Calculator input methods
![A simple four-function calculator program.](/Images/godic/202501/03/Gcalctool-basic4728.png")
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![Screenshot of an RPN Calculator program on GNU+Linux.](/Images/godic/202501/03/Screenshot_from_2015-09-10_21-56-234728.png")
![Scientific calculator displaying fractions and their decimal equivalents](/Images/godic/202501/03/CalculatorFractions-5550x4728.jpg")
There are various ways in which calculators interpret keystrokes.
One can categorize calculators into two main types: 1) single-step or immediate execution calculators and 2) expression or formula calculators.
On a formula calculator one types in an expression and then presses 'Enter' to evaluate the expression. There are various systems for typing in an expression: infix, postfix, natural display, etc.