Source code control systems (also known as Version Control Systems) let you manage projects effectively. They're very useful for one person and essential for a group. They track all changes in different versions so that no code is ever lost and meaning can be assigned to changes. One can create throw-away and debugging code with confidence with a source code control system, since the code you modify is kept carefully separate from committed, official code that will be shared with the team or released.
I was late to appreciate the benefits of source code control systems but now I wouldn't live without one even on a one-person project. Generally they are necessary when you have team working on the same code base. However, they have another great advantage: they encourage thinking about the code as a growing, organic system. Since each change is marked as a new revision with a new name or number, one begins to think of the software as a visibly progressive series of improvements. I think this is especially useful for beginners.
A good technique for using a source code control system is to stay within a few days of being up-to-date at all time. Code that can't be finished in a few days is checked in, but in a way that it is inactive and will not be called, and therefore not create any problems for anybody else. Committing a mistake that slows down your team-mates is a serious error; it is often taboo.
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