HP 35s HP 35s scientific calculator - User Guide - Page 187

Using RPN operations (which work with the stack, as explained in chapter 2). Using ALG operations (as explained in appendix C). Using equations (as explained in chapter 6). The previous example used a series of RPN operations to calculate the area of the circle. Instead, you could have used an equation in the program. (An example follows later in this chapter.) Many programs are a combination of RPN and equations, using the strengths of both. Strengths of RPN Operations Use less memory. Execute faster. Strengths of Equations and ALG Operations Easier to write and read. Can automatically prompt. When a program executes a line containing an equation, the equation is evaluated in the same way that  evaluates an equation in the equation list. For program evaluation, "=" in an equation is essentially treated as "-". (There's no programmable equivalent to  for an assignment equation - other than writing the equation as an expression, then using STO to store the value in a variable.) For both types of calculations, you can include RPN instructions to control input, output, and program flow. Data Input and Output For programs that need more than one input or return more than one output, you can decide how you want the program to enter and return information. For input, you can prompt for a variable with the INPUT instruction, you can get an equation to prompt for its variables, or you can take values entered in advance onto the stack. Simple Programming 13-5