Matlab isn't ploting the function

Hi everyone!

I'm trying to get Matlab to fetch the values of an excel cell in a particular sheet in a particular workbook, then computing a piecewise function and then presenting the results in a logarithmic scale. All fine and cool but then, when the graph shows up, nothing is on it.

I'm very very new to Matlab (like two days new) and i've been strugling with it. Any help?

eta_l = xlsread('calculos.xlsx','Theoretical Predictions','S2');
eta_0 = xlsread('calculos.xlsx','Theoretical Predictions','S2');
l_c = xlsread('calculos.xlsx','Theoretical Predictions','P3');
sigma_f = xlsread('calculos.xlsx','Base Values','B3');
v_f = xlsread('calculos.xlsx','Burn','E23');
E_m = xlsread('calculos.xlsx','Base Values','D2');
E_f = xlsread('calculos.xlsx','Base Values','B2');
d = xlsread('calculos.xlsx','Base Values','B5');
for x=logspace(-6,2)
    if x < l_c
       y1=((eta_l.*eta_0.*((v_f.*sigma_f.*E_m)./(E_f.*d.*sqrt(3))).*x)+((sigma_f.*E_m.*(1-v_f))./E_f));
    else
       y2=(eta_l.*eta_0.*(((-sigma_f.*v_f.*l_c)./2).*(1./x)+(sigma_f.*v_f)))+(((sigma_f.*E_m)./E_f).*(1-v_f));
    end
end
semilogx(x,y1,x,y2);
grid on

Thanks in advance!

P.S.:

The function is as follows if you're having trouble reading it from the code

where "l" is "x".

[Mathieu NOE]

hello

are you sure that x, y1 and y2 exist ? and are not NaN ?

what do you get in your workspace when you type whos ?

[João Novais]

Yes they do exist. For instance this is the graph of the whole function y2 by it's own

The thing is that i'm having trouble ploting them together. When i type "whos" it appears that all my variables appear as "double"

Name         Size             Bytes  Class     Attributes

  E_f          1x1                  8  double              
  E_m          1x1                  8  double              
  d            1x1                  8  double              
  eta_0        1x1                  8  double              
  eta_l        1x1                  8  double              
  l_c          1x1                  8  double              
  sigma_f      1x1                  8  double              
  v_f          1x1                  8  double              
  x            1x1                  8  double              
  y            1x100              800  double              
  y1           1x1                  8  double              
  y2           1x1                  8  double              

[Mathieu NOE]

there is something wrong probably in this for loop

for x=logspace(-6,2)
    if x < l_c
       y1=((eta_l.*eta_0.*((v_f.*sigma_f.*E_m)./(E_f.*d.*sqrt(3))).*x)+((sigma_f.*E_m.*(1-v_f))./E_f));
    else
       y2=(eta_l.*eta_0.*(((-sigma_f.*v_f.*l_c)./2).*(1./x)+(sigma_f.*v_f)))+(((sigma_f.*E_m)./E_f).*(1-v_f));
    end
end

at the end , x, y1 and y2 are scalars (see dimensions displayed in your workspace) and not vectors (should be same size as y ? )

the for loop must be rewritten with indexes for x and y1 , y2 - not the way you do it

suggestion (I cannot test it without your data and I am not sure that initializing y1 and y2 with zeros is appropriate - you should double check that) :

N = 100;
x =logspace(-6,2,N) ;
y1 = zeros(1,N);
y2 = zeros(1,N);

for ci = 1:N
    if x(ci) < l_c
       y1(ci)=((eta_l.*eta_0.*((v_f.*sigma_f.*E_m)./(E_f.*d.*sqrt(3))).*x(ci))+((sigma_f.*E_m.*(1-v_f))./E_f));
    else
       y2(ci)=(eta_l.*eta_0.*(((-sigma_f.*v_f.*l_c)./2).*(1./x(ci))+(sigma_f.*v_f)))+(((sigma_f.*E_m)./E_f).*(1-v_f));
    end
end

Answer 1

[Star Strider]

I do not have any of the data, however I believe that I understand the problem:

for x=logspace(-6,2)
    y = x.^2;
end

figure
plot(x, y, '.-')

However this produces a different result:

x=logspace(-6,2)

x = 1×50

0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0001 0.0001 0.0001 0.0002 0.0003 0.0004 0.0006 0.0009 0.0013 0.0018 0.0027 0.0039 0.0057 0.0083 0.0121 0.0176 0.0256 0.0373 0.0543

y = x.^2;

figure
plot(x, y, '.-')

% return

Try something like this —

x=logspace(-6,2)
idx = x < l_c
y1=((eta_l.*eta_0.*((v_f.*sigma_f.*E_m)./(E_f.*d.*sqrt(3))).*x(idx))+((sigma_f.*E_m.*(1-v_f))./E_f));
y2=(eta_l.*eta_0.*(((-sigma_f.*v_f.*l_c)./2).*(1./x(~idx))+(sigma_f.*v_f)))+(((sigma_f.*E_m)./E_f).*(1-v_f));

Be sure that all the ‘x’ references are ‘x(idx)’ or ‘x(~idx)’. I only found one in each assignment, and changed those.

.

[João Novais]

Thank you for your answer!

i wrote that code down like this but it told me that the vectors need to be the same length.

x=logspace(-6,2);
idx = x < l_c;
y1=((eta_l.*eta_0.*((v_f.*sigma_f.*E_m)./(E_f.*d.*sqrt(3))).*x(idx))+((sigma_f.*E_m.*(1-v_f))./E_f));
y2=(eta_l.*eta_0.*(((-sigma_f.*v_f.*l_c)./2).*(1./x(~idx))+(sigma_f.*v_f)))+(((sigma_f.*E_m)./E_f).*(1-v_f));
plot(x,y1,x,y2)

And then on the command window

Error using plot
Vectors must be the same length.

Error in kellytysonsimu (line 13)
plot(x,y1,x,y2)

[Star Strider]

The ‘x’ vectors need to change as well:

x=logspace(-6,2);
idx = x < l_c;
y1=((eta_l.*eta_0.*((v_f.*sigma_f.*E_m)./(E_f.*d.*sqrt(3))).*x(idx))+((sigma_f.*E_m.*(1-v_f))./E_f));
y2=(eta_l.*eta_0.*(((-sigma_f.*v_f.*l_c)./2).*(1./x(~idx))+(sigma_f.*v_f)))+(((sigma_f.*E_m)./E_f).*(1-v_f));
plot(x(idx),y1,x(~idx),y2)

That should work.

.

[João Novais]

Thank you so much!!

[Star Strider]

As always, my pleasure!

.