C++ Array unexpected bug

Question

I want to make a simple programm that shows a square of 16 'X' and that you can change specific coordinates. Everything is finished and i dont get any bugreports but in the programm things are very strange. what i should get is easy

 XXXX                              
 XXXX
 XXXX
 XXXX

and if i change 1,1 to A i should have this

 AXXX
 XXXX
 XXXX
 XXXX

but what i see when i run the program is this:

 XXXX
 XXXX
 XXXX
 @

well what is with the fourth line? there is a smiley in the first spot (i used an @) and the rest is empty. when i change 1,1 to anything, 3,4 changes too. everything else works properly. and if i try to change something in the fourth line the program crashes. if you want to test it yourself i can upload the .exe #include

using namespace std;

int main()
{
    for (;;) {
    int eingabe1;
    int eingabe2;
    char neueingabe;
    char array[4][4];
    array[1][1] = 'X';
    array[1][2] = 'X';
    array[1][3] = 'X';
    array[1][4] = 'X';
    array[2][1] = 'X';
    array[2][2] = 'X';
    array[2][3] = 'X';
    array[2][4] = 'X';
    array[3][1] = 'X';
    array[3][2] = 'X';   
    array[3][3] = 'X';
    array[3][4] = 'X';
    array[4][1] = 'X';
    array[4][2] = 'X';
    array[4][3] = 'X';
    array[4][4] = 'X';
    cout << "koordinaten eingeben" << endl; 
    cin >> eingabe1;
    cin >> eingabe2;
    cout << array[eingabe1][eingabe2] << endl;
    cout << "neueingabe eingeben" << endl;
    cin >> neueingabe;
    array[eingabe1][eingabe2] = neueingabe;
    cout << array[eingabe1][eingabe2]<< endl;
    }
    return 0;
}

i hope you can help

You should start by reading some [C++ book](http://stackoverflow.com/questions/388242/the-definitive-c-book-guide-and-list), really. — , Jan 23 '13 at 14:35
Also note that "bug reports" generally come from humans, or other entities *testing* your software. The compiler can only generate warnings and errors if your code doesn't comply with the language specifications, but it cannot tell you "it doesn't do what it should". The behavior you're describing is clearly a bug, so that's a bug report right there. — unwind, Jan 23 '13 at 14:37

score 3 · Answer 1 · edited Jan 23 '13 at 14:43

3

YOU ARE WRITING PAST YOUR ARRAY!

Arrays in C/C++/C#/Java and most other languages start their numbering with 0.

So for

int arr[4];
// you have:
arr[0]=1;
arr[1]=2;
arr[2]=3;
arr[3]=4;
//arr[4]= A REALLY BIG ERROR;

edited Jan 23 '13 at 14:43

Tadeusz Kopec for Ukraine

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answered Jan 23 '13 at 14:32

Dariusz

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WHY DO YOU WRITE IN CAPS? – Jan 23 '13 at 14:35
1

I couldn't stress that sentence enough, sorry. – Dariusz Jan 23 '13 at 14:37
@TadeuszKopec Hope I don't get -1'd for this stressing of mine (and yours) ;) – Dariusz Jan 23 '13 at 14:45
well i actually deserver getting caps'd when i make such a mistake that someone even suggests me to read a book while thats what im doing – Robert Wegner Jan 23 '13 at 14:47
1

That's the reason why we should use `std::vector` in C++. – Skalli Jan 23 '13 at 15:20
@skalli that's something completely different; both have their uses, some of them overlap, but not all – Dariusz Jan 23 '13 at 15:23
1

Of course there are reasons not to use `std::vector`. But there are no reasons to use a raw array. A `std::array` is a viable alternative in this case. – Skalli Jan 23 '13 at 15:25
in Smalltalk, most arrays start at 1 – Tobias Langner Jan 23 '13 at 16:49

score 1 · Answer 2 · answered Jan 23 '13 at 14:32

1

Start the indices at 0. So the max index is 3,3; the lowest is 0,0. Accessing other values leads to random behavior - in the best case it crashes your program.

answered Jan 23 '13 at 14:32

Tobias Langner

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C++ Array unexpected bug

2 Answers2