Why is nothing coming up on my plot?

 for x = -5:0.01:5
if x <= -1
    y = 20;
elseif x > -1 && x <= 1
    y = -5 * x + 10;
elseif x > 1 && x <= 3
    y = -10 * x^2 + 35 * x - 20;
elseif x > 3 && x >= 4
    y = -5 * x +10;
else
    y = -10;
end
plot(x,y)
axis([-5,5,-100,100])
xlabel('x')
ylabel('y')
end

Answer 1

[Stephen23]

MATLAB is a high-level language, so forget about loops and use logical indexing:

x = -5:0.01:5;
y = -10+zeros(1,numel(x));
y(x<=-1) = 20;
idx = (x>-1 & x<=1) | (x>3 & x<=4);
y(idx) = -5*x(idx) + 10;
idx = (x>1 & x<=3);
y(idx) = -10*x(idx).^2 + 35*x(idx)-20;

And lets have a look at it:

>> plot(x,y,'-o')

Answer 2

[madhan ravi]

no need of loop

x = -5:0.01:5;
y =ones(1,numel(x)).*(-10);
y(x<=-1)=20;
y((x > -1 & x <= 1) | (x > 3 & x <= 4))=-5 .* x((x > -1 & x <= 1) | (x > 3 & x <= 4))+ 10;
y(x > 1 & x <= 3) = -10 .* x(x > 1 & x <= 3).^2 + 35 .* x(x > 1 & x <= 3) - 20;
plot(x,y)
axis([-5,5,-100,100])
xlabel('x')
ylabel('y')

your corrected loop way:

x = -5:0.01:5
 for i = 1:numel(x)
if x(i) <= -1              
    y(i) = 20;    %note here (i) is put in order to avoid overwriting 
elseif x(i) > -1 & x(i) <= 1
    y(i) = -5 * x(i) + 10;
elseif x(i) > 1 & x(i) <= 3
  y(i) = -10 .* x(i).^2 + 35 .* x(i) - 20;
elseif x(i) > 3 & x(i) <= 4
    y(i) = -5 * x(i) +10;
else
    y(i) = -10;
end
end
plot(x,y)
axis([-5,5,-100,100])
xlabel('x')
ylabel('y')

[madhan ravi]

if this is what you are looking for accept the answer so that people know the question is solved else let know whats additionally required , I can see that you haven't responded to the previous(question) answerer

Answer 3

[SEUNG RHI CHOI]

You didn't save y values.

i=1;
for x = -5:0.01:5
if x <= -1
  y(i) = 20;
elseif x > -1 && x <= 1
  y(i) = -5 * x + 10;
elseif x > 1 && x <= 3
  y(i) = -10 * x^2 + 35 * x - 20;
elseif x > 3 && x >= 4
  y(i) = -5 * x +10;
else
  y(i) = -10;
end
i = i+1;
end
plot(-5:0.01:5,y);
axis([-5,5,-100,100])
xlabel('x');
ylabel('y');