How do I use eval?

Question

Kathleen 2024-2-9

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https://ww2.mathworks.cn/matlabcentral/answers/2080146-how-do-i-use-eval

评论： Kathleen 2024-2-9

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I have errors using the eval function on line 102 and 110. How do I fix that? Thanks!

clear; clc; close all

%% normal stress

fprintf('\n ====== Exercise 2.1 a=======\n\n')

====== Exercise 2.1 a=======

tau = [-30 -20; -20 -40]; %2D stress tensor (Mpa)

theta = 10;

fhat = [sind(theta) , cosd(theta)];

nhat = [ cosd(theta) , -sind(theta)];

tnhat = tau * nhat.';

tn = nhat * tnhat %normal stress

tn = -23.4611

if 1

figure(1); hold on; grid on; axis equal

x = [ 0 1 ; 1 0];

for th = [ 90 180 -90 -180];

X= [cosd(th) sind(th); -sind(th) cosd(th)];

yp = X * x;

quiver(0,0,yp(1),yp(2),0,'r-');

end

%% shear stress

ts = fhat * tnhat

ts = -17.0838

fprintf('\n ====== Exercise 2.1 a end=======\n\n')

====== Exercise 2.1 a end=======

%% principle stresses

fprintf('\n ====== Exercise 2.1 b =======\n\n')

====== Exercise 2.1 b =======

I = [1 0; 0 1];

[V,D] =eig( tau );

V

V = 2×2

0.6154 -0.7882 0.7882 0.6154

fprintf('\nEigenvalues: 1= %.1f 2=%.1f\n', diag(D))

Eigenvalues: 1= -55.6 2=-14.4

eva(1) = atan2d( V(1,1), V(2,1) ); %eigenvec-1 angle +cw w.r.t. North

eva(2) = 180 + atan2d( V(1,2), V(2,2) ); %eigenvec-2 angle +cw w.r.t. North

fprintf('\nEvec angles reckoned +cw w.r.t N: 1= %.1f 2= %.1f (deg) \n\n', eva)

Evec angles reckoned +cw w.r.t N: 1= 38.0 2= 128.0 (deg)

ang = [0:45:360];

figure(2); hold on; grid on; axis equal

for i = 1: length(ang)

x = cosd(ang(i));

y = sind(ang(i));

a1 = atan2d(y,x); % +CCW wrt East (+x-hat)

a2 = atan2d(x,y); % +CW wrt North (+y-hat)

if 0 %this block will use modulus to map angles from -pi/pi to 0-2*pi domain

a1 = mod(a1,360);

a2 = mod(a2,360);

end

fprintf('x/y= (%5.1f %5.1f) a1/a2 =%6.1f %6.1f \n',x,y,a1, a2)

plot([0 x],[0 y],'k-')

tt = compose('a1= %d a2=%d', a1,a2);

text(x,y,tt, 'fontsize',15)

end

x/y= ( 1.0 0.0) a1/a2 = 0.0 90.0 x/y= ( 0.7 0.7) a1/a2 = 45.0 45.0 x/y= ( 0.0 1.0) a1/a2 = 90.0 0.0 x/y= ( -0.7 0.7) a1/a2 = 135.0 -45.0 x/y= ( -1.0 0.0) a1/a2 = 180.0 -90.0 x/y= ( -0.7 -0.7) a1/a2 =-135.0 -135.0 x/y= ( 0.0 -1.0) a1/a2 = -90.0 180.0 x/y= ( 0.7 -0.7) a1/a2 = -45.0 135.0 x/y= ( 1.0 0.0) a1/a2 = 0.0 90.0

figure(3); hold on; grid on

set(0,'DefaultLineLineWidth',5,'DefaultAxesFontSize',12)

theta=10;yy=[-40,30];% x-coordinates of the line passing through the center

x1_fault=tand(theta)*yy; % y-coordinates of the line passing through the

plot(x1_fault, yy,'r--','LineWidth', 2);hold on

nn = 20*nhat; ff= 20*fhat;

quiver(0,0, nn(1), nn(2), 0, 'b-','linewidth',3)

quiver(0,0, ff(1), ff(2), 0, 'g-','linewidth',3)

ee = eval(1,1) * evec(:,1);

Error using eval
Argument must be a text scalar.

quiver(0,0,ee(1,1),ee(2,1),'k-','linewidth',2)

quiver(0,0,evec(1,1)*20,evec(2,1)*20,'m-','linewidth',2)

fprintf('max principle stress axis = %.1f (deg)\n', atand(ee(1)/ee(2)) )

ee = eval(2,2) * evec(:,2); %min principle eigen-vec

quiver(0,0,ee(1),ee(2),'k-','linewidth',2)

quiver(0,0,evec(1,2)*20,evec(2,2)*20,'m-','linewidth',2)

xline(0, 'linewidth',0.5); yline(0, 'linewidth',0.5)

% Set x-axis and y-axis limits

xlim([-40, 30]);

ylim([-40, 30]);

% Set grid intervals to be multiples of 10

set(gca, 'XTick', -40:10:30);

set(gca, 'YTick', -40:10:30);

lgd = legend('','','','','fault','fault normal','fault parallel','eval1*evec-1','evec1', 'eval2*evec-2','evec2');

lgd.FontSize = 15; lgd.Location = 'northwest';

title('Exercise 2.1'); xlabel('x-axis (MPa)'); ylabel('y-axis (MPa)')

fprintf('\n ====== Exercise 2.1 b end=======\n\n')

3 个评论
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the cyclist 2024-2-9

I can't figure out how to fix your code. But I don't think you want the MATLAB eval function, which is completely unrelated to what you are doing.

It looks like whoever wrote this code -- you? -- intended to calculate the eigenvalue (eval?) and eigenvector (evec?), and do something with them.

Earlier in the code, I see a variable named eva (not eval). And I see ee. I just can't quite figure out what is intended.

But since this is homework, I guess that's your job, not mine.

Kathleen 2024-2-9

yeah, I wrote the code from scratch. I am horrible with mathlab but I try my best. It had to be eva and not eval. Stupid me, I figured it out. Thanks!

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