argument in meshgrid - confused

Question

Tatte Berklee 2022-8-31

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链接

此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1791840-argument-in-meshgrid-confused

编辑： Bruno Luong 2022-8-31

采纳的回答： Bruno Luong

Hey folks!

I am trying to use -meshgrid- and use -contourf- to plot a contourp map.

The two-variable function I am interested in studying is:

for

I tried this code: Version 1

%Meshgrid Ver 1

a = -1:.2:1;

b = 0:.2:1;

[X,Y]= meshgrid(a,b);

Z = X./(Y.*(1-X)-1);

contourf(X,Y,Z);

Next, I tried a similar code, except I changed the -meshgrid- argument:

%Meshgrid Ver 2

[X,Y]= meshgrid(-1:1);

Z = X./(Y.*(1-X)-1);

contourf(X,Y,Z);

I looked at the MATLAB document here: https://www.mathworks.com/help/matlab/ref/meshgrid.html

My questions:

(1) Why are they giving different contour plots?

(2) Version 1 is the correct contour plot corresponding to the origina equation Z, right?

Thanks, guys!

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Answer 1

Bruno Luong 2022-8-31

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You need to make finer grid, you meshgrid is too coarse.

The version 1 is the top half of version 2.

%Meshgrid Ver 1

a = -1:.01:1;

b = 0:.01:1;

[X,Y]= meshgrid(a,b);

Z = X./(Y.*(1-X)-1);

ax1 = subplot(2,1,1);

contourf(X,Y,Z);

title('Version 1')

%Meshgrid Ver 2

[X,Y]= meshgrid(-1:0.01:1);

Z = X./(Y.*(1-X)-1);

ax2 = subplot(2,1,2);

contourf(X,Y,Z);

title('Version 2')

linkaxes([ax1 ax2])

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Bruno Luong 2022-8-31

编辑：Bruno Luong 2022-8-31

Because your grid is too coarse. Matlab draws the contour line as sequence of straight lines that cut the grid. The real function and contour lines are smooth.

Tatte Berklee 2022-8-31

Gocha! Thanks, Bruno!!

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Answer 2

David Hill 2022-8-31

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1791840-argument-in-meshgrid-confused#answer_1038905

在 MATLAB Online 中打开

Your domain for X is -1:1 and your domain for Y is 0:1. The first code choses the correct domains, the second assigns the same domain to both X and Y (-1:1). If you want more data points:

a = -1:.001:1;

b = 0:.001:1;

[X,Y]= meshgrid(a,b);

Z = X./(Y.*(1-X)-1);

contourf(X,Y,Z);