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The GNU Awk User's Guide

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13.2.1 Implementing nextfile as a Function

The nextfile statement, presented in Using gawk's nextfile Statement, is a gawk-specific extension--it is not available in most other implementations of awk. This section shows two versions of a nextfile function that you can use to simulate gawk's nextfile statement if you cannot use gawk.

A first attempt at writing a nextfile function is as follows:

 
# nextfile --- skip remaining records in current file
# this should be read in before the "main" awk program

function nextfile()    { _abandon_ = FILENAME; next }
_abandon_ == FILENAME  { next }

Because it supplies a rule that must be executed first, this file should be included before the main program. This rule compares the current data file's name (which is always in the FILENAME variable) to a private variable named _abandon_. If the file name matches, then the action part of the rule executes a next statement to go on to the next record. (The use of `_' in the variable name is a convention. It is discussed more fully in Naming Library Function Global Variables.)

The use of the next statement effectively creates a loop that reads all the records from the current data file. The end of the file is eventually reached and a new data file is opened, changing the value of FILENAME. Once this happens, the comparison of _abandon_ to FILENAME fails, and execution continues with the first rule of the "real" program.

The nextfile function itself simply sets the value of _abandon_ and then executes a next statement to start the loop.

This initial version has a subtle problem. If the same data file is listed twice on the commandline, one right after the other or even with just a variable assignment between them, this code skips right through the file a second time, even though it should stop when it gets to the end of the first occurrence. A second version of nextfile that remedies this problem is shown here:

 
# nextfile --- skip remaining records in current file
# correctly handle successive occurrences of the same file
# this should be read in before the "main" awk program

function nextfile()   { _abandon_ = FILENAME; next }

_abandon_ == FILENAME {
      if (FNR == 1)
          _abandon_ = ""
      else
          next
}

The nextfile function has not changed. It makes _abandon_ equal to the current file name and then executes a next statement. The next statement reads the next record and increments FNR so that FNR is guaranteed to have a value of at least two. However, if nextfile is called for the last record in the file, then awk closes the current data file and moves on to the next one. Upon doing so, FILENAME is set to the name of the new file and FNR is reset to one. If this next file is the same as the previous one, _abandon_ is still equal to FILENAME. However, FNR is equal to one, telling us that this is a new occurrence of the file and not the one we were reading when the nextfile function was executed. In that case, _abandon_ is reset to the empty string, so that further executions of this rule fail (until the next time that nextfile is called).

If FNR is not one, then we are still in the original data file and the program executes a next statement to skip through it.

An important question to ask at this point is: given that the functionality of nextfile can be provided with a library file, why is it built into gawk? Adding features for little reason leads to larger, slower programs that are harder to maintain. The answer is that building nextfile into gawk provides significant gains in efficiency. If the nextfile function is executed at the beginning of a large data file, awk still has to scan the entire file, splitting it up into records, just to skip over it. The built-in nextfile can simply close the file immediately and proceed to the next one, which saves a lot of time. This is particularly important in awk, because awk programs are generally I/O-bound (i.e., they spend most of their time doing input and output, instead of performing computations).


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