How do I plot my code? I got Blank Graph

Question

hgrlk 2019-4-14

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https://ww2.mathworks.cn/matlabcentral/answers/456244-how-do-i-plot-my-code-i-got-blank-graph

评论： Geoff Hayes 2019-4-15

Hello,

I want to plot my code. It's newton-raphson method code. But I musn't change the while loop. In the question it is given that "use while and if statement".

When I write the plot, Matlab gives me a blank graph. So I exclude the " plot" when I'm sharing the code with you.

NOTE : . 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|')
Ea = 100;
m = 0 ;
x = 0;
while abs(Ea) > 10^(-3)
    if m <= 50
        f = @(x) x^3 - x - 3 ;
        diff = @(x) 3*(x^2) - 1 ; 
        xnew = x - (f(x) / diff(x));
        Ea = (((xnew-x)/xnew)*100);
        fprintf('%10.4f %10.4f %10.4f %10.4f %10.4f\n',m,x,f(x),diff(x),abs(Ea))
        x = xnew;
         
    else
        break
    end
    m = m+1;
    
    
end

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Answer 1

Geoff Hayes 2019-4-14

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https://ww2.mathworks.cn/matlabcentral/answers/456244-how-do-i-plot-my-code-i-got-blank-graph#answer_370582

在 MATLAB Online 中打开

hgrlk - store each value of x from each iteration of your loop. Then, when the loop concludes, you can plot the data. Perhaps somethig like

Ea = 100;
maxIterations = 50;
m = 1;
x = zeros(maxIterations,1);
x(m) = 12;
f = @(x) x.^3 - x - 3 ;
diff = @(x) 3*(x.^2) - 1 ; 
while abs(Ea) > 10^(-3) && m < maxIterations
    xnew = x(m) - (f(x(m)) / diff(x(m)));
    Ea = (((xnew-x(m))/xnew)*100);
    fprintf('%d %10.4f %10.4f %10.4f %10.4f\n',m,x(m),f(x(m)),diff(x(m)),abs(Ea));
    m = m + 1;  
    x(m) = xnew;
end
x = x(1:m);
figure;
plot(x);
figure;
plot(f(x));

Note how we combine the two conditions that you have - that for the error and that for the maximum number of iterations. The f and diff are defined outside of the while loop since we can then have access to f when we plot (once complete). On each iteration of the loop, we store the new estimate in an array so that we can plot all values of x and f(x). See plot LineSpec properties for details on how to add colour, style, and markers to your plot.

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hgrlk 2019-4-14

I understand that solution I guess, but I must use if condition, because it is given in the question. Thats why I wrote while abs(Ea) > 10^(-3) and if m <= 50 in my code.

How can I write that without change the if condition?

when I add plot in my code there is nothing in the graph..

Geoff Hayes 2019-4-15

You can put the if back then if that is what you need (same as you originally had). As for nothing in the graph, I was able to see the results plotted in each figure (using the code I posted).

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