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Top Forums UNIX for Dummies Questions & Answers Faster than nested while read loops? Post 302545778 by alister on Tuesday 9th of August 2011 01:22:37 PM
Old 08-09-2011
Quote:
Originally Posted by chstr_14
I am thinking that a perl or an awk script could do this, which are much faster than while loops.
The while loops are not your problem; it's the repeated reading of the same file over and over again (file3 = L1*L2, file2 = L1, file1 = 1). Even if the entire file's contents are stored in the operating system's cache, while reading will be much faster than hitting the disk, a switch to kernel mode is still required (this is expensive compared to accessing process memory).

The easiest way to speed this up would be to store each file's lines in an array. While it's not necessary to use perl or awk for that purpose, since modern shells support arrays, it's a typical approach.

Quote:
Originally Posted by jim mcnamara
Why not use the join command or maybe paste?
join(1) may not be appropriate either, as it requires both a common field (not present in the sample data) and pre-sorted data (which the sample data is, but the real data may not be).

Regards,
Alister
 

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JOIN(1) 						      General Commands Manual							   JOIN(1)

NAME
join - relational database operator SYNOPSIS
join [ options ] file1 file2 DESCRIPTION
Join forms, on the standard output, a join of the two relations specified by the lines of file1 and file2. If file1 is `-', the standard input is used. File1 and file2 must be sorted in increasing ASCII collating sequence on the fields on which they are to be joined, normally the first in each line. There is one line in the output for each pair of lines in file1 and file2 that have identical join fields. The output line normally con- sists of the common field, then the rest of the line from file1, then the rest of the line from file2. Fields are normally separated by blank, tab or newline. In this case, multiple separators count as one, and leading separators are dis- carded. These options are recognized: -an In addition to the normal output, produce a line for each unpairable line in file n, where n is 1 or 2. -e s Replace empty output fields by string s. -jn m Join on the mth field of file n. If n is missing, use the mth field in each file. -o list Each output line comprises the fields specified in list, each element of which has the form n.m, where n is a file number and m is a field number. -tc Use character c as a separator (tab character). Every appearance of c in a line is significant. SEE ALSO
sort(1), comm(1), awk(1) BUGS
With default field separation, the collating sequence is that of sort -b; with -t, the sequence is that of a plain sort. The conventions of join, sort, comm, uniq, look and awk(1) are wildly incongruous. 7th Edition April 29, 1985 JOIN(1)
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