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Help with piecewise function? Can't use else/if?
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AJ
el 18 de Feb. de 2014
Comentada: Carlos Guerrero García
el 14 de Nov. de 2022
Hi, I am having trouble on a piece-wise homework problem I am having.
This is my code. I plot it and in the middle of the graph from negative pi to positive pi where y should be the cosine of x isn't right. I just have a straight line at y=-1 across the graph. I know I need to make it a vector or a loop it so it doesn't use if/else and skip the middle cosine function.
Whenever I try to get a function command I get this: Function definitions are not permitted in this context.
code so far:
clear all
close all
clc
x=-2*pi:.01:2*pi
if (x<-pi)
y=-1;
elseif(x>=-pi&x<=pi)
y=cos(x);
else(x>pi)
y=-1;
plot(x,y)
end
This is what I'm getting: The middle part (cosine function) is wrong. I was told it's just graphing the part after else.
Help please?
Thank you.
2 comentarios
Matt Tearle
el 18 de Feb. de 2014
Congratulations on writing a great (homework) question!
Respuesta aceptada
Karan Gill
el 1 de Dic. de 2016
Editada: Karan Gill
el 17 de Oct. de 2017
If you have R2016b, you can just use the piecewise function: https://www.mathworks.com/help/symbolic/piecewise.html.
2 comentarios
Sarah Palfreyman
el 14 de Dic. de 2016
Editada: Sarah Palfreyman
el 14 de Dic. de 2016
Here is the example.
john Snori
el 1 de Sept. de 2021
The second method involves the use of if-else statements along with for loop. In this method, we’ll define all the sub-functions along with the constraints using if-else statements and then we will plot the piecewise function.
Source : https://www.entechin.com/how-to-plot-a-piecewise-function-in-matlab/
Más respuestas (3)
AJ
el 18 de Feb. de 2014
2 comentarios
Jos (10584)
el 18 de Feb. de 2014
Take a look at this:
% Step 1: initialise x and y
x = -10:2:10
y = zeros(size(x))
% Step 2: selective processing
tf = x < -5
y(tf) = -5
tf = x > -5 & x < 5
y(tf) = -2 + 2 * x(tf)
tf = x > 5
y(tf) = 5
% step 3: visualisation
plot(x,y,'bo-')
Image Analyst
el 18 de Feb. de 2014
Editada: Image Analyst
el 18 de Feb. de 2014
Use two &:
elseif x(k) >=-pi && x(k) <= pi
and you need to make an array out of y=-1:
y(k) = -1;
otherwise it's just a single number, not an array of -1s. Plus you need to have it in a loop over k like I mentioned
for k = 1 : length(x)
then everything inside the look has a (k) index. Of course there is a vectorized way to do it, if you want that.
6 comentarios
Jenik Skapa
el 26 de Mzo. de 2019
Editada: Jenik Skapa
el 26 de Mzo. de 2019
I would use this construction without the "for" loop:
x = -2*pi : pi/5 : 2 * pi;
y = NaN * ones(size(x));
y(x < -pi) = -1;
y((x >= -pi) & (x < pi)) = cos(x((x >= -pi) & (x < pi)));
y(x >= pi) = -1;
plot(x, y, 'r*-'); grid on;
Steven Lord
el 26 de Mzo. de 2019
FYI you can use NaN to create an array without needing to first create a ones array.
y = NaN(size(x));
Carlos Guerrero García
el 13 de Nov. de 2022
And what about this code ???
x=-5:0.1:5; g=-1+(abs(x)<=pi).*(1+cos(x)); plot(x,g)
Do you like it ???
4 comentarios
Image Analyst
el 14 de Nov. de 2022
I guess you are choosing not to take the suggestions, but I'll do them here:
x = -2 * pi : 0.01 : 2 * pi;
g = -1 + (abs(x) <= pi) .* (1 + cos(x));
plot(x, g, 'b-', 'LineWidth', 2)
grid on;
xlabel('x');
ylabel('g');
Carlos Guerrero García
el 14 de Nov. de 2022
Thanks to Image Analyst....I have no time enough to make the corrections suggested but thanks to your improvemets to my basic script!!!!
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