Convert column to matrix format using awk

Question

I have a gridded data file in column format as:

ifile.txt
x     y     value
20.5  20.5  -4.1
21.5  20.5  -6.2
22.5  20.5   0.0
20.5  21.5   1.2
21.5  21.5   4.3
         


        

Answer 1

The following awk script handles :

• any size of matrix
• no relation between row and column indices so it keeps track of them separately.
• If a certain row column index does not appear, the value will default to zero.

This is done in this way:

awk '
BEGIN{PROCINFO["sorted_in"] = "@ind_num_asc"}
(NR==1){next}
{row[$1]=1;col[$2]=1;val[$1" "$2]=$3}
END { printf "%8s",""; for (j in col) { printf "%8.3f",j }; printf "\n"
      for (i in row) {
        printf "%8.3f",i; for (j in col) { printf "%8.3f",val[i" "j] }; printf "\n"
      }
    }' 

How does it work:

• PROCINFO["sorted_in"] = "@ind_num_asc", states that all arrays are sorted numerically by index.
• (NR==1){next} : skip the first line
• {row[$1]=1;col[$2]=1;val[$1" "$2]=$3}, process the line by storing the row and column index and accompanying value.
• The end statement does all the printing.

This outputs:

          20.500  21.500  22.500
  20.500  -4.100   1.200   7.000
  21.500  -6.200   4.300  10.400
  22.500   0.000   6.000  16.700

note: the usage of PROCINFO is a gawk feature.

However, if you make a couple of assumptions, you can do it much shorter:

• the file contains all possible entries, no missing values
• you do not want the indices of the rows and columns printed out:
• the indices are sorted in column-major-order

The you can use the following short versions:

sort -g  | awk '($1+0!=$1){next}
                      ($1!=o)&&(NR!=1){printf "\n"}
                      {printf "%8.3f",$3; o=$1 }'

which outputs

  -4.100   1.200   7.000
  -6.200   4.300  10.400
   0.000   6.000  16.700

or for the transposed:

awk '(NR==1){next}
     ($2!=o)&&(NR!=2){printf "\n"}
     {printf "%8.3f",$3; o=$2 }' 

This outputs

  -4.100  -6.200   0.000
   1.200   4.300   6.000
   7.000  10.400  16.700