# Simpson's Rule Illustration - How to create those parabolas?

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Dominic el 27 de Mzo. de 2013
Comentada: Richard Treuren el 17 de Abr. de 2014
I managed to create the rectangle and trapezium strips, but stuck on the parabola strips for Simpson's Rule like the one shown below.
In this code, the user has to input the strip width, function, limits,
Here's my code for the RECTANGLE STRIPS
% 7. Display figure
clf, hold on;
plot([a b], [0 0], 'k' ), plot([0 0], [min( y_x(x) ) max( y_x(x))], 'k') % This shows a black line for the x-axis (y=0) and the y-axis (x=0).
xlabel('x', 'FontWeight', 'bold'), ylabel('y(x)', 'FontWeight', 'bold')
title(['Numeric integration through Rectangle Rule of y(x)=' , y_xs , ' with ', num2str(n), ' slices ||| Result is ', num2str(S_r) '.'] , 'FontWeight', 'bold')
% 8. To create the rectangular strips
for x=a:dx:(b-dx);
y_x(x);
left = x; right = x+dx; bottom = 0; top = y_x(x);
X = [left left right right]; Y = [bottom top top bottom]; %to create the shape
fill(X,Y, 'b', 'FaceAlpha', 0.3)
end
% 9. Display a smooth line in the graph
x = a:dx/100:b;
plot(x, y_x(x), 'k')
Here's my code for the TRAPEZIUM STRIPS:
% 7. Display figure
clf, hold on;
plot(x, y_x(x), 'k--')
plot([a b], [0 0], 'k'), plot([0 0], [min( y_x(x) ) max( y_x(x))], 'k')
xlabel('x', 'FontWeight', 'bold'), ylabel('y(x)', 'FontWeight', 'bold')
title(['Numeric integration through Trapezium Rule of y(x)=' , y_xs , ' with ', num2str(n), ' slices ||| Result is ', num2str(S_t) '.'] , 'FontWeight', 'bold')
% 8. Display Trapezium strips
for x=a:dx:(b-dx);
y_x(x);
left = x; right = x+dx; bottom = 0; top1 = y_x(x); top2 = y_x(x+dx);
X = [left left right right]; Y = [bottom top1 top2 bottom]; %to create the shape
fill(X,Y, 'b', 'FaceAlpha', 0.3)
end
% 9. Display a smooth line in the graph
x = a:dx/100:b;
plot(x, y_x(x), 'b')
Comments on how to optimise and improve brevity this code would also be appreciated! Cheers
##### 6 comentariosMostrar 4 comentarios más antiguosOcultar 4 comentarios más antiguos
Dominic el 29 de Mzo. de 2013
Let me know if you want to have a look at the other parts of this code.
Charles el 26 de Abr. de 2013
let's have a look at rest of the code

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### Respuestas (3)

Richard Treuren el 17 de Abr. de 2014
Editada: Richard Treuren el 17 de Abr. de 2014
I changed the script of proecsm a bit so that it does work for different step sizes and some other changes in the area plot
clc;clear; close all
steps = 5; % number of steps
x = linspace(0,2*pi,steps*12); % create data
xs = linspace(0,2*pi,2*steps+1); % sample points
f = sin(x);
fs = sin(xs); % sample function
c(steps,3)=0;
for k = 1:steps
c(k,:) = polyfit(xs(2*k-1:2*k+1),fs(2*k-1:2*k+1),2); %fit coefficients
hold on
z = linspace(xs(2*k-1),xs(2*k+1),12);
y = c(k,1).*z.^2+c(k,2).*z+c(k,3);
area(z,y,'FaceColor',[0.6,1,1])
end
hold on
plot(x,f,'r','LineWidth',2) % plot function
hold on
plot(xs,fs,'bo','LineWidth',2) % plot sample points
##### 1 comentarioMostrar -1 comentarios más antiguosOcultar -1 comentarios más antiguos
Richard Treuren el 17 de Abr. de 2014
if you want to calculate the area (using simpson's rule) you can add the next lines to the script:
sim=0;
for i=1:steps
sim = sim + (xs(2*i+1)-xs(2*i-1))/6*(fs(2*i-1)+4*fs(2*i)+fs(2*i+1));
simp(i)= sim; %variable for plotting
end
sim

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bym el 31 de Mzo. de 2013
here is what I have done
clc;clear; close all
x = linspace(0,4*pi); % create data
f = sin(x);
xs = linspace(0,4*pi,11); % sample points
fs = sin(xs); % sample function
plot(x,f,'g') % plot function
hold on
plot(xs,fs,'bo') % plot sample points
xp = reshape(x,[],5)'; % prepare for plotting
xp(5,21)=x(end); % duplicate end point
xp(1:4,21)=xp(2:5,1); % duplicate end points
c(5,3)=0; %preallocate
for k = 1:5
c(k,:) = polyfit(xs(2*k-1:2*k+1),fs(2*k-1:2*k+1),2); %fit coefficients
fill([xp(k,1) xp(k,:) xp(k,end)],[0 polyval(c(k,:),xp(k,:)) 0] ...
,'c', 'FaceAlpha',.1)
end
ylim([-1.25,1.25])
##### 4 comentariosMostrar 2 comentarios más antiguosOcultar 2 comentarios más antiguos
Dominic el 2 de Abr. de 2013
This is the figure that is showing up with mine...
Here is the code, I am not sure what to replace the numerical values I am asking above, so I just copied what you gave. Surprise, surprise, it didn't show as the way I want it to.
Here is my try
% 4. Create the strips (with parabolic arcs)
x_s = linspace(a,b); % create x-data
y_s = y_x(x_s); % create y-data
x_arcs = linspace(a,b,n+1); % sample points
y_arcs = y_x(x_arcs); % sample function
plot(x_arcs, y_arcs, 'bo-') % plot sample points
xp = reshape(x_s, [], 5)'; % !!! prepare for plotting
xp(5,21)=x(end); % !!! duplicate end point
xp(1:4,21)=xp(2:5,1); % !!! duplicate end points
c(5, 3)=0; % !!! preallocate
for k = 1:5
c(k,:)=polyfit(x_arcs(2*k-1:2*k+1), y_arcs(2*k-1:2*k+1), 2); %fit coefficients
fill([xp(k,1) xp(k,:) xp(k,end)], [0 polyval(c(k,:), xp(k,:)) 0], 'c', 'FaceAlpha', 0.1)
end
Dominic el 2 de Abr. de 2013
please refer to the variable list above

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Dominic el 4 de Abr. de 2013
##### 1 comentarioMostrar -1 comentarios más antiguosOcultar -1 comentarios más antiguos
Dominic el 6 de Abr. de 2013
bump.

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