plotting a simple constant
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Robert
le 17 Sep 2016
Réponse apportée : Sam Chak
le 2 Mar 2024
Matlab strikes again with stupidity
Been using matlab for years and still fighting ridiculous problems
x = [1:.5:10]
y = x.*4;
Z = 4
plot(x,y,'blue'); hold on
plot (x,Z,'red')
Why won't this give me a simple plot with both functions on it. Totally insane. It gives me the x*4 plot but will not give me the constant 4
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Réponse acceptée
Anatoly Kozlov
le 6 Avr 2020
Modifié(e) : Anatoly Kozlov
le 6 Avr 2020
x = 0:0.001:1;
c=5;
const = @(x)(c).*x.^(0);
plot(x, const(x))
1 commentaire
Anatoly Kozlov
le 6 Avr 2020
Modifié(e) : Anatoly Kozlov
le 6 Avr 2020
Note: const = @(x)(c); doesn't work
Plus de réponses (3)
Image Analyst
le 17 Sep 2016
Sometime in your years of using MATLAB you probably ran across ones() function but forgot about it. You need to use it so that, for each value of x, you have a value for Z. Here is the correct way to do it.
x = [1 : 0.5 : 10]
y = x .* 4
% Now declare a constant array Z
% with one element for each element of x.
Z = 4 * ones(1, length(x));
plot(x, y, 'b', 'LineWidth', 2);
hold on
plot(x, Z, 'r', 'LineWidth', 2)
grid on;
Otherwise, your Z had only 1 element, not 1 for every value of x so it won't plot a point at every value of x.
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Paul
le 2 Mar 2024
x = [1:.5:10];
y = x.*4;
Z = 4;
plot(x,y,'blue'); hold on
%plot (x,Z,'red')
yline(Z,'red')
sunny
le 2 Mar 2024
x =1:.5:10;
y = x.*4;
Z = 4;
m=5:.5:14;
n=m-x;
plot(x,y,'blue');
hold on
plot (x,n,'red');
hold off;
0 commentaires
Sam Chak
le 2 Mar 2024
Hi @Robert
Before I discovered other special non-math functions like ones() and yline(), I used to rely on certain math tricks, such as the sign function, to plot a constant y-value over a specified x range. The concept was to treat plotting
as if it were any other vector in a finite-dimensional Euclidean space. However, this trick had a fatal flaw when attempting to plot the constant y-value over
, as
. Therefore, it was necessary to adjust or shift the 'goalpost' to overcome this limitation.
![](https://www.mathworks.com/matlabcentral/answers/uploaded_files/1633351/image.png)
![](https://www.mathworks.com/matlabcentral/answers/uploaded_files/1633356/image.png)
![](https://www.mathworks.com/matlabcentral/answers/uploaded_files/1633361/image.png)
![](https://www.mathworks.com/matlabcentral/answers/uploaded_files/1633366/image.png)
Example 1: Using the sign function
x = 1:0.5:10;
y1 = 4*x;
y2 = 4*sign(x.^2);
figure(1)
plot(x, y1, 'linewidth', 2), hold on
plot(x, y2, 'linewidth', 2), grid on
xlim([x(1), x(end)])
xlabel x, ylabel y
title('Example 1: Using the sign function')
legend('y_{1}', 'y_{2}', 'fontsize', 16, 'location', 'best')
Example 2: Fatal flaw when crossing ![](https://www.mathworks.com/matlabcentral/answers/uploaded_files/1633376/image.png)
![](https://www.mathworks.com/matlabcentral/answers/uploaded_files/1633376/image.png)
x = -2:0.5:2;
y1 = 4*x;
y2 = 4*sign((x - 0).^2);
figure(2)
plot(x, y1, 'linewidth', 2), hold on
plot(x, y2, 'linewidth', 2), grid on
xlim([x(1), x(end)])
xlabel x, ylabel y
title('Example 2: Fatal flaw when crossing x = 0')
legend('y_{1}', 'y_{2}', 'fontsize', 16, 'location', 'best')
Example 3: Shifting the goalpost
x = -2:0.5:2;
y1 = 4*x;
y2 = 4*sign((x - 2*x(1)).^2);
figure(3)
plot(x, y1, 'linewidth', 2), hold on
plot(x, y2, 'linewidth', 2), grid on
xlim([x(1), x(end)])
xlabel x, ylabel y
title('Example 3: Shifting the goalpost')
legend('y_{1}', 'y_{2}', 'fontsize', 16, 'location', 'best')
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