Programming Forums > Application Development > C++

ZenOswyn · Nov 20th, 2006, 7:57 PM

Edit: I was changing a value that didn't exist. It's been a long day. Mods, feel free to delete this.

Hi all. I have no idea why this is happening, but whenever I change a value in my array to 1, it also changes the value in another place. It only does this when I try to change the last item on the first row.

Here's a bit of code I made to illustrate what's happening in my larger program.

(Toggle Plain Text)

#include <iostream>
using namespace std;

int main() {
	int matrix[3][3];
	
	matrix[0][3] = 1;
	cout << matrix[0][3] << " " << matrix[1][0];
	return 0;
}

That outputs two 1's on my screen

I tried this wth mingw under windows and g++ under linux, same thing. Which leads me to believe I'm probably missing something really simple.

Thanks for the help

Xyhm · Nov 20th, 2006, 8:10 PM

Quote:

Originally Posted by ZenOswyn

Hi all. I have no idea why this is happening, but whenever I change a value in my array to 1, it also changes the value in another place. It only does this when I try to change the last item on the first row.

Here's a bit of code I made to illustrate what's happening in my larger program.

(Toggle Plain Text)

#include <iostream>
using namespace std;

int main() {
	int matrix[3][3];
	
	matrix[0][3] = 1;
	cout << matrix[0][3] << " " << matrix[1][0];
	return 0;
}

That outputs two 1's on my screen

I tried this wth mingw under windows and g++ under linux, same thing. Which leads me to believe I'm probably missing something really simple.

Thanks for the help

Well, you're assigning a value outside of the range of the array. When you declare an array as, say, int test[3], then the highest index you can assign is test[2]. test[3] is undeclared, and using that part can yield errors of various kinds, depending on the rest of the program.

So, for example, if you try the following you will see that matrix[1][0] will not be identical to the value that is assigned to matrix[0][3]:

(Toggle Plain Text)

#include <iostream>
using namespace std;

int main() {
	int matrix[4][4];
	
	matrix[1][0] = 3;
	matrix[0][3] = 2;
	cout << matrix[0][3] << " " << matrix[1][0];
	return 0;
}

mrynit · Nov 20th, 2006, 10:33 PM

arrays start counting on 0. 0,1,2,3... but we normaly count on 1. 1,2,3,4... so when you say int list[3]; the varius indexes are list[0], list [1], list[2].

Nov 20th, 2006, 7:57 PM	#1
ZenOswyn Newbie Join Date: Nov 2006 Posts: 5 Rep Power: 0	Strange bug with a 2d array Edit: I was changing a value that didn't exist. It's been a long day. Mods, feel free to delete this. Hi all. I have no idea why this is happening, but whenever I change a value in my array to 1, it also changes the value in another place. It only does this when I try to change the last item on the first row. Here's a bit of code I made to illustrate what's happening in my larger program. (Toggle Plain Text) #include <iostream> using namespace std; int main() { int matrix[3][3]; matrix[0][3] = 1; cout << matrix[0][3] << " " << matrix[1][0]; return 0; } #include <iostream> using namespace std; int main() { int matrix[3][3]; matrix[0][3] = 1; cout << matrix[0][3] << " " << matrix[1][0]; return 0; } That outputs two 1's on my screen I tried this wth mingw under windows and g++ under linux, same thing. Which leads me to believe I'm probably missing something really simple. Thanks for the help Last edited by ZenOswyn; Nov 20th, 2006 at 8:08 PM. Reason: I was being stupid

Nov 20th, 2006, 10:33 PM	#3
mrynit Hobbyist Programmer Join Date: Mar 2006 Location: WA, USA Posts: 343 Rep Power: 3	arrays start counting on 0. 0,1,2,3... but we normaly count on 1. 1,2,3,4... so when you say int list[3]; the varius indexes are list[0], list [1], list[2]. __________________ i dont know much about programming but i try to help

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Programming Forums > Application Development > C++

Strange bug with a 2d array