awk in multiple files


 
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# 1  
Old 01-24-2011
awk in multiple files

Hello all,
I have two files:
one that looks like this
 
string1
string2
string3
string6
and another that looks like this
 
Wstring1
apple
Wstring2
vegetable
Wstring3
mouse
Wstring4
globe
Wstring5
night
Wstring6
letter
cup
table
the word string in the second file always starts with the capital W

I want to find, for every string that is written in the first file, the corresponding lines under this string in the second file.

For this example the output would be:
 
Wstring1
apple
Wstring2
vegetable
Wstring3
mouse
Wstring6
letter
cup
table

I suppose it is an awk command...
# 2  
Old 01-24-2011
It is a filtering join (not an outer join (all) or cartesian product (3 lines and 4 lines yields 12 possible lines) join), so the first question is, is it many to many. Second question is, are they sorted the same (or can be). Many to many requires multiple passes or storage. Sorting is necessary for a merge join, which is very popular and robust.

Awk or one of the more advanced shells with associative arrays can do it, for modest amounts of data and speed, even if they are not sorted and 0/1 to 1/many, just put one file into the associative array and then look it up using the data from the other file.
# 3  
Old 01-25-2011
I did not understand your answer...
# 4  
Old 01-25-2011
Join is a RDBMS / set logic term.

If you start by readin a line from file 1 into one variable and two lines from file 2 into 2 other variables, then you are started into a merge. If the values match, you print, but then which do you read next?




It will not work unless the technique and sometimes data accomodate any data challenges. You know the data. (pick one):
  • you store all of file1 in an associative array and then look up as you go through file 2.
  • both files are sorted by string, and the values in one file are unique, so when one file has a low string, that is where you read. We could do the sort, but two line records in file 2 need to be merged to one line. The join command can do this for you.
  • you test every line of one file for every record in the other file, M x N comparisons, which is OK for small files.

Last edited by DGPickett; 01-25-2011 at 11:11 AM..
 
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