AWK: problem getline inside while loop

jjhall · 02-27-2008, 11:33 AM

Hello,

I am having a disconcerting problem regarding awk. I am trying to read certain lines from a data file. The line numbers which I am reading are stored in a separate file.

Code:

BEGIN {
  file1 = "extract_t0.0500.dat"
  i = 29
}

{
  for (j=1;j<i;j++) {
    file_in = "pointlist_x"bin[j]".txt"
    while ((getline < file_in) > 0) {
      command = "tail +"$1" " file1 " | head -1"
      print command
      command | getline 
      print $1 " " NF
      printf("%5.4f \t%5.4f\n",$5,$7)
    }
  }
}

Note that "pointlist_x*.txt" are files with a single column of integers that correspond to the line numbers that I want to read in file1. The output of this script gives, for example:

Code:

tail +1050 extract_t0.0500.dat | head -1

(The output of the printf statement is a blank line, or null string.) Now, if I run the command

Code:

tail +1050 extract_t0.0500.dat | head -1 | awk '{printf("%5.4f \t%5.4f\n",$5,$7)}'

in the shell, I get floating point values as I expect.

Can someone explain why getline is behaving oddly in this situation?

Thanks in advance for the help.

jjhall · 02-27-2008, 12:43 PM

I may have figured out the problem. The for loop is in the body of the script, and would therefore be applied to each line of the input file. The getline command seems to be operating on the input file and ignoring the command that was supplied to it.

By putting an END statement before that chunk of code, I can fix the problem. I have noticed, however, that I must still supply an input file, e.g.

Code:

awk -f test.awk extract_t0.0500.dat

Is an input file required on the command line for invoking awk?

jim mcnamara · 02-27-2008, 03:19 PM

or use associative arrays -

Code:

awk '
      FILENAME=="file2" { arr[sprintf("%d",FNR)]=$0; }
      FILENAME=="file1" { if(arr[sprintf("%d", $1)]) { 
      	        	    print arr[sprintf("%d", $1)]
      	                }
      }
    ' file2 file1

input and output:

Quote:

/home/jmcnama> cat file1
1
3
6
8
/home/jmcnama> cat file2
this is line 1
this is line 2
this is line 3
this is line 4
this is line 5
this is line 6
/home/jmcnama> t.awk
this is line 1
this is line 3
this is line 6