I need help plotting this, i Cant figure out what is wrong

Question

ragheed idrees 2019-6-5

0
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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/465742-i-need-help-plotting-this-i-cant-figure-out-what-is-wrong

评论： ragheed idrees 2019-6-6

fun = @(x) x.^3 - 10*x.^2 + 16*x +80;
a = 0;
b = 6;
for n = [2, 3, 4, 5, 6, 8, 10, 12, 14, 16, 18, 20]
    
I = A9P1TRAPIdrees(fun,a,b,n);
err = abs((372-I)/372)*100;
fprintf('n = %d value=%f error= %f\n', n, I, err);
end
subplot(3,1,1); %f(x) vs. x
fplot(fun,[0 6])
title({['Ragheed Idrees  ',datestr(now)];'';'f(x) vs x)'})
xlabel('x')
ylabel('f(x)')
subplot(3,1,2); %Integral vs number of sugments
plot(n,I)
title('Integral vs number of sugments')
xlabel('number of segments')
ylabel('Integral')
subplot(3,1,3); %Percent error vs number of sugments 
loglog(n,err)
title('Percent error vs number of sugments')
xlabel('number of segments')
ylabel('Percent Error')

-----------------------------------------------------------------------------A9P1TRAPIdrees-------------------------------------------------------------------------------------------------------------

function I = A9P1TRAPIdrees(fun,a,b,n)
%this function uses the trapezoidal rule to integrate a function with given
%boundaries and number of segments
%defining the output
h = (b-a)/n;
s=0.5 * (fun(a)+fun(b));
I=0;
for jj = 1 : n-1
    s = s +fun(a + jj*h);
end
I = h *s;

it gives me a warning "Function behaves unexpectedly on array inputs. To improve performance, properly vectorize your function to return an output with the same size

and shape as the input arguments. "

and the last 2 subplots wont plot!

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Answer 1

Walter Roberson 2019-6-6

1
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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/465742-i-need-help-plotting-this-i-cant-figure-out-what-is-wrong#answer_378046

在 MATLAB Online 中打开

fun = @(x) x^3 - 10*x.^2 + 16*x +80;

should be

fun = @(x) x.^3 - 10*x.^2 + 16*x +80;

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Walter Roberson 2019-6-6

在 MATLAB Online 中打开

nvals = [2, 3, 4, 5, 6, 8, 10, 12, 14, 16, 18, 20];
numn = length(nvals);
err = zeros(1, numn);
I = zeros(1, numn);
for nidx = 1 : numn
    n = nvals(nidx);
    I(nidx) = A9P1TRAPIdrees(fun,a,b,n);
    err(nidx) = abs((372-I(nidx))/372)*100;
    fprintf('n = %d value=%f error= %f\n', n, I(nidx), err(nidx));
end
subplot(3,1,1); %f(x) vs. x
fplot(fun,[0 6])
title({['Ragheed Idrees  ',datestr(now)];'';'f(x) vs x)'})
xlabel('x')
ylabel('f(x)')
subplot(3,1,2); %Integral vs number of sugments
plot(nvals, I)
title('Integral vs number of sugments')
xlabel('number of segments')
ylabel('Integral')
subplot(3,1,3); %Percent error vs number of sugments 
loglog(nvals, err)
title('Percent error vs number of sugments')
xlabel('number of segments')
ylabel('Percent Error')
-----------------------------------------------------------------------------A9P1TRAPIdrees-------------------------------------------------------------------------------------------------------------