There are various ways in which calculators interpret keystrokes. These can be categorized into two main types:
The immediate execution mode of operation (also known as singlestep, algebraic entry system (AES)^{[7]} or chain calculation mode) is commonly employed on most generalpurpose calculators. In most simple fourfunction calculators, such as the Windows calculator in Standard mode and those included with most early operating systems, each binary operation is executed as soon as the next operator is pressed, and therefore the order of operations in a mathematical expression is not taken into account. Scientific calculators, including the Scientific mode in the Windows calculator and most modern software calculators, have buttons for brackets and can take order of operation into account. Also, for unary operations, like ? or x^{2}, the number is entered first, then the operator; this is largely because the display screens on these kinds of calculators are generally composed entirely of sevensegment characters and thus capable of displaying only numbers, not the functions associated with them. This mode of operation also makes it impossible to change the expression being input without clearing the display entirely.
Formula  keystrokes  keystroke count 


6
6  

8  
4  
12 
The first and second example have been given twice. The first version is for simple calculators, showing how it is necessary to rearrange operands in order to get the correct result. The second version is for scientific calculators, where operator precedence is observed.
Immediateexecution calculators are based on a mixture of infix and postfix notation: binary operations are done as infix, but unary operations are postfix. Because operators are applied oneatatime, the user must work out which operator key to use at each stage, and this can lead to problems.^{[8]}^{[9]} When discussing these problems, Harold Thimbleby has pointed out that buttonoperated calculators "require numbers and operation signs to be punched in a certain order, and mistakes are easy to make and hard to spot".^{[10]}
Problems can occur because, for anything but the simplest calculation, in order to work out the value of a written formula, the user of a buttonoperated calculator is required to:
Mistakes can be hard to spot because:
The simplest example given by Thimbleby of a possible problem when using an immediateexecution calculator is 4 × (5).^{[11]} As a written formula the value of this is 20 because the minus sign is intended to indicate a negative number, rather than a subtraction, and this is the way that it would be interpreted by a formula calculator.
On an immediateexecution calculator, depending on which keys are used and the order in which they are pressed, the result for this calculation may be different. Also there are differences between calculators in the way a given sequence of button presses is interpreted.^{[12]} The result can be:
The effects of operator precedence, parentheses and noncommutative operators, on the sequence of button presses, are illustrated by:
These are only simple examples, but immediateexecution calculators can present even greater problems in more complex cases. In fact, Thimbleby claims that users may have been conditioned to avoid them for all but the simplest calculations.^{[14]}
The potential problems with immediateexecution calculators stem from the fact that they are imperative.^{[15]} This means that the user must provide details of how the calculation has to be performed.
Thimbleby has identified the need for a calculator that is more automatic and therefore easier to use, and he states that such a calculator should be more declarative.^{[16]} This means that the user should be able to specify only what has to be done, not how, and in which order, it has to be done.
Formula calculators are more declarative because the typedin formula specifies what is to be done, and the user does not have to provide any details of the stepbystep order in which the calculation has to be performed.
Declarative solutions are easier to understand than imperative solutions,^{[16]}^{[17]} and there has been a longterm trend from imperative to declarative methods.^{[18]}^{[19]} Formula calculators are part of this trend.
Many software tools for the general user, such as spreadsheets, are declarative.^{[20]} Formula calculators are examples of such tools.
Software calculators that simulate handheld, immediate execution calculators do not use the full power of the computer: "A computer is a far more powerful device than a handheld calculator, and thus it is illogical and limiting to duplicate handheld calculators on a computer." (Haxial Software Pty Ltd)^{[21]} Formula calculators use more of the computer's power because, besides calculating the value of a formula, they work out the order in which things should be done.
Infix notation is a method where unary operations are input into the calculator in the same order as they are written on paper. Different forms of this input scheme exist. In the algebraic entry system with hierarchy (AESH),^{[7]} the precedence of basic mathematical operators is taken into account,^{[7]} whereas calculators with algebraic entry system with parentheses (AESP)^{[7]} support the entry of parentheses.^{[7]} An input scheme known as algebraic operating system (AOS)^{[7]} combines both.^{[7]}
Calculators that use infix notation tend to incorporate a dotmatrix display to display the expression being entered, frequently accompanied by a sevensegment display for the result of the expression. Because the expression is not evaluated until it is fully entered, there is provision for editing the entered expression at any point prior to evaluation, as well as replaying entered expressions and their answers from memory.
Most graphing calculators by Casio and Texas Instruments use this method. On its scientific calculators, Sharp calls this method Direct Algebraic Logic (D.A.L.),^{[22]} and Casio calls this method the Visually Perfect Algebraic Method (V.P.A.M.).^{[23]}
Formula  keystrokes  keystroke count 

6  
8  
12  
12 
In reverse Polish notation,^{[7]} also known as postfix notation, all operations are entered after the operands on which the operation is performed. Reverse Polish notation is parenthesisfree, which usually leads to fewer button presses needed to perform an operation. By the use of a stack, one can enter formulas without the need to rearrange operands.
HewlettPackard's calculators are wellknown examples among calculators which use RPN. Early models, such as the HP35, used RPN entirely without any alternative methods. Later models, such as the HP 35s, also had infix notation and could conveniently allow users to switch between it and RPN.
Formula  keystrokes  keystroke count 


7 6  
7  
9  

12 11 (RPL and Entry RPN)^{[24]} 10 (Classical RPN)^{[24]} 
Note: The first example illustrates one of the few cases where reverse Polish notation does not use the fewest button presses  provided one does not rearrange operands. If one would do so then only six keystrokes would be needed.
BASIC notation is a particular implementation of infix notation where functions require their parameters to be in brackets.
This method was used from the 1980s to the 1990s in BASIC programmable calculators and pocket computers. Texas Instruments would later implement the method in many of its graphing calculators, including the TI83 and TI84 Plus series. Most computer algebra systems also use this as the default input method.
In BASIC notation, the formula is entered as it would be entered in BASIC, using the PRINT
command  the PRINT
command itself being optional. On pressing "ENTER" or "=", the result would be displayed. As with standard infix notation, typing mistakes in the entered formula could be corrected using the same editor function as the one used when programming the calculator.
Formula  keystrokes  keystroke count 

6  

12
16 
For the second example, two options are given depending on if the BASIC programmable pocket computers have dedicated trigonometric keys^{[25]} or not.^{[26]}
The tenkey notation input method first became popular with accountants' paper tape adding machines. It generally makes the assumption that entered numbers are being summed, although other operations are supported. Each number entered is followed by its sign (+/), and a running total is kept. An assumption is made that the last operand can be implicitly used next, so by just entering another + (for example), one will reuse the most recent operand. Ten key input mode is available in printing calculators from companies such as Sharp,^{[27]} and in software calculators like Judy's TenKey^{[28]} used by accounting firms. Online tenkey training and certification tools are available as well,^{[29]}^{[30]} and some businesses use ten key typing speed as an employment criterion.
Formula  keystrokes  keystroke count 

8  
8  
5  
9 
Modern computer algebra systems, as well as many scientific and graphing calculators, allow for "prettyprinting", that is, entry of equations such that fractions, surds and integrals, etc. are displayed in the way they would normally be written. Such calculators are generally similar in appearance to those using infix notation, but feature a full dotmatrix display and templates for entering expressions, which are navigated using arrow keys on the calculator. The templates contain spaces for values or expressions to be entered, and empty values would typically result in a syntax error, making it more cumbersome to navigate than standard infix notation; standard infix notation is often an option on such calculators as well.
Casio used to call this feature Natural Display or Natural textbook display,^{[31]}^{[32]} but now uses NaturalVPAM.^{[33]} Sharp calls this WriteView^{[34]} on its scientific calculators and simply Equation Editor on its graphing calculators.^{[35]} HP calls this its Textbook display setting,^{[36]} which can be used in both RPN and Algebraic mode and in both the Stack and in the Equation Writer application.^{[37]}Mathematica calls this SemanticFaithful Typesetting.^{[38]}Mathcad calls this standard math notation.^{[39]}Maple has a Math Equation Editor,^{[40]} but does not have a special name for this input method. Texas Instruments calls it MathPrint,^{[41]} incorporating it in its highend calculators, such as the TINspire series, and in 2011 added the feature to its TI84 series with the 2.55 OS update.^{[42]}
Formula  keystrokes  keystroke count 

6  

9
12  
4  
12 
For the second example, two options are given, depending on whether the calculators will automatically insert needed parentheses or not. Machines equipped with an alphanumeric display will display SIN(30)×COS(30)
before is pressed.