Not getting the right output for my if statement? [piecewise function]

(Ajay deleted this so I (MF) am restoring it).

    % code
  function y = FcnEval(x)
  if x < -2
      y= -1./ ((x.^2) + 1)
  elseif -2 <= x < -1
      y= sin(x)
  elseif -1 <= x < 1
      y= x.^2
  elseif 1 <= x < 2
      y=2
  else  x >= 2     
      y=1./(x+1)
  end

but when I run the test case: x1 = 3; x2 = -2:2;

y1 = FcnEval(x1)

y2 = FcnEval(x2)

I'm supposed to get y1 = 0.2500

y2= -0.9093 1.0000 0 2.0000 0.3333

what am i doing wrong?

[Walter Roberson]

Meaningful part of the question has been edited out of existence by the original poster :(

[Walter Roberson]

There is no such thing as an "if loop", just if statements.

[Matt Fig]

Saved from google cache:

    % code
function y = FcnEval(x)
if x < -2
    y= -1./ ((x.^2) + 1)
elseif -2 <= x < -1
    y= sin(x)
elseif -1 <= x < 1
    y= x.^2
elseif 1 <= x < 2
    y=2
else  x >= 2     
    y=1./(x+1)
end

but when I run the test case: x1 = 3; x2 = -2:2;

y1 = FcnEval(x1)

y2 = FcnEval(x2)

I'm supposed to get y1 = 0.2500

y2= -0.9093 1.0000 0 2.0000 0.3333

what am i doing wrong?

Answer 1

[Daniel Shub]

First off, I doubt your code runs since else if is not valid syntax. Second, I am pretty sure that 1 <= x < 2 is not doing what you think it is doing.

[Ajay]

i fixed my syntax and it is running, but i'm not getting the correct answers (everything outputs to "2") I still don't get what is wrong with the function?

[Daniel Shub]

Again, 1 <= x < 2 is not doing what you think. Do you expect 1 <= 10 < 2 to be true (1) or false (0)? What does MATLAB say? Do you understand why?

[Matt Fig]

Daniel is exactly right.

MATLAB evaluates

1 <= x < 2

as:

(1 <= x) < 2

So no matter whether x is greater than or less than 1, the result of evaluating the expression in parenthesis is always less than 2! Thus all of these are true:

tf = 1 <= -inf < 2
tf = 1 <= inf < 2
tf = 1 <= 0 < 2

[Ajay]

thanks guys i just fixed it. I wasn't aware that the notation for a piecewise function was differently portrayed in matlab. I'm still not quite getting the right answer for the last part, but it's good to know I'm on the right track!

[Daniel Shub]

What do you think MATLAB should do with

if 1 <= -2:2 & -2:2 < 2
  disp('true');
else
  disp('false');
end

MATLAB does exactly what you tell it to do. You need to be careful with arrays ...