Computer Science Canada

Array out of bounds?
Author:  SJ [ Tue Dec 30, 2008 1:03 pm ]
Post subject:  Array out of bounds?
I've came across this today

c++:
    vector <int> v;
    v.push_back (5);

    // version1, this doens't give an error
    for (int i = 0; i+2 < v.size(); i++)
      cout << v[i+2] << endl;
   
    // version2, this goes nuts!
    for (int i = 0; i < v.size()-2; i++)
      cout << v[i+2] << endl;


version 1 seem to ignore the out of bounds element, whereas version 2 print out a bunch of random numbers then freeze up. I would predict a segmentation fault for both cases, but why does the first one just (seemingly) ignore it?
Author:  A.J [ Tue Dec 30, 2008 1:25 pm ]
Post subject:  Re: Array out of bounds?
Keep in mind that you only pushed 1 element (namely '5'). So, v.size() will return 1.

Version 1 exits when i + 2 >= v.size(). But since i = 0, and v.size() = 1, it doesn't output anything (because i + 2 > v.size() ).

Although in Version 2, you want i >= v.size() - 2 for the loop to exit.....however, since i = 0 initially and v.size() - 2 = -1, the loop goes on forever (because i increases by 1, and therefore can never by less than or equal to -1)

That's why it crashes....
Author:  SJ [ Tue Dec 30, 2008 1:32 pm ]
Post subject:  RE:Array out of bounds?
ahh right, thank you Smile
Author:  A.J [ Tue Dec 30, 2008 1:52 pm ]
Post subject:  Re: Array out of bounds?
no probs Smile
Author:  md [ Tue Dec 30, 2008 3:33 pm ]
Post subject:  RE:Array out of bounds?
Errm... 0 >= -1 so the second loop should exit right away... this is one of those times a debugger might help - or not waking up at 3:00 pm...

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