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Postfix + + Postfix + + is a stack - based computer language directed at the evaluation of arithmetic expressions. You will implement a Postfix
Postfix
Postfix is a stackbased computer language directed at the evaluation of arithmetic expressions. You will implement a Postfix interpreter that can evaluate Postfix code lineby line, as entered, for example, on a mobile device.
Postfix arithmetic
Operators, in postfix arithmetical expressions, follow operands. For example, means The postfix expression is evaluated as follows:
replace with the result of adding to
replace with
Postfix expressions are conveniently evaluated using a stack. An expression consisting of operands and operators collectivelytokens is read from left to right. Successive operands are pushed on a stack until an operator arrives. The appropriate number of operands are then popped from the stack, combined with the operator, and the result is pushed back on the stack. The result of a calculation is always to be found at the top of the stack. A stack is notated a b c in the following example; the stack top is the leftmost token.
Tokens Stack before Action Stack after
Read
Read
Read
Pop twice, evaluate, and push
Pop two twice, evaluate, and push
Postfix with variables
The refers to an enhanced form of postfix: a postfix with variables. Expressions can contain variable names; the value of the variable is used for the calculation. The assignment operator assigns a value to a variable. For example
A
will set the value of A to It is equivalent to the infix assignment statement A that is to be found in many computer languages.
Tokens Stack before Action Stack after
A Read A A
A Read A
A Pop twice and set the value of A to
An interactive session might proceed as follows
A
B
A B
Symbol Table
A symbol table data structure associates keys with values. For example,
Example Key Value
Phone book Name Telephone number
DNS URL IP address
Education Student ID Module grades
Compiler Variable name Memory address
Dictionary Word Definition
A generic symbol table should support two operations, INSERT and SEARCH. There may also be a DELETE operation. No assumptions are made on the type and format of keys and values.
You will need a symbol table in your Postfix interpreter. The table will map variable name key to value.
The target hardware the javascript code
A Postfix interpreter might run on a small device with very limited memory. This means that the variable namespace is small, for example, AZ
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