Re: simple (not for me) operation on matrix

[Jonathan C. Webster]

Paolo Ariano wrote:
> hi *
>
> ive a big matrix (750 cols* 140 rows) for example:
>
> 1 2 3 2 4 5 6
> 3 4 6 5 3 2 1
> 2 4 5 6 7 8 8
> 6 7 5 4 3 2 1
>
> i need to obtain a new matrix where i sum line 1 and line 2, then line 3
> and line 4 and so on as:
>
> (1+3) (2+4)
> (2+6) (4+7)

Sometimes reshape and transpose can be faster than looping 
and indexing.
Try this.  I left out the numerical responses.
#origional matrix
aa = rand(6, 3)

# reshape transpose
raap = reshape (aa',6,3)

#You now wish to .+ the top 3 rows to the bottom 3 rows
# and transpose the result

 qq1 = raap(1:3,:)
 qq2 = raap(4:6,:)
 ssum = ( qq1 .+ qq2)'

You can probably get that down to one or two lines.

Hope that helps,
Jonathan



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