iCALCULA is based on Reverse Polish Notation (RPN) and the use of a four registers stack. Automatic storage of intermediate results is the reason that a RPN calculator easily processes complex calculations without the need of using parenthesis. The key to automatic storage is the RPN memory stack.
RPN works with unary (√, sin(), ... )and binary operators (+, ÷, ...). Algebraic notation places operators between operands. An addition of two numbers can be expressed:
2 + 3 = 5
Then, in an algebraic calculator you type:
|2| |+| |3| |=|
RPN notation places operators after the numbers or variables they operate:
2 3 +
So, in a RPN calculator like iCALCULA you type:
|2| |ENTER| |3| |+|
One of the most visible difference between both notations is that RPN doesn't use parenthesis or |=| key at all. This can be an advantage in more complex calculations:
The contents of the stack move up and down automatically as new numbers enter the x-register (lifting the stack) and as operators combine two numbers in the x and y-register to produce one new number in the x-register (dropping the stack).
Suppose the stack is filled with the numbers 2,5,6 and 4. See how the stacks drops and lifts its contents while calculating 6-4+8:
t 2 2 2 2
z 5 2 5 2
y 6 5 2 5
x 4 |-| 2 |8| 8 |+| 10
1 2 3
1. The stack drops its content. The t-register replicates its content.
2. The stack lifts its content. The t-register's content is lost.
3. The stack drops
Most functions and operators prepare the stack to lift its content when the next number enters the x-register. But there are some exceptions, for example |ENTER|.
THE |ENTER| KEY
|ENTER| key separates two numbers keyed in one after the other. It lifts the stack and replicates x-register. It also prepares the stack so that x-register will be overwritten with next input. Suppose the stack is again filled with 2,5,6 and 4. Now enter and add two new numbers:
t 2 5 6 6 6
z 5 6 4 4 6
y 6 4 5 5 4
x 4 |5| 5 |ENTER| 5 |6| 6 |+| 11
1 2 3 4
1. Lifts the stack
2. Lifts the stack and replicates the x-register
3. Does not lift the stack
4. Drops the stack and replicates the t-register
The replicating feature of |ENTER| can be interesting if you want to enter a number and apply an operator to itself.
|3| |ENTER| |×|
gives you the square of 3.
The replicating effect of |ENTER| together with the replicating effect of stack drop (from t into z-register) allows you to fill the stack with a numeric constant for calculations.
|3| |ENTER| |ENTER| |ENTER| |×| |×| |×| |×| ...
gives you successive powers of 3.
THE |LAST x| REGISTER
The |Last x| register is a companion to the stack: it holds the number that was in the x-register before the last operation was executed. This ability to retrieve the "last x" has two main uses: correcting errors and reusing a number in a calculation.