Storing variables from a for loop

I'm trying to plot a piecewise function using a for loop and if else if statements but the loop only remembers the last value of the variable so only plots one point on the graph. How can I make it plot all the points???

clear

x = -5:0.1:5;

for x = -5:0.1:5

    if (x < -3 | x > 3)

        y = 3/(x^2-9);
    elseif (x == -3 | x == 3)
        y = 9;
    else
        y = 9/(x^2-9);
    end

end

scatter(x,y,'r');

axis([-5 5 -15 15])

xlabel('x axis');

ylabel('y axis');

Answer 1

[Rik]

You overwrote everything in your loop. You could have used a loop, but then you should have used indices. This problem can be better tackled by using logical indexing (for which you also need to not forget element-wise operation).

x = -5:0.1:5;
y=9*ones(size(x));
condition1=x < -3 | x > 3;
y(condition1)=3./(x(condition1).^2-9);
condition2=x == -3 | x == 3;
y(condition2) = 9;
condition3=~(condition1 | condition2);
y(condition3) = 9./(x(condition3).^2-9);
scatter(x,y,'r');

Using a loop would result in this code:

x = -5:0.1:5;
y=9*ones(size(x));
for i=1:numel(x)
    if (x(i) < -3 || x(i) > 3)
        y(i) = 3/(x(i)^2-9);
    elseif (x(i) == -3 || x(i) == 3)
        y(i) = 9;
    else
        y(i) = 9/(x(i)^2-9);
    end
end
scatter(x,y,'r');

[Rik]

Did this work for you? If not, feel free to post comments here with your remaining questions. If it did solve your problem, please mark it as accepted answer. It will make it easier for other people with the same question to find an answer.

Answer 2

[Star Strider]

Subscript ‘x’ and ‘y’:

The loop becomes:

for k = 1:numel(x)

      if (x(k) < -3 | x(k) > 3)

          y(k) = 3/(x(k)^2-9);
      elseif (x == -3 | x == 3)
          y(k) = 9;
      else
          y(k) = 9/(x(k)^2-9);
      end       

end

You can use ‘logical indexing’ to do this in a single anonymous function:

yfcn = @(x) ((x < -3) | (x > 3)).*(3./(x.^2-9)) + ((x == -3) | (x == 3)).*9 + ((x>-3) | (x<3)).*((9./(x.^2-9)));

figure(2)
scatter(x, yfcn(x))
xlabel('x axis')
ylabel('y axis')