"Error using quiver"???

Question

Ana Vlajnic 2015-2-24

0
链接

此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/180157-error-using-quiver

回答： Alka Nair 2015-2-25

The error I get from my code is as follows:

    Error using quiver (line 44)
    DOUBLE cannot convert the input expression into a double array.

If the input expression contains a symbolic variable, use VPA.

CODE:

    clear all;
    syms y t;
    f = (2*y - t)/ (2*t - y);
    c = (abs(t - y))/ (abs((t + y)^3));
    l = sqrt(f.^2 + 1);
    [t, y] = meshgrid(-5:0.2:5, -5:0.2:5);
    quiver(t, y, 1./l, f./l, 0.5), axis equal tight
    xlabel 't', ylabel 'y'

END CODE

Please tell me what' I'm doing wrong!

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Answer 1

Alka Nair 2015-2-25

0
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/180157-error-using-quiver#answer_169192

When the input argument to DOUBLE function is a symbolic expression, MATLAB will output the error message you received.

In this particular case, the arguments to 'quiver' '1./l' and 'f./l' are symbolic expressions. To workaround this issue, Use 'EVAL' to evaluate the symbolic expression.