bicgstab with convergence monitor / modify MATLAB's .m files

Hi everybody,

the thing I want to achieve is an iterative matrix solver (bicgstab) that delivers a convergence monitor during run time, i.e., a semi-log plot with the iteration number as x-axis and the achieved relative error of the respective iterated solution as logarithmic y-axis.

The way I was trying to do it was I edited the bicgstab.m file from the Matlab sparse matrix toolbox and just inserted a plot command into the for loop that performs the iterations.

But then, the following error occured:

Undefined function 'iterchk' for input
arguments of type 'double'.
Error in bicgstab_monitor (line 75)
[atype,afun,afcnstr] = iterchk(A);

iterchk is a function that is called from within the bicgstab.m-function, but when I want to call it from the command window, it returns the same error.

It seems that I cannot use the same functions that are called from within the bicgstab.m-function if I just copy and paste it to another place. Can you tell me the reason and maybe a solution to this issue?