The last variable in an array is incorrect [closed] - c++

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Closed 10 years ago.
So, I'm creating a program to create a (sort of) simulation of a melee deathmatch video game (not actually making a video game at the moment, just making simple AIs with goals to kill each other). In order to do this, I am using a tile-based, turn-based system.
Now the introduction is out of the way, here is the specific problem: in one of the arrays I am using, the last value is stored incorrectly in RAM, no matter how many variables in the array. Here is the relevant code:
(I will post all the code I have at the bottom of this, but the problem is in here)
#include "stdafx.h"
#include <iostream>
int _tmain(int argc, _TCHAR* argv[])
{
using namespace std;
int npcTileAttacker[] = { 0, 0, 0, 0, 0};
int s = 0;
while (s < 6)
{
cout << "The value that is being selected from the array is " << s << endl;
cout << npcTileAttacker[s] << endl;
s++;
cout << "The value of s has now been set to " << s << endl;
}
This outputs:
The value that is being selected from the array is 0
0
The value of s has now been set to 1
The value that is being selected from the array is 1
0
The value of s has now been set to 2
The value that is being selected from the array is 2
0
The value of s has now been set to 3
The value that is being selected from the array is 3
0
The value of s has now been set to 4
The value that is being selected from the array is 4
0
The value of s has now been set to 5
The value that is being selected from the array is 5
-858993640
The value of s has now been set to 6
Obviously, this last value from the array is incorrect. What I want to know is why this would be happening.
In addition to this, when the program ends, I get an error message:
"Run-Time Check Failure #2 - Stack around the variable 'npcTileAttacker' was corrupted."
I have tried placing the output values of s and the array piece of code around other arrays in the program, resulting in the same problem occuring.
Here is my full code, if required:
#include "stdafx.h"
#include <iostream>
int _tmain(int argc, _TCHAR* argv[])
{
using namespace std;
int numberOfNPCs = 5;
//Remember whose alive (so we can skip the dead's turns)
int npcAlive[5] = { 1, 1, 1, 1, 1 };
/*This determines what action is going to be carried out on this square. For the moment:
if npcTileActivity[n] = 1;
the goals is death
WARNING! THIS WILL RESULT IN BUGS!!! I need to figure out a way that allows multiple activities on a tile
(maybe apply actions onto NPCs directly, rather than onto their tiles)
*/
int npcTileActivity[] = { 0, 0, 0, 0, 0};
//This tells you who is doing the action on this tile
int npcTileAttacker[5] = { 1, 2, 3, 4, 0 };
int s = 0;
while (s < 6)
{
cout << "The value that is being selected from the array is " << s << endl;
cout << npcTileAttacker[s] << endl;
s++;
cout << "The value of s has now been set to " << s << endl;
}
//This determines whether or not the NPC will fight back. Currently pointless, as this will just kill them.
int npcPacifism[5] = { 0 };
//This is their HP
int npcDefense[5] = {5, 5, 5, 5, 5};
//This is the default damage (presumably this is done with knives)
int npcOffense[5] = {1, 1, 1, 1, 1};
/*This determines what each NPC wants to do.
1 - Kill Target
*/
int npcGoal[5] = {1, 1, 1, 1, 1};
//This is the NPC they are aiming at
int npcTarget[5] = {1, 2, 3, 4, 0};
/*The x coord for their target. In the future:
-I want this to be able to be changed during the sim
-This should be disabled until the NPC can find out where their target is
*/
int npcGoalLocationX[5] = {4, 1, 4, 3, 1};
/* The Y coord for their target
*/
int npcGoalLocationY[5] = {2, 3, 4, 2, 1};
/*Their x coord.
This will change, then the all npcGoalLocations need to be updated
*/
int npcLocationX[5] = {1, 4, 1, 4, 3};
/* Their y coord.
*/
int npcLocationY[5] = {1, 2, 3, 4, 2};
int m = 1;
while (m != 0)
{
int i = 0;
//Loop until every npc has had a turn
while (i < (numberOfNPCs + 1))
{
/*npcGoalLocationY[i] = npcLocationY[npcTarget[i]];
npcGoalLocationY[i] = npcLocationY[npcTarget[i]];*/
if (npcAlive[i] = 1)
{
/* Tile activities:
1 - Kill occupant
*/
if (npcTileActivity[i] = 1)
{
cout << "This shouldn't be the first thing." << endl;
//This gets the attack and defense values for the appropriate acting NPC
int j = 0;
while (j < (numberOfNPCs + 1))
{
if (npcTileAttacker[i] = j)
{
//Defender's HP - Attacker's damage
int rollAttack1 = npcDefense[i] - npcOffense[j];
if (rollAttack1 > 0)
{
npcDefense[i] = rollAttack1;
cout << "NPC " << j << " attacks NPC " << i << endl;
if (npcPacifism[i] = 0)
{
//Defender retaliates
int rollAttack2 = npcDefense[j] - npcOffense[i];
if (rollAttack2 > 0)
{
npcDefense[j] = rollAttack2;
cout << "NPC " << i << " retaliates" << endl;
}else
{
npcAlive[j] = 0;
cout << "NPC " << j << " dies" << endl;
}
}
}else
{
npcAlive[i] = 0;
cout << "NPC " << i << " dies" << endl;
}
}
j++;
}
}else
{
cout << "This should be the first." << endl;
if (npcGoal[i] != 0)
{
if (npcGoalLocationX[i] = npcLocationX[i])
{
if (npcGoalLocationY[i] = npcLocationY[i])
{
//The Tile Activity of the current NPC's target is set to whatever the current NPC's goal is
npcTileActivity[npcTarget[i]] = npcGoal[i];
}
}
if (npcGoalLocationX[i] > npcLocationX[i])
{
npcLocationX[i]++;
}
if (npcGoalLocationX[i] < npcLocationX[i])
{
npcLocationX[i]--;
}
if (npcGoalLocationY[i] > npcLocationY[i])
{
npcLocationY[i]++;
}
if (npcGoalLocationY[i] < npcLocationY[i])
{
npcLocationY[i]--;
}
}
}
}
i++;
}
cin >> m;
}
return 0;
}
Also, I get a problem (around the lines which cout "This should be first" and "This shouldn't be the first thing"): The one which shouldn't be first is first and the one which should be first never even executes. This is probably related to the array error, however.
Thanks for your assistance.

Answer 1

Your condition is off by one:
while (s < 6)
should be
while (s < 5)
The array { 0, 0, 0, 0, 0} has five elements, so valid indexes are 0,1,2,3,4.
Your condition stops when s < 6 is false, so it's still true for s == 5.

Answer 2

Your array has only 5 cells:
int npcTileAttacker[] = { 0, 0, 0, 0, 0};
That is, s should go from 0 to 4, instead of 0 to 5.
The "random" value that you are seeing is in fact whatever value was there on the stack after npcTileAttacker array since you are overflowing that array.

Answer 3

The size of your array is 5. Hence the valid indices are from 0-4. So, npcTileAttacker[5] will always post garbage.

Answer 4

You're out by 1 in your while loop expression.
You'd also be better off using a for loop and not hard coding the length of the array. Try something like this:
int npcTileAttacker[] = { 0, 0, 0, 0, 0};
int npcTileAttackerLength = sizeof(npcTileAttacker)/sizeof(npcTileAttacker[0]);
for(int s=0; s<npcTileAttackerLength; s++)
{
cout << "The value that is being selected from the array is " << s << endl;
cout << npcTileAttacker[s] << endl;
}
This way the length variable will always hold the number of items in the array.