# BrainFuck part 10 - RPN calc tool

DPAmar

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## First operators

Let's add 3 operators to our system now :

- addition (code 43)
- subtraction (code 45)
- multiplication (code 42)

## Let's start

- Memory: empty
- Cursor: first cell
- Input: space separated integers and operators

## Process

- Leave an empty space (current number on stack is null)
- While there is a char to read
- subtract 32 and set else flag
- if char is null : it's space
- reset else flag, move to the right

- subtract 10 (total 42), if char is null : it's multiply
- reset else flag, multiply the 2 operands

- subtract 1 (total 43), if char is null : it's addition
- reset else flag, sum the 2 operands

- subtract 2 (total 45), if char is null : it's subtraction
- reset else flag, subtract the 2 operands

- otherwise
- subtract 3 (to get the digit value), and not 16 anymore (target total is 48, so 45+3 and not 32+16)
- add current number 10 times to current digit to get new current number

- loop

## Code

```
>,[ leave empty space and read char
let's track our target values here
space 32
multiply 42
add 43
subtract 45
digit 48 to 57
>++++[-<-------->]+< subtract 32
[---------- subtract 10
[- subtract 1
[-- subtract 2
[ it's a digit
--->-< subtract 3 to get digit value (and reset else flag)
<[->++++++++++<] add current number 10 times to current digit
>[-<+>] replace current number by current digit
]>[- now unwind cases: it's subtraction
<<<[-<->]> do the subtraction
]<]>[- next case : it's add
<<<[-<+>]> do the addition
]<]>[- next case : it's multiply
<<<[->>+<<]>>[-<<< do the multiplication
[->+>+<<]>[-<+>]>> ** part 2 **
]<<<[-]>>[-<<+>>] ** part 3 **
]<]>[- next case : it's space
> move to next stack entry
] end of switch/case
<,] read char and loop
```

## Final state

- Memory: stack of integers
- Cursor: after stack
- Input: empty (read)
- Output: unchanged

*Note*: We can see how to add cases now:

- get the target code
- put somewhere in cases switch the code to match
- add implementation when we unwind the cases stack

*Note 2*: as all the digits are processed the same way, we will continue to process them as the default case (avoids 10 different implementations for 10 codes)

## Test program

This programs reads integers and the 3 operands, all space-separated, and then prints values on the stack as chars

```
> add empty cell
>,[>++++[-<-------->]+<[----------[-[--[--->-<<[->+++++++ calc tool
+++<]>[-<+>]]>[-<<<[-<->]>]<]>[-<<<[-<+>]>]<]>[-<<<[->>+< ** part 2 **
<]>>[-<<<[->+>+<<]>[-<+>]>>]<<<[-]>>[-<<+>>]]<]>[->]<,] ** part 3 **
<[<]>[.>] print stack as chars
```

You can try with input `6 8 * 7 7 * 1 7 7 * +`

for example. 6x8 = 48, char 0; then 7x7=49, char 1; and finally 1+(7x7)=50, char 2; so it prints 012

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