How do I use eval?

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
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')




 

[Matt J]

How do I fix that?

That depends. What are you trying to do in that line?

To put it another way, your usage of eval doesn't resemble any syntax mentioned in the eval documentation, so it's not clear what you are trying to do.

[the cyclist]

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]

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!