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awk question - print columns with names

 
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# 1  
Old 08-05-2014
awk question - print columns with names
I found this command and would like to know what it means:

Code:
gawk 'NR==1{for(i=1;i<=NF;i++)if($i~/FE/)f[n++]=i}{for(i=0;i<n;i++)printf"%s%s",i?" ":"",$f[i];print""}'

It seems to mean if the row =1 assign i to that row, and then if FE is in the top row /column then increment and print the row.

I am at a loss as to what i<n means. And also the f[n++]. Anybody familiar with awk? The command seems to print a column based on the name of the column instead of the number of the column
# 2  
Old 08-05-2014
Code:
gawk 'NR==1{ # For the very first line ( NR is line number )
        for(i=1;i<=NF;i++) # Loop over all columns from 1 to NF (number of fields)
                if($i~/FE/)f[n++]=i # If the field contains NE, set F[n]=(field number).  n increments itself because of the ++.
                # I wonder if that line is supposed to be f[n++]=$i, to save the column itself.
        }
        { # For every line
                for(i=0;i<n;i++) # Loop from 0 to n, from the block above
                        printf "%s%s",i?" ":"",$f[i] # Print each column in array f in order.
                        # For the first column, print nothing before it.
                        # For every other column, print : first.
                        print "" # Print a blank line, just to put a newline at the end
        }'

This User Gave Thanks to Corona688 For This Post:
newbie2010
# 3  
Old 08-05-2014
Quote:
Originally Posted by Corona688
Code:
gawk 'NR==1{ # For the very first line ( NR is line number )
        for(i=1;i<=NF;i++) # Loop over all columns from 1 to NF (number of fields)
                if($i~/FE/)f[n++]=i # If the field contains NE, set F[n]=(field number).  n increments itself because of the ++.
                # I wonder if that line is supposed to be f[n++]=$i, to save the column itself.
        }
        { # For every line
                for(i=0;i<n;i++) # Loop from 0 to n, from the block above
                        printf "%s%s",i?" ":"",$f[i] # Print each column in array f in order.
                        # For the first column, print nothing before it.
                        # For every other column, print : first.
                        print "" # Print a blank line, just to put a newline at the end
        }'

Close. Comments below are related to the text marked in red above:
  1. "NE" should be "FE".
  2. No. F[n++]=i is correct. F[0] will be the field number of the 1st field with a header containing "FE"; F[1] will be the field number of the 2nd field with a header containing "FE", ... F[n-1] will be the field number of the nth field containing "FE". Since there is no next command associated with the NR==1 condition, the header for all fields containing "FE" will be printed by the next clause.
  3. The character printed before the 1st field output is an empty string; the output field separator output before other fields is a single <space> character.
This User Gave Thanks to Don Cragun For This Post:
Corona688
 
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