Calc tool

Context

Let's implement a calculation tool. Inputs will be operations as [operand 1][operator][operand 2] and output will be the operation with its result

Initial state

• Memory: empty
• Cursor: first cell
• Input: an operation [operand][operator][operand] where operatands are numbers in base 10, and operator one of + - * /

Process

• Read input - memory after reading should be operand operator_flag operand 0 0 0 0 ....
• Print char (faster than rebuilding it afterwards)
• Use a switch/case
• If it's a *, set operator flag to 0 (do nothing) and start reading a new integer
• Same for +, - and * but set operator flag to 1, 2 or 3
• Otherwise, it's a digit: build integer
• multiply previously read integer by 10
• add the new digit
• Move operator to have operand 0 operand operator flag switch_flag
• Set switch_flag to 1 and use another switch/case
• Flag is 0: multiply numbers and store result in second cell
• Flag is 1 or 2: same, but sums up or subtract numbers
• Flag is 3: move operand 2, then perform division
• Display = symbol
• Display result
• If it's 0, then display '0'
• Otherwise, display the computed integer

Code - try it

Codes:
Multpily 42
Add 43
Subtract 45
Divide 47

>,[
  print char
  .
  subtract 42
  >++++++[-<------->]+<
  [
    not a multiplication
    -
    [
      not an addition
      --
      [
        not a subtraction
        --
        [
          not a division
          this is a digit
          build number
          ->++++++++[-<<[->+>>+<<<]>>>[-<<<+>>>]<]<[-<+>]
        ]>[-
          this is a division
          set operation flag to 3 and start new number
          <+++>>>
        ]<
      ]>[-
        this is a subtraction
        set operation flag to 2 and start new number
        <++>>>
      ]<
    ]>[-
      this is an addition
      set operation flag to 2 and start new number
      <+>>>
    ]<
  ]>[-
    this is a multiplication
    start new number
    >>
  ]
  read next char
  <,
]
move operator
<<[->>+<<]>>
>+<[
  not a multiplication
  -
  [
    not an addition
    -
    [->-<
      division
      move operand 2
      <[->>+<<]
      divide
      <<[->+>>+>-[<-]<[->>+<<<<[->>>+<<<]>]<<]
      clear and move result
      >[-]>>>[-]>[-<<<<+>>>>]<<
    ]>[-
      subtraction
      <<<<[->+<]>>[-<->]>>
    ]<
  ]>[-
    addition
    <<[-<+>]<<[->+<]>>>>
  ]<
]>[-
  multiplication
  <<<<[->>[-<+>>+<]>[-<+>]<<<]
  clean operands
  >>[-]>>
]

print =
+++++++++[-<+++++++>]<--.[-]

<+<[>-<
  print result if not 0
  [>>>>++++++++++<<<<[->+>>+>-[<-]<[->>+<<<<[->>>+<<<]>]<<]>+[-<+>]>>>[-]>[-<<<<+>>>>]<<<<]<[>++++++[<++++++++>-]<-.[-]<]
]>[
  or print 0
  +++++++[-<++++++>]<.[-]
]

Final state

• Memory: empty
• Cursor: second cell
• Input: empty
• Output: the operation followed by equal sign and its result

Note: of course, this is a calc tool valid in the Z/256Z group, meaning that all inputs and results are taken modulo 256. Moreover, the division is actually an euclidean division, so result is not displayed completely (only the quotient is)