RPN input mode
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.
On an immediate execution calculator, the user presses a key for each operation, by pressing keys to calculate all the intermediate results, before the final value is shown.
With the immediate execution mode (also known as algebraic entry system (AES) or chain calculation mode) of operation each binary operation is executed as soon as the next operator is pressed, therefore the order of operations in a mathematical expression is not taken into account. Scientific calculators have buttons for brackets and these calculators can take order of operation into account. Also for unary operations like √ or x2 the number is entered first then the operator. Simple four-function calculators, such as those included with most operating systems, usually use this input method.
1 + 2 × 3 =
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