howeverit' educational to see this important use of stacksand the issues raised are interesting in their own right as it turns outit' fairly difficultat least for computer algorithmto evaluate an arithmetic expression directly it' easier for the algorithm to use two-step process transform the arithmetic expression into different formatcalled postfix notation evaluate the postfix expression step is bit involvedbut step is easy in any casethis two-step approach results in simpler algorithm than trying to parse the arithmetic expression directly of coursefor human it' easier to parse the ordinary arithmetic expression we'll return to the difference between the human and computer approaches in moment before we delve into the details of steps and we'll introduce postfix notation postfix notation everyday arithmetic expressions are written with an operator (+-*or /placed between two operands (numbersor symbols that stand for numbersthis is called infix notationbecause the operator is written inside the operands thus we say + and / orusing letters to stand for numbersa+ and / in postfix notation (which is also called reverse polish notationor rpnbecause it was invented by polish mathematician)the operator follows the two operands thus + becomes ab+and / becomes abmore complex infix expressions can likewise be translated into postfix notationas shown in table we'll explain how the postfix expressions are generated in moment table infix and postfix expressions infix postfix + - ab+ca* / ab*ca+ * abc* * + ab*ca*( +cabc+ * + * ab*cd*( + )*( -dab+cd-(( + )* )- ab+ *