How do I correct this plot code?

Asked by Joe Johnson on 21 Aug 2012
Latest activity Answered by Matt Fig on 21 Aug 2012

I am trying to plot an equilibrium equation on Matlab, but I am getting an error of "Error using *, inner matrix dimensions must agree". I have copied the code below.

g = 2.79
k = 1.5
b = 2
a = 3
m = 1
x = -2*pi:.1:2*pi;
y = 3*g*k*((g.^2 - 2*g*a*cos(x) + a.^2).^(1/2) - b)*sin(x)*(1/(m*a))*(1/((g.^2 - 2*g*a*cos(x) + a.^2).^(1/2)));
p = plot(x,y)
set(gca,'XTick',-2*pi:pi/2:2*pi)
set(gca,'XTickLabel',{'-2pi','-3pi/2','-pi','-pi/2','0','pi/2','pi','3pi/2','2pi'})
xlabel('-2\pi \leq \Theta \leq 2\pi')
ylabel('stability')
title('Plot of Stability of Buckling Bar')
text(pi,sin(pi)),'\leftarrow sin(\pi\)',...
     'HorizontalAlignment','left')
text(0,sin(0)),'\leftarrow sin(0)',...
     'HorizontalAlignment','left')
set(p,'Color','red','LineWidth',2)

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Joe Johnson

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2 Answers

Answer by Azzi Abdelmalek on 21 Aug 2012
Edited by Azzi Abdelmalek on 21 Aug 2012

you have forgoten some .* or ./

   g = 2.79 ;k = 1.5; b = 2; a = 3 ;m = 1 ;x = -2*pi:.1:2*pi;
    y = 3*g*k*((g.^2 - 2*g*a*cos(x) + a.^2).^(1/2) - b).*sin(x)*(1/(m*a)).*(1./((g.^2 - 2*g*a*cos(x) + a.^2).^(1/2)));
    p = plot(x,y) 
    set(gca,'XTick',-2*pi:pi/2:2*pi) 
    set(gca,'XTickLabel',{'-2pi','-3pi/2','-pi','-pi/2','0','pi/2','pi','3pi/2','2pi'})
    xlabel('-2\pi \leq \Theta \leq 2\pi') 
    ylabel('stability') ,
    title('Plot of Stability of Buckling Bar')
    text(pi,sin(pi),'\leftarrow sin(\pi)','HorizontalAlignment','left')
    text(0,sin(0),'\leftarrow sin(\0)','HorizontalAlignment','left') 
    set(p,'Color','red','LineWidth',2)

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Azzi Abdelmalek
Answer by Matt Fig on 21 Aug 2012
Edited by Matt Fig on 21 Aug 2012

Whenever you mean to multiply (or divide) a vector by a vector on an element-by-element basis, you must use the (.*) or (./) operator instead of (*) or (/). Remember MATLAB was originally a matrix language, so it interprets the latter operators as matrix multiply.

Try this to fully see what I mean:

A = [1 2 3];
B = [4 5 6];

A.*B    % = [A(1)*B(1),A(2)*B(2),A(3)*B(3)]
A*B     % error because a a 1-by-3 cannot multiply a 1-by-3
A*(B')  % = A(1)*B(1) + A(2)*B(2) + A(3)*B(3) matrix multiplication.

To avoid this it is always best to just use all .* when doing non-matrix multiplication and division.

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Matt Fig

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