I can not plot my Matlab code

Question

hgrlk 2019-4-11

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https://ww2.mathworks.cn/matlabcentral/answers/455842-i-can-not-plot-my-matlab-code

评论： Adam Danz 2019-4-15

Hello,

I want to plot my code, but when i write the code there is no line in the graph. I want 2 graphs, one is f(x) versus i and one is x versus i. Also how can I write a function for the circle mark for each data point in the function? ( I use Matlab R2015a )

Thank you..

clc
clear all
close all
fprintf('%10s %10s %10s %10s %10s\n','i','xi','f(xi)','diff_f(xi)','|Ea|')
x = 0;
i = 1;
while i <= 50
   
  f = @(x) x^3 - x - 3 ;
  diff = @(x) 3*(x^2) - 1 ; 
  xnew = x - (f(x) / diff(x));
  if abs(((xnew-x)/xnew)*100) < 10^(-3)
      break
  end
  
  
 fprintf('%10.4f %10.4f %10.4f %10.4f %10.4f\n',i,x,f(x),diff(x),abs((xnew-x)/xnew)*100)
    
  x = xnew;
  i =i+1;
    
end

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Adam Danz 2019-4-11

The code you shared doesn't produce a fugure at all. Also, 'xi' is nowhere present in your code.

hgrlk 2019-4-11

I know, I erase the code of plot that i wrote. Because it didn't give me anything.

xi is x in my code, I didn't mentioned sorry about that.

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Answer 1

Adam Danz 2019-4-11

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https://ww2.mathworks.cn/matlabcentral/answers/455842-i-can-not-plot-my-matlab-code#answer_370265

编辑：Adam Danz 2019-4-12

在 MATLAB Online 中打开

On each iteration of your while-loop you were overwriting the x variable instead of stored the value for each iteration.

Here is a re-write of your code with the following significant chages. I tested the first and last row of the outputs for my version and your version and they are identical.

I moved the anonymous functions outside of the loop and changed the internal variable names to z
I changed your function name "diff" to "dif" since matlab already has a function named "diff"
Your x variable is allocated prior to the loop so it can store the value from each iteration.
The while-loop was replaced with a for-loop.
Everything within the loop was restructured and simplified.
At the end of the for-loop, we're storing the x variable value for each iteration.

fprintf('%10s %10s %10s %10s %10s\n','i','xi','f(xi)','diff_f(xi)','|Ea|')
f = @(z) z^3 - z - 3 ;
dif = @(z) 3*(z^2) - 1 ;
x = NaN(1,50);
for i = 1:50
    if i == 1
        tempX = 0; 
    else
        tempX = xnew; 
    end
    xnew = tempX - (f(tempX) / dif(tempX));
    if abs(((xnew-tempX)/xnew)*100) < 10^(-3)
        break
    end
    fprintf('%10.4f %10.4f %10.4f %10.4f %10.4f\n',i,tempX,f(tempX),dif(tempX),abs((xnew-tempX)/xnew)*100)
    x(i) = xnew; %
end

If you want to plot f(x) vs i,

subplot(1,2,1)
plot(1:50, f([0, x(1:end-1)]))
xlabel('i')
ylabel('f(x)')

If you want to plot x vs i,

subplot(1,2,2)
plot(1:50, x)
xlabel('i')
ylabel('x')

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hgrlk 2019-4-14

thank you, but in my code i musn't change the while loop as for. because in the question it is given "use while and if". But i think it will work with"while loop" too.