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BrainFuck part 2 - Working with arrays

DPAmar
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Get array lengh

This is one of the most common operations generally performed on arrays : how many items are stored into ? But it's not really straightforward to implement...

We need to count items in an array and store the result after the array itself. It can be done by

  • having 2 arrays, the left part for "not-counted-yet" items, and the right part for "already-counted" items
  • go to the rightmost part of the 1st array, move it to the leftmost part of the 2nd one and increment counter
  • repeat until the 1st array is empty
  • and do some cleansing: put the whole array at its original place

Let's start

  • Memory: 0, Array, 0, 0, 0
  • Cursor: on 0 cell after the array
  • Input: any

Process

  • invariant: memory is 0, ArrayA, 0, ArrayB, 0, N with N = length(arrayB) and Array = ArrayA + ArrayB
  • while last cell of ArrayA is not null
    • move the the right (first cell of ArrayB)
    • go to the end of ArrayB and increase N by 1
    • go back to the end of ArrayA
  • loop
  • memory is 0, 0, Array, 0, length(Array)

Code

<[         while last cell of ArrayA is not null
  [->+<]   move the the right (first cell of ArrayB)
  >[>]>+   go to the end of ArrayB and increase N by 1
  <<[<]<   go back to the end of ArrayA
]          loop

Minified version

<[[->+<]>[>]>+<<[<]<]

Final state

  • Memory: 0, 0, Array, 0, length(Array)
  • Cursor: on first 0 cell
  • Input: unchanged
  • Output: unchanged

Variant

Cleansing : put back the array at its original location; final memory : 0, Array, 0, 0, length(Array)

<[[->+<]>[>]>+<<[<]<]   compute length
>>[[-<+>]>]             move array back

Test program

This program reads up to 9 chars, store them into an array, then print length (limited to 9 because number to decimal string conversion will be needed otherwise)

>,[>,]                  read chars
<[[->+<]>[>]>+<<[<]<]   compute length
>>[[-<+>]>]             move array back
++++++++[->++++++<]>.   add 48 to length (48 is '0' ASCII code; it converts an integer into a decimal digit); and print length
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