generation of data plot using an equation

Hi Friends, I am a newbie in this world. Major of my work consists of generation of data plot. Recently I am need for a code in which I can vary r_shell for a fixed value of r_core to find out g from the given equation:

• g=[(r_core+r_shell)^3-r_shell^3]/(r_core+r_shell)^3
• The range of r_shell is 0 to 5.2 with gap 0.26. In each case, the value of r_core is fixed at 5. Plot should be r_shell in x-axis and g in y-axis.I also need the data in tabulated form.My code was:

if true
clear all;
close all;
dbstop if error;
r_shell=vertcat(0:.26:5.2);
r_core=5;
for i=0:length(r_shell); 
  a = (r_core+i).^ (3)-(i).^ (3);
  g= a/(r_core+i).^3; 
end 
temp=[i;g]';
save('temp','-ascii')
figure
plot(temp(:,1),temp(:,2));
xlabel('Shell thickness(nm)')
ylabel('Murray g factor');

save('diel.tab','temp','-ascii') % code end

I wrote the code but due to my lack of knowledge it remain failed to made it. Please suggest me. I am in deep need of it. Thank you all and Merry Christmas for you and family. Hirak

Answer 1

[Hirak]

Thank you Niklas. It worked nicely. Now, if I want to vary r_core also, say, for the spread 1:0.1:10, and make a three dimensional plot,what should I do? I want to learn MATLAB. From where should I start? There is no one here to teach me MATLAB. Please suggest me. With warm regards, Hirak.

Answer 2

[Niklas Nylén]

I can see a few issues in your code:

The use of vertcat when defining r_shell seems unnecessary, this should give the same result:

r_shell=0:.26:5.2;

Matlab indexes start at 1, not 0, and you also use the counting variable i instead of the value r_shell(i), but in this case a for loop is unnecessary. A matrix operation is faster:

g=((r_core+r_shell).^3-r_shell.^3)./(r_core+r_shell).^3

To plot the data:

figure
plot(r_shell, g);
xlabel('Shell thickness(nm)')
ylabel('Murray g factor');

And save the data in an ascii file

data = [r_shell; g]:
save('diel.tab','tmp','-ascii')

[Hirak]

Thank you Niklas. It worked nicely. Now, if I want to vary r_core also, say, for the spread 1:0.1:10, and make a three dimensional plot,what should I do? I want to learn MATLAB. From where should I start? There is no one here to teach me MATLAB. Please suggest me. With warm regards, Hirak.

[Niklas Nylén]

What you will want to do is to create two vectors where all combinations of the two original vectors are present. I will use variable names x and y in this example with some limited amount of data for readability.

Step 1: Create the original vectors (this is r_shell and r_core in your case)

>> x = 1:0.5:2;
>> y = 8:1:10;

Step 2: Create all combinations of the two vectors x and y by using the function meshgrid

 >> [x1,y1] = meshgrid(x,y)

This will create two matrices where all combinations are available

x1 = 1    1.5    2
     1    1.5    2
     1    1.5    2

y1 =  8     8     8
      9     9     9
     10    10    10

Although you can use these matrices as direct inputs to your calculation, for future plotting it would be nice to have them as vectors instead.

>> x1 = x1(:)
>> y1 = y1(:)

x1 =  1  1   1  1.5  1.5  1.5   2   2   2
y1 =  8  9  10    8    9   10   8   9  10

As you can see, if you combine the elements with the same index in the two vectors, you get all possible combinations. Now you can use the vectors in a calculation, for example

>> z = (x1 + y1.^x1)./x1

z = 9.00 10.00 11.00 16.08 19.00 22.08 33.00 41.50 51.00

To plot the 3d data, use the command plot3

>> figure
>> plot3(x1,y1,z,'.')
>> grid on

It looks like the answer you accepted above was your own response, please click accept on my answer instead.

As for learning matlab, I would suggest either searching online for some small projects that you can create. There are also a number of beginner tutorials available.