, that makes more sense. Thanks for the tip :)