How to assign 'null values' to certain ranges of an equation based on a criteria?

Hi guys,

I have two equations. I want one of those (Eq2) to only have valid values when Eq1 is yielding positive values. For all the negative values of Eq1 for given x,y, I want Eq2 to give null or invalid values.

Here's the sample:

x=[-10:10];
y=[-10:10];

Eq1 = @(x,y) x - y;
Eq2 = @(x,y) x + y;

[X,Y] = meshgrid(x,y);

Z1 = Eq1(X,Y);
Z2 = Eq2(X,Y);

Basically: I want Eq2 to have values for only those x,y where Eq1 is >= 0. For all x,y where Eq1 is < 0, I want Eq2 to have invalid/null values. So that when I go to graph Eq2 or just pull values from it, it only does it for areas where Eq1 is >= 0.

Is this possible?

Thank you

Answer 1

[Stephen23]

The simplest solution is to use indexing, as per Guillaume's solution. But this requires the array of values to be defined first, which is not optimal for a function. If you really want this as a function, rather than a fixed matrix of values, then try this:

Eq1 = @(x,y) x - y;
Eq2 = @(x,y) x + y;
Eq3 = @(x,y) Eq2(x,y) + rem(0,Eq1(x,y)>=0);

which can be tested in the command window:

>> [X,Y] = meshgrid(-4:4,-4:4);
>> Eq1(X,Y)
ans =
     0     1     2     3     4     5     6     7     8
    -1     0     1     2     3     4     5     6     7
    -2    -1     0     1     2     3     4     5     6
    -3    -2    -1     0     1     2     3     4     5
    -4    -3    -2    -1     0     1     2     3     4
    -5    -4    -3    -2    -1     0     1     2     3
    -6    -5    -4    -3    -2    -1     0     1     2
    -7    -6    -5    -4    -3    -2    -1     0     1
    -8    -7    -6    -5    -4    -3    -2    -1     0
>> Eq3(X,Y)
ans =
    -8    -7    -6    -5    -4    -3    -2    -1     0
   NaN    -6    -5    -4    -3    -2    -1     0     1
   NaN   NaN    -4    -3    -2    -1     0     1     2
   NaN   NaN   NaN    -2    -1     0     1     2     3
   NaN   NaN   NaN   NaN     0     1     2     3     4
   NaN   NaN   NaN   NaN   NaN     2     3     4     5
   NaN   NaN   NaN   NaN   NaN   NaN     4     5     6
   NaN   NaN   NaN   NaN   NaN   NaN   NaN     6     7
   NaN   NaN   NaN   NaN   NaN   NaN   NaN   NaN     8

[A]

This is exactly it because it affects the equation directly.

[A]

Thanks!

Answer 2

[Guillaume]

There's no concept of null values in matrices, but you have NaN (Not a Number) instead.

Z1 = Eq1(X, Y);
Z2 = Eq2(X, Y);
Z2(Z1 < 0) = NaN; %set values of Z2 where Z1 is < 0 to NaN.

[A]

Thank you for this!