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Wierd Array Errors

Started by darkcrobat Apr 24, 2010 at 5:14 PM 3 replies 1.1k views

Original Post

Apr 24, 2010 at 5:14 PM

Hi guys. I'm working on a little rogue-like game, and have run into a wierd error. This compiles fine, but when save_map() is called, an error is found and the application exits with an error message. Running through the program with a debugger, this error message is displayed: Unhandled exception at 0x010f1ad0 in Supa Basic.exe: 0xC0000005: Access violation reading location 0x010f8000. The 0x010f80000 location is my map[20][40] array, but I'm not sure how to fix this error.


void save_map() {
message("Saving...");
int x=0;
int y=0;
do {
	do {
		switch (map[y][x])
			{
			message("4");
			//floor
			case 0: 
				
				save<<"."; 
			break;
			//wall
			case 1: 
				save<<"#"; 
			break;
			
			//down stairs
			case 2:
				save<<">";
			break;

			//goblin
			case 3:
				save<<"g";
			}
				
		x++;
		} while (x!=40); 

	save<<"\n";
	y++;
} while (y!=20);
message("Game Saved!");
}


int map[20][40]= {
0,1,0,0,3,0,0,0,0,0,0,0,
...
};

fastcall22

Mod

751

fastcall22

Apr 24, 2010 at 5:19 PM

When you loop the second time when y=1, x doesn't get set to zero; so you go from [0][0] to [0][39], then the next row from [1][40] to [1][79]. Also, use for-loops instead of do-whiles; the intention is clearer with for-loops than do-whiles:

for ( int y = 0; y < 20; ++y ) {     for ( int x = 0; x < 40; ++x ) {        /* ... */    }}

*Epic bug fix for great justice!

[Edited by - _fastcall on May 7, 2010 4:19:16 PM]

darkcrobat

Author

darkcrobat

Apr 24, 2010 at 5:21 PM

Ah, thanks. No wonder the problem was with array values. And there's no subscript checking in C++!

fastcall22

Mod

751

fastcall22

Apr 24, 2010 at 5:22 PM

Quote:
Original post by darkcrobat
And there's no subscript checking in C++!

If you want subscript checking in C++, you'd use std::vector or boost::multi_array instead of raw arrays.

Also,

Quote:

Unhandled exception at 0x010f1ad0 in Supa Basic.exe: 0xC0000005: Access violation reading location 0x010f8000.

If you're using your degbugger correctly, you can see which line of code is causing this error (which should be at the switch( map[y][x] )). Given the description of the error, you can deduce that either y or x is an invalid value. Then you can look at the auto/local/watch/immediate mode window to inspect the values of y and x. In which case, y should be 1 and x should be 40 at this point. Then from here, you can make the appropriate fixes: Set x to 0 after incrementing y => change the do-while loops to for-loops.

BrianLarsen

Apr 24, 2010 at 10:50 PM

instead of -> int map[20][40] and company,

#define MAXWIDTH 20
#define MAXHEIGHT 40

int map[MAXWIDTH][MAXHEIGHT];

save_map()
{

int y=0;

while (y < MAXWIDTH)
{
int x=0;

while (x{

// switch code...

x++;
}

y++;
}

}

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