Plot Piecewise function graph

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L
L 2024-3-9
评论： Alexander 2024-3-10
Ran in:
Here's my code but i can't get the graph as shown in the picture.
% Use conditional statements and loops to calculate function values for
% different input ranges
x = -2.99*pi: 0.01: 3*pi; % Create domain from -2.99pi to 3pi, at increment of 0.01
y = zeros(1,length(x));
for i = 1: length(x) % For each statement i, it will pass through each value of x
if x(i) > -3*pi && x(i) < -pi % If x lies at −3pi < x <-pi,
y(i) = 1./log(2)*sin(x(i)); % the function is y = 1/(ln2)(sinx)
elseif x(i) >= -pi && x(i) <=2 % If x lies at -pi <= x <= 2,
y(i) = abs(x(i)) - 3; % the function is y = |x| -3
else
y(i) = exp(1); % If x doesn't lies at above interval, the function is y = e
end
end
plot(x, y);
text(3.2\pi, 0.2 , 'x'); text(0.2\pi, 3.5, 'y');
axis([-4\pi 4\pi -4 4]);
xticklabels({'-3\pi', '-2\pi', '-\pi', '0', '\pi', '2\pi', '3\pi'}); % Display tick marks along the x-axis by specifying the text to show pi symbol
yticks(-3: 1: 3); % Display tick marks along the y-axis 0.5,1 and 1.5
box off;
**This is the graph should be look like.

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回答（3 个）

Paul
Paul 2024-3-9
This line
y(i) = 1./log(2)*sin(x(i));
is incorrect.
The order of operations for multiplication and division is that they are evaluated in order from left to right. So that line could also be expressed as
y(i) = ( 1./log(2) ) * sin(x(i));
i.e., the division is done first, and the result of the division becomes the first operand for the multiplication.
Hopefully that gives you a clue as to how to fix the code (at least that part of it).
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Paul
Paul 2024-3-10
Yes, those two lines do yield the same result. The first line is used in the code in the question. I used the extra parentheses in the second to emphasize the order of operations used to evaluate the first.
Neither of those lines are a correct implementation of
which was the point I was trying to make to lead the OP to the correct Matlab expression. I think the OP did eventually get the correct expression, but that comment has since been deleted.

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Star Strider
Star Strider 2024-3-9
Ran in:
To do this in the Symbolic Math Toolbox, just write it essentially as in your original post —
syms x
f(x) = piecewise((-3*pi<x<-pi), 1/(log(2)*sin(x)), (-pi<=x<=2), abs(x)-3, (2<x<=3*pi), exp(1))
f(x) = 
figure
fplot(f, [-4*pi 4*pi], 'MeshDensity',1E2)
grid
ylim([-1 1]*5)
.
  2 个评论
L
L 2024-3-10
how to remove the asymptote for x=-2pi?
Star Strider
Star Strider 2024-3-10
Ran in:
First, turn all of them off, then re-draw the ones you want using the xline funciton —
syms x
f(x) = piecewise((-3*pi<x<-pi), 1/(log(2)*sin(x)), (-pi<=x<=2), abs(x)-3, (2<x<=3*pi), exp(1))
f(x) = 
figure
hfp = fplot(f, [-4*pi 4*pi], 'MeshDensity',1E2);
grid
hfp.ShowPoles = 'off'; % Turn Off All Asymptotes
xline(-3*pi, '--k') % Draw Asymptote At -3*pi
ylim([-1 1]*5)
.

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Alexander
Alexander 2024-3-10
编辑：Alexander 2024-3-10
You should change
axis([-4\pi 4\pi -4 4]);
to
axis([-4*pi 4*pi -4 4]);
in the first step. Than you can see your complete function.

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