Re: Constructing arrays

[Martin Helm]

Am Montag, 26. Juli 2010 14:56:48 schrieb dirac:
> Hi thanks for the reply,
> 
> I see how this can work, but I am having problems with my data. The problem
> I think is that I have a matrix which I pre-defined as zeros, then replaced
> by numbers from a for loop calculation. I don't know how many rows I will
>  be using in the calculation (in the for loop) so I pre allocated more than
>  enough...Could this be a problem? Should I remove the zeros on the end of
>  the vector with the indecies in?
> 
> Also could I have something like:
> 
> a=[1:10]
> b=matrix
> col=some pre defined column from the matrix
> plot(a(1:length(b),col))
> 
> it is b that has the zeros and indecies in.
> 
> Sorry if this isn't very clear!
> 
> Thanks
> Martin
> 
> -----
> Pretty much convinced Octave>Excel for scientific data analysis!
> 
If I understand this correctly and you want to use a column of the matrix for 
the index it should read like this

plot(a(b(:, col)))

b(:, col) extracts the column number col from the matrix (so the result is a 
vector of indexes).

Remark: I would remove the trailing zeros or if you don't want that and have 
for example the value 

m = number of nonzero values in the column 

you can also do

plot(a(b(1:m, col)))

otherwise you will have an error like this (if index 0 is accessed)

error: subscript indices must be either positive integers or logicals.


- mh