RPN versus algebraic mode
When the HP 50G is turned on for the first time, it operates in algebraic mode, which matches the input style used by most non-HP calculators. In algebraic mode, binary operators such as + and * are written infix (e.g., 5+3), while functions are written in prefix notation (e.g., SIN(3.14); the parentheses around the argument are added automatically when you hit the SIN key). Moreover, the usual rules of operator precedence apply (e.g., 5+3*2 is taken to mean 5+(3*2) = 11), and parentheses are used to group expressions and override default precedences (e.g., 2*(5+3) = 16). The expression to be calculated is entered on a single input line and is evaluated in one go when you hit the ENTER key. The main advantage of this input method is that it is close to "textbook-style" formulas and is familiar to users who haven't used HP calculators before.
The HP 50G also has a second input method, called RPN a.k.a. reverse polish notation in which binary operators as well as all other functions and commands are written postfix (e.g., 5 3 +). When using this method, the ENTER key is used to separate consecutive operands, so the example above would actually be typed on the calculator as: 5 ENTER 3 +. RPN is HP's traditional input method which has been used on most older HP calculators and is preferred by most dedicated HP calculator users. The main advantages are that you don't need any parentheses, usually resulting in fewer keystrokes and making it much easier to enter complicated expressions, and that the calculation is done in a stepwise fashion so that intermediate results can be reviewed (and, if necessary, corrected) during the calculation. Calculator programming is also done in RPN mode, which is the native mode of all internal operations within the calculator.
You can easily switch between algebraic and RPN mode using the following commands: MODE +/- ENTER. Once you've done this, the new mode setting "sticks" even if you turn the calculator off, until you change it again.
Algebraic or RPN, what should I use?
With most older HP calculators you actually didn't have a choice, since they only provided RPN. However, the 49 series calculators including the 50G also have a very nice algebraic mode, which raises the question which one to use as the primary interface to the calculator. HP has made the algebraic mode the default because that will make it easier for new users to start using the calculator right away, but that doesn't mean that you will always be able or wish to get along without learning RPN.
Although RPN might feel a bit "backwards" at first, most if not all seasoned HP calulator users will assure you that it becomes quite natural once you're familiar with it, and makes calculations easier and more effective. It's also one of the distinguishing features of HP calculators, and facilitates access to some of the calculator's more advanced functionality. Since RPN is so popular among HP calculator users, a lot of third-party documentation will assume that you're well familiar with it. Last but not least, even in RPN mode it is possible to enter expressions in the algebraic way using algebraics and the equation editor.
On the other hand, the algebraic mode of the HP 50G also works quite well for many problems, and this method will be easier for newbies and casual users who are not familiar with RPN. Another advantage of the algebraic mode is that it gives you a history of past inputs and results. So it makes sense to familiarize yourself with both input styles and use them as you see fit. Most seasoned HP calculator users prefer RPN, however, and will use that as their primary interface to the calculator.
See also
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