Nyquist criterion - undefined variable in algorithm - matlab

Question

I write an algorithm that checks the stability of the closed system of the Nyquist criterion (http://en.wikipedia.org/wiki/Nyquist_stability_criterion)
function answear=stability(re,im)
%% Function check stability of system
%re is real part of transmitation
%im is imagine part of transmitation
%% Check number of vectors elements
re(end +1:5) = 0;
im(end +1:5) = 0;
if( length(re) > length(im))
root = length(re);
else
root = length(im);
end
for w=1:root
tran(w) = re(1) + re(2)*w.^1 + re(3)*w.^2 + re(4)*w.^3 + re(5)*w.^4 +1i*(...
im(1) + im(2)*w.^1 + im(3)*w.^2 + im(4)*w.^3 +im(5)*w.^4);
end
%% Algorithm
switch root
case 0
exist('Write nonzero numbers', 'var')
case 1
for w=1:length(w)
if( real(tran(w)) > 0 && imag(tran(w)) > 0)
answear=1;
else
answear=0;
end
end
case 2
for w=1:length(w)
if( real(tran(w)) > 0 && imag(tran(w)) > 0)
if( real(tran(w)) < 0 && imag(tran(w)) > 0)
answear=1;
else
answear=0;
end
end
end
case 3
for w=1:length(w)
if( real(tran(w)) > 0 && imag(tran(w)) > 0)
if( real(tran(w)) < 0 && imag(tran(w)) > 0)
if( real(tran(w)) < 0 && imag(tran(w)) < 0)
answear=1;
else
answear=0;
end
end
end
end
case 4
for w=1:length(w)
if( real(tran(w)) > 0 && imag(tran(w)) > 0)
if( real(tran(w)) < 0 && imag(tran(w)) > 0)
if( real(tran(w)) < 0 && imag(tran(w)) < 0)
if( real(tran(w)) > 0 && imag(tran(w)) < 0)
answear=1;
else
answear=0;
end
end
end
end
end
end
%% Answear
if answear==1
disp('System unstable')
else
disp('System stable')
end
plot(real(tran),imag(tran))
grid on
end
Function returns
Undefined function or variable "answear".
Error in stability (line 87) if answear==1
So the algorithm is badly written?

Answer 1

Your code could use a lot of cleanup:
Instead of an if-statement such as this one:
if( real(tran(w)) > 0 && imag(tran(w)) > 0)
answear=1;
else
answear=0;
end
you could write a boolean assignment:
answear = real(tran(w)) > 0 & imag(tran(w)) > 0);
Why do you have three (almost) identical nested if-statements at all?
if( real(tran(w)) > 0 && imag(tran(w)) > 0)
if( real(tran(w)) < 0 && imag(tran(w)) > 0)
if( real(tran(w)) < 0 && imag(tran(w)) < 0)
First of all, you could replace everything with one if-statement. But what are you actually testing with this? It seems that those nested if statements are never executed. For instance, real(tran(w)) cannot be both positive and negative at the same time (unless it is a vector you are working on, in which case you shouldn't be using the operator &&).
Also, this is probably your code triggers the error regarding variable answear. Accessing it is impossible since it hasn't been assigned a value (none of the if-statements have been executed).
What is tran, and what is w? Are they global variables? If they are, pass them as input parameters. Your function is probably poorly designed if it relies on external states and variables.
I haven't actually run your code, but these suggestions should make it easier for you to debug it.
P.S:
Please fix the annoying spelling error (it is "answer" and not "answear") :)