AWK/BASH: get nth line from a file by getline feed to actions in a same awk line

cristalp · 11-23-2011, 10:25 AM

Dear All,

I am asking if it is possilbe to get the nth from a file then apply the a matrix dot multiplication on that line. The matrix is from another file.

I have a xyzfile with lines like:

Code:

x1 y1 z1
x2 y2 z2
x3 y3 z3
...
...
x12 y12 z12
x13 y13 z13
...
...

I have a matrix saved in file called matrix with lines:

Code:

a11 a12 a13
a21 a22 a23
aa31 a32 a33

I found I can do dot multiplication between a matrix and a list of coordinates by:

Code:

awk 'NR==FNR {
        for(i=1;i<=NF;i++)A[NR,i]=$i
        next
      }
      {
        for(i=1;i<=NF;i++){
          t=0
          for(j=1;j<=NF;j++)
            t+=A[i,j]*$j
          printf ("%f%s", t, FS)
        }
        print ""
      }' matrixfile xyzfile

This will do the do multiplication line by line until the end of xyzfile.

However, my question is I want to dot multiply only the No.i line in the xyzfile by the matrix.

I do not want to use another awk and then redirect to a file include that line, then multiply it with matrix by the above code. I do not want any redundant file generated. I hope to do everything in a single awk simply and the running speed should be fast.

How could I change the above code slightly to achieve my goal? Is that possible to use getline in this case?

I would thank you for your kind help!!!

grail · 11-23-2011, 10:47 AM

Quote:

However, my question is I want to dot multiply only the No.i line in the xyzfile by the matrix.

Please explain further? Or at least how this differs from using FNR?

cristalp · 11-23-2011, 11:03 AM

Quote:

Originally Posted by grail

Please explain further? Or at least how this differs from using FNR?

OK, but I am worried that will make it more complicated.

The point is that the matrix file is generated from the No.i(th) line of the xyzfile. So, for each line in xyzfile the matrix will be different!!! So, I can not simply apply a single matrix to every line of the xyzfile. I must use THAT matrix generated by THAT line in the xyzfile dot multiply on THAT line itself!!! Do you get the point?? Thanks!!!

crts · 11-23-2011, 11:38 AM

Quote:

Originally Posted by cristalp

OK, but I am worried that will make it more complicated.

The point is that the matrix file is generated from the No.i(th) line of the xyzfile. So, for each line in xyzfile the matrix will be different!!! So, I can not simply apply a single matrix to every line of the xyzfile. I must use THAT matrix generated by THAT line in the xyzfile dot multiply on THAT line itself!!! Do you get the point?? Thanks!!!

Looks like I am a bit slow on comprehending. So do you mean that the matrix starts at line 'i'? You do know that matrices can vary in dimension, right? Even "normal" vectors can be viewed as matrix. The dot product of two vectors

Code:

a1     b1
a2  *  b2  = a1*b1 + a2*b2 + a3*b3
a3     b3

can be interpreted as matrix multiplication of

Code:

                b1
( a1 a2 a3 ) *  b2
                b3

So, from your example I would guess that we are talking about three dimensional matrices. Is that correct? So when you say the matrix that start at line 'i' you actually mean the matrix that spans from line 'i' to line 'i+2', right?

Quote:

I must use THAT matrix generated by THAT line in the xyzfile dot multiply on THAT line itself!

Which means that line 'i' must be replaced? What about the other two (if my assumption about three dimensions is correct) lines?
Please give an example where your matrix is marked (bold) in the first file and how the computation would look like on *paper*. If one file is indeed to be edited then show this result, too.