# Contour Plot with Boolean Indices (z must be at least a 2x2 matrix)

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Robert Zukowski am 24 Apr. 2023
Kommentiert: Robert Zukowski am 24 Apr. 2023
I'm trying to create a contour plot where the output depends on a condition from the input. An example is shown below.
The code is supposed to assign Z=Y when Y<=5 and Z=X when Y>5.
X = 1:1:5;
Y = 1:1:10;
[X,Y] = meshgrid(X, Y);
bool = Y <= 5;
Z(bool) = Y(bool);
Z(~bool) = X(~bool);
contour(X,Y,Z)
I know that the bool variable is getting initialized correctly, but for some reason, Z becomes a really long vector instead of keeping its dimensions. (which gives the error using contour: Z must be at least a 2x2 matrix)
I'm not sure how to approach this problem differently, so any insight is greatly appreciated.
Thanks!
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### Akzeptierte Antwort

Walter Roberson am 24 Apr. 2023
X = 1:1:5;
Y = 1:1:10;
[X,Y] = meshgrid(X, Y);
Z = X;
bool = Y <= 5;
Z(bool) = Y(bool);
contour(X,Y,Z)
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### Weitere Antworten (1)

LeoAiE am 24 Apr. 2023
The issue you're facing is due to the fact that when you use logical indexing with an uninitialized array like Z, MATLAB automatically converts it into a vector. To fix this issue, you can initialize the Z array with the same size as X and Y using the zeros function.
X = 1:1:5;
Y = 1:1:10;
[X,Y] = meshgrid(X, Y);
bool = Y <= 5;
Z = zeros(size(X));
Z(bool) = Y(bool);
Z(~bool) = X(~bool);
contour(X,Y,Z)
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Robert Zukowski am 24 Apr. 2023
Thanks for the explanation as well

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