How to define and plot a piecewise function using if/elseif

Question

Iakovos Antonoglou 2018-6-18

0
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https://ww2.mathworks.cn/matlabcentral/answers/406065-how-to-define-and-plot-a-piecewise-function-using-if-elseif

评论： Walter Roberson 2018-6-18

采纳的回答： Walter Roberson

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Hello, i am trying to define and plot a piecewise fucntion specifically using if/elseif. The function is

f(x)=1,sin(x)>=1/2 f(x)=-1,sin(x)<=-1/2 f(x)=0,elsewise

I've trying using syms or function but i seem to be ending up with a problem, most commonly being any function i try to define inside an if gives me an error "x function not defined"

My latest attempt that produces no error but still doesnt work is

function gga(x)
x=linspace(0,4*pi)
if sin(x)>=1/2
    y=1
elseif sin(x)<=-1/2
    y=-1
else y=0
end
plot(x,y)
end

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Answer 1

Walter Roberson 2018-6-18

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https://ww2.mathworks.cn/matlabcentral/answers/406065-how-to-define-and-plot-a-piecewise-function-using-if-elseif#answer_325038

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Symbolic form:

syms x
f(x) = piecewise(sin(x) >= 1/2, 1, sin(x) <= -1/2, -1, 0);
fplot(f, [0 4*pi])

Numeric form:

y = zeros(size(x));
sx = sin(x);
mask = sx <= -1/2;
y(mask) = -1;
mask = sx >= 1/2;
y(mask) = 1;

It is also possible to construct it as a single numeric expression, but that takes a bit of thinking.

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Iakovos Antonoglou 2018-6-18

Hello, I am looking to use if/else if specifically,not the piecewise command. Thank you for your time

Walter Roberson 2018-6-18

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In order to do this using if/elseif/else you will need to loop over the elements.

for idx = 1 : length(x)
  if sin(x(idx)) ...

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Answer 2

Ankita Bansal 2018-6-18

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https://ww2.mathworks.cn/matlabcentral/answers/406065-how-to-define-and-plot-a-piecewise-function-using-if-elseif#answer_325071

编辑：Ankita Bansal 2018-6-18

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Hi Iakovos, the solution given by Walter can be rewritten as

x=linspace(0,4*pi);
y = zeros(size(x));
y(sin(x) <= -1/2) = -1;
y(sin(x) >= 1/2) = 1;
plot(x,y)

However, if you want to use if/else in your code then you can do this as

function gga
x=linspace(0,4*pi);
l=numel(x);
for i=1:l
    if sin(x(i))>=1/2
    y(i)=1;
elseif sin(x(i))<=-1/2
    y(i)=-1;
else y(i)=0;
    end
end
plot(x,y)
end