matlab - almost done! simple mistake in function graphing ?

Hi ,

i am trying to graph kx, ky, K (constant line for k) on the y axis, and the Length (L) from 30 to 140 on the xaxis, as of right now I get the constant line from 30 to 140, but the other values for kx and ky start from zero. I need the values for kx and ky to go from 30 to 140 as well.

Any ideas?

clear
clc

mfp = 31;           % mean free path (nm)
k = 316.72;         % conductivity (W / m*k)

% Equations

Lmax = 140;
dL=10;
L=30;

while L<=Lmax

kx(L)=k*(1-((2*mfp)/(3*3.14*L)))

ky(L)=k*(1-(mfp/(3*L)));

L= L+dL;

end

kx(~kx)=[];  % removes all zeros from kx vector
ky(~ky)=[];  % removes all zeros from ky vector
plot(kx,'-'),hold on, plot(ky,'--'), hold on, plot([30,140],[316.72,316.72]);
grid on;

%plot(kx,'-'),hold on, plot(ky,'--'), hold on, plot(k);
title(['Conductivity (W / m*k ) ']);
xlabel('Thickness (nm)','FontSize',14);
ylabel('Conductivity','FontSize',14);
leg1=legend('kx','ky');

this was given to me for the last time i asked, and it makes the graph curve like it should. it just needs to be from 30 to 140 isntead of 0 to 11 or which one it goes too.

kx(~kx)=[];  % removes all zeros from kx vector
ky(~ky)=[];

Answer 1

[Oleg Komarov]

If you remove the zeros then your kx and ky will have less values and if you omit the x-values when calling plot by default it sets them to 1:numel(y-values). Therefore, say you have 12 values after removing the zeros it will plot kx from 1 to 12.

What you have to do is find at which positions your non-zero values are and call plot(X,Y).

Another remark, since you know how many iterations you'll have to do in the loop, call explicitly for.

mfp = 31; % mean free path (nm)
k   = 316.72; % conductivity (W / m*k)

% Preallocate kx and ky (search for preallocation for details)

kx = zeros(Lmax,1);
ky = zeros(Lmax,1);

% Loop from 30 to 140 with a step of 10.

for ii = 30:10:140
    kx(ii) = k*(1-(2*mfp/(3*3.14*ii)));
    ky(ii) = k*(1-(mfp/(3*ii)));
end

% Find the positions (x coordinates) of the non zero entries in kx

idx = find(kx);
plot(idx,kx(idx),'-')
hold on

% Find the positions (x coordinates) of the non zero entries in ky

idx = find(ky);
plot(idx,ky(idx),'--')
plot([30,140],[316.72,316.72]);
grid on

EDIT

A vectorized alternative:

mfp = 31; % mean free path (nm)
k   = 316.72; % conductivity (W / m*k)

L = 30:10:140;
kx = k*(1-(2*mfp./(3*3.14*L)));
ky = k*(1-(mfp./(3*L)));

x = 1:numel(L);
plot(x,kx,'-',x,ky,'--',[x(1) x(end)],[316.72,316.72])
set(gca,'Xlim',[x(1) x(end)],'XtickLabel',L)

[Chris]

Could you explain what has changed, it does work. But i dont understand some of the additions.

[Chris]

I am confused, at:

kx = zeros(Lmax,1);

ky = kx;

idx = find(kx);

idx = find(ky);

[Chris]

Is there not an easier way to just use my while loop and get it to go from 30 to 140?

[Oleg Komarov]

You can keep your while loop but that's not the core of your problem.

There are tons of solutions and I presented you with an easy one. Find returns the positions of all non zero elements (for more details read the documentation). This way you keep the reference to the [0 140].

[Walter Roberson]

Your while loop already goes from 30 to 140. It is the graph that is the difficulty. Oleg's code should make the graph reflect 30 to 140.