The word diff, in many computer circles, refers to the concept of comparing two items and displaying, in some manner, the differences between the two items. Most frequently, it is a comparison of two files. If the output is in text, the Unix tradition is to display the differences in terms of the changes made to the first file to achieve a file similar to the second file.

Often in a GUI application, coloring or other techniques are used to convey more information about what changed. In some applications, entire lines are highlighted, while in other, particular characters are highlighted.

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See diff in Tcl

The code that was here was crap (according to the author) and has been removed.

Arjen Markus We have faced a slightly different problem: two files that should be compared with special care for (floating-point) numbers. The solution was simple in design:

• Read the files line by line (all lines should be comparable, we did not need to deal with inserted or deleted lines)
• Split the lines into words and compare the words as either strings or as numbers.
• By using [string is float] we identified if the "word" is actually a number and if so, we compared them numerically (even allowing a certain tolerance if required).

This way you are immune to numbers formatted in different ways: 0.1, +.1, 1.0E-01, +1.00e-001 all spell the same number and you can encounter all of these forms (sometimes you have less than perfect control over the precise format).

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Arjen Markus Question: would not this be a nice addition for the fileutil module in Tcllib?

GPS maybe it would...

Arjen Markus If so, it would benefit (in my opinion) from two custom procedures:

• A procedure one can supply to compare the lines (for instance: ignore white-space or interpret numbers as numbers - my original problem)
• A procedure to process the output (in a manner as Tkdiff does for instance)

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Arjen Markus A few thoughts for improving the performance:

• Store the lines as {lineno content}
• Sort by content (lsort has this ability via "-index")
• Use binary search to replace the inner loop.

This would bring back the number of iterations from O(N^2) to O(NlogN). But perhaps it is not worth the trouble :-)

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See also Using Snit to glue diff, patch, and md5sum.

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CL has received mild testimonials about "Active File Compare" available through http://formulasoft.com There's no particular Tcl connection; it's just been valuable to me as a Tcl developer when working under Windows.

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From comp.lang.tcl on Aug 22, 2007, we find this note:

A unofficial patched version of XDelta3 binary diff compression package is available on

http://downloads.sourceforge.net/tcldbrcs/xdelta30q-prepatched-tcl.tar.gz?use_mirror=osdn

[and] supports a simple but flexible callback interface to feed/extract data to/from the compressor.

TCL Examples included.

Jean-Samuel Gauthier

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comparing files in tcl

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