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

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A
A 2015-7-13
评论： A 2015-7-13
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

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采纳的回答

Stephen23
Stephen23 2015-7-13
编辑：Stephen23 2015-7-13
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

更多回答（1 个）

Guillaume
Guillaume 2015-7-13
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.

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