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I have a vb.net program in which I must compare three text files(two against one) and verify that they are all same. Even if there is one change I must know where the change is, which text file and which line. The format of the text file is like this

timestamp|ab|someval timestamp|ab|someval1 timestamp|bc|someval2 timestamp|bc|someval2

All the text files are like this with a same format, but the values might be in a different order. For instance

text1.txt ts|av|2 ts|ab|3 ts|av|4 text2.txt ts|av|4 ts|ab|3 ts|av|2

This should not fail as they have the same values. Can any one tell me how can I go about this?

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  • Have you considered using an existing diff program, and simply invoking it from your program? Commented Apr 18, 2011 at 3:02
  • With the constraint that the lines in the file can appear out-of-order, your choice of third party programs probably becomes limited. You could pre-sort the files, but you'd lose information as to which line numbers were in error. Commented Apr 18, 2011 at 17:24

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So what you have, in effect, is potentially three different permutations of the same items. So if the text files were files of integers, then these three would be considered identical:

1,2,3 3,2,1 2,1,3

but 1,2,4 would not be.

If the file is small enough to fit into memory, then you can use a simple HashSet(of String) (I hope I got the VB syntax right). Note that you only have to keep the contents of ONE file in memory. The others are read line-by-line.

For the first file, read each line into an object (or perhaps just save it as a string) and add it to your HashSet. Now, for each of the other two files (assuming the hashSet is called file1Data):

int lineNum = 0; foreach (var line in File.ReadLines(filename)) { ++lineNum; if (!file1Data.Contains(line)) { // error here } }

If the files aren't small enough to fit into memory, then I see no other option than to do an external sort on each file, then either use an existing diff program, or write a simple merge comparison.

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3 Comments

Thanks... I dont think you have the right syntax for vb.net here. Also I dont think i need to use a hashset. A simple array list would do the trick for me. But i was dumb enough not to think of this at first. Thanks again
@swordfish: If you have large files, a program using a hashset would run much faster, and really isn't (much) more complicated.
@swordfish: If you use an array then you'll have to sort it, and every check will require a binary search. If you don't sort it then every lookup will be a linear search and that will take a very long time. A HashSet will be much faster. Sorry for the C#, by the way. But you should be able to convert without much trouble, since it's mostly just using BCL objects.

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