elimination of element outside of matrix dimension

[irfan20uk]

Hello,
In a function I trying to call elements of a matrix one at time according to method of step forward and step backward in space. My problem is in the statement when I mention that to consider the element one step forward and one step backward as well so it will take obviously also that are logically not existed or you can say that are located outside the dimension of the matrix. I seems to be a very simple problem but :( I failed to tackle down at least... I just want to get rid of these elements during process and also the resulting value. Means both the elements and the associated result. Following is the function structure what I approached untill now but could not work well besides it compiles. Thanks in advance for helping hand

void Lattice::particle()
{
int dim = 50;

for(i=0; i<dim; i++)
{
for(j=0;j<dim; j++)
{
if(!Pressure_x[-1][j] || !Pressure_y[-1][j])
{
x_diff = help->x - particle_x[i-1][j]; //help is pointer
y_diff = help->y - Pressure_y[i-1][j];

func[i-1][j] = (1-(x_diff/del_x))*(1-(y_diff/del_y));

//del_y & del_y are values in another function associated to i & j position.
}

if(!Pressure_x[dim][j] || !Pressure_y[dim][j])
{
x_diff = help->x - particle_x[i+1][j]; //help is pointer
y_diff = help->y - Pressure_y[i+1][j];

func[i+1][j] = (1-(x_diff/del_x))*(1-(y_diff/del_y));
}

if(!Pressure_x[i][-1] || !Pressure_y[i][-1])
{
x_diff = help->x - particle_x[i][j-1]; //help is pointer
y_diff = help->y - Pressure_y[i][j-1];

func[i][j-1] = (1-(x_diff/del_x))*(1-(y_diff/del_y));
}

if(!Pressure_x[i][dim] || !Pressure_y[i][dim])
{
x_diff = help->x - particle_x[i][j+1]; //help is pointer
y_diff = help->y - Pressure_y[i][j+1];

func[i][j+1] = (1-(x_diff/del_x))*(1-(y_diff/del_y));
}

if(!func[-1][j] || !func[dim][j] || !func[i][-1] || !func[i][dim])
{
if(!value[-1][j] || !value[dim][j] || !value[i][-1] || !value[i][dim])
F = (value[i-1][j]*func[i-1][j]) + (value[i+1][j]*func[i+1][j])
+ (value[i][j-1]*func[i][j-1]) + (value[i][j+1]*func[i][j+1]);
help->points = F;
}
}
}
}

Well it will be really nice if somebody have a look on my function and suggest me how to ignore the values or elements like pressure_x[-1][j], pressure_y[-1][j], pressure_x[dim][j], pressure_y[dim][j], pressure_x[i][-1], pressure_y[i][-1], pressure_x[i][dim], pressure_y[i][dim], and same as with func[][] and value[][] terms.