

Post date: December 31, 2007, 01:12
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Tutorial quote: If anybody within earshot of you is struggling to learn sed and ever remarks "This is like learning Klingon! Could they make it any more cryptic?", you can always point them to dc.
dc is the commandline Unix "ReversePolish Notation"(RPN) calculator. The name stands for "desktop calculator". OK, so what is a Polish calculator and why would you want to reverse one? The math that you do in school uses infix notation, with the operator between the numbers (3 + 5). Prefix notation puts the operator first (+ 3 5) and is what the Lisp language uses. The prefix notation is known as "Polish notation" after the nationality of Jan Łukasiewicz who invented it. Postfix notation, then, has the operator at the rear (3 5 +), and so is also the reverse of Polish notation.
What's the difference? In computer programming, you have to specify what order you want a multipart calculation to be in. Everybody is familiar with the old problem of A*BC. For A=5, B=3, C=2, multiplying first and subtracting second gives you 13, while subtracting first and multiplying second gives you 5. To specify which operations you want performed first, you have to either memorize the complex orders of operations (which change from one language to the next) or use a lot of parenthesis ((A*B)C) or (A*(BC)). Hey, that's what everybody complains about in all those languages that use parenthesis! 
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