why does it not plot the function?

Question

shamma aljaberi 2023-11-1

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/2041621-why-does-it-not-plot-the-function

评论： Voss 2023-11-1

hi! im trying to plot the real(x) and imaginary parts(y) of R_P. but the plot doesnt show any points in the graph. here is my code:

clc, clear

% Values from table for even ID number

global L1 L2 L3 L4 AP delta theta1

L1=5;

L2=1;

L3=5;

L4=7;

AP=5;

delta=50;

theta1=0;

for i=0:360/200:360

%loop variables

R2=L2*exp(1i*deg2rad(i));

%R3=L3*exp(1i*deg2rad(theta3));

%R4=L4*exp(1i*deg2rad(theta4));

R1=L1*exp(1i*deg2rad(theta1));

Z=R1-R2;

Z_con=conj(Z);

%variables of the quadratic equation

a=L4*Z_con;

b=Z.*Z_con+L4^2-L3^2;

c=Z*L4;

T=(-b+sqrt(-(b.^2)-(4.*a.*c)))/(2.*a);

S=(L4*T+Z)/L3;

%angular positions

theta4=rad2deg(angle(T));

theta3=rad2deg(angle(S));

theta_AP=theta3-delta;

%path coordinates of A and P

R3=L3*exp(1i*deg2rad(theta3));

R4=L4*exp(1i*deg2rad(theta4)); %not needed

R_AP=AP*exp(1i*deg2rad(theta_AP));

%assuming that the global axis is at point O2

R_A=R2;

R_P=R2+R_AP;

end

x=real(R_P)

x = 4.4335

y=imag(R_P)

y = 3.6347

plot(x,y)

there is no error shown. thank you!

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Dyuman Joshi 2023-11-1

在 MATLAB Online 中打开

Remove this line, as it is redundant. And it is best to avoid the use of global variables.

global L1 L2 L3 L4 AP delta theta1

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Answer 1

Voss 2023-11-1

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链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/2041621-why-does-it-not-plot-the-function#answer_1344891

在 MATLAB Online 中打开

Each iteration of the for loop, you are overwriting the value of R_P, so after the loop R_P is a scalar (i.e., it only has one element: the value from the final iteration of the loop).

(Arrays with only one element don't show up when you plot them, unless you use a data marker, so that's why the plot doesn't show up.)

To store the values from all iterations, make R_P a vector and store one element in it each time through the loop.

Something like this:

% Values from table for even ID number

global L1 L2 L3 L4 AP delta theta1

L1=5;

L2=1;

L3=5;

L4=7;

AP=5;

delta=50;

theta1=0;

vals = 0:360/200:360;

n_vals = numel(vals);

R_P = zeros(1,n_vals);

for i=1:n_vals

%loop variables

R2=L2*exp(1i*deg2rad(vals(i)));

%R3=L3*exp(1i*deg2rad(theta3));

%R4=L4*exp(1i*deg2rad(theta4));

R1=L1*exp(1i*deg2rad(theta1));

Z=R1-R2;

Z_con=conj(Z);

%variables of the quadratic equation

a=L4*Z_con;

b=Z.*Z_con+L4^2-L3^2;

c=Z*L4;

T=(-b+sqrt(-(b.^2)-(4.*a.*c)))/(2.*a);

S=(L4*T+Z)/L3;

%angular positions

theta4=rad2deg(angle(T));

theta3=rad2deg(angle(S));

theta_AP=theta3-delta;

%path coordinates of A and P

R3=L3*exp(1i*deg2rad(theta3));

R4=L4*exp(1i*deg2rad(theta4)); %not needed

R_AP=AP*exp(1i*deg2rad(theta_AP));

%assuming that the global axis is at point O2

R_A=R2;

R_P(i)=R2+R_AP;

end

x=real(R_P)

x = 1×201

4.4335 4.4617 4.4894 4.5163 4.5424 4.5678 4.5922 4.6156 4.6381 4.6595 4.6797 4.6988 4.7167 4.7333 4.7487 4.7627 4.7753 4.7866 4.7965 4.8049 4.8120 4.8176 4.8218 4.8245 4.8259 4.8258 4.8243 4.8214 4.8172 4.8116

y=imag(R_P)

y = 1×201

3.6347 3.6388 3.6420 3.6444 3.6460 3.6469 3.6470 3.6463 3.6450 3.6429 3.6402 3.6369 3.6330 3.6285 3.6234 3.6179 3.6119 3.6054 3.5986 3.5914 3.5838 3.5760 3.5678 3.5594 3.5508 3.5420 3.5330 3.5238 3.5146 3.5052

plot(x,y)