View source for Common Matlab/Octave errors

== Common Matlab/Octave errors ==
=== Function outputs ===
Remember that when you define a function with an output:
 function thisout = functionname (thisin)
you have to give the output (<tt>thisout</tt>) a value. Results of calculations stay
within functions unless their explicitly set to an output.

=== Undefined variables ===
By default, variables are private to the functions in which they are used. Therefore, if you want
to use a variable within a function, pass it as an input. If you want to use a variable outside of a
function, set it as an output.

=== Size of derivative vectors ===
The following error,
<pre>
error: lsode: evaluation of user-supplied function failed
error: lsode: inconsistent sizes for state and derivative vectors
</pre>
can be caused by two problems - the most likely being a mismatch between your system differentials'
output size and the starting value vector size, when calling <tt>lsode</tt>. Make sure that there is
a starting value for each of the states for which your system function calculates a derivative.

The second case is when you have a derivative output which (for some obscure reason) grows in size 
(dimensions).

=== Strange size errors ===
A very common error that can be hard to track down is shown below:
<pre>
octave> a = [1 -(1+2); 3]
error: number of columns must match (1 != 2)
</pre>

The problem is that Octave is interpreting the - as a negation (making it negative) rather than a subtraction, so there are actually two elements in the first row.  The best way to avoid this problem is to use spaces on both sides of + and - signs at all times.

=== Plotting problems ===
Most common plotting problem: you plot but "nothing happens".  The plot window appears but is blank.  You are very probably plotting scalar values instead of vectors.  Check this by repeating the plot using a more visible marker style like 
<pre>
plot(x, y, 's') % squares
</pre>
If you see a single dot, you have forgotten to add elements to your vectors.  

A common plotting pattern is
<pre>
x = []
for i = 1:10
    newx = f(); % newx is calculated somehow
    x = [x newx]; % add element to vector
end
plot(x)
</pre>
The single dot plot means you have probably forgotten to add elements to the vector, so the code looks like 
<pre>
x = newx
</pre> 

=== Using fsolve ===
fsolve is a numeric root finding routine.  If fsolve is not working for you, you probably have one of the following problems:

# Mistake in equations.  If you can, work out the root manually and substitute the root into your function.  If it does not evaluate to zero, the equation may be wrong.
# Wrong initial guess.  The results that you obtain are highly dependant on your initial guess, so if