One day I was in a piece of code, and I thought, "this is all scrunched together; a little space would make this a lot nicer." And I set about adding blank lines where I thought they improved readability. Later, I was looking at the revision history for the file, and I saw that I had basically undone the changes of the last programmer. That programmer must have thought, "this is all spaced out; I could see more of it if it were condensed..."
Generally, I try to make the least change I can. Sometimes I find something that I think is so horrible that I do change it. In those cases, I'm thinking that the style is bad enough that it's no longer a matter of taste. Even then, I prefer to keep those changes separate from actual functional changes. I want future generations to be able to sort out the important stuff from me imposing my horrifying style on innocent code.
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