Two Guys Arguing

bookkeeping

Posted in bf by youngnh on 04.16.09

bf has a tendency to push the maximum limit of Things I Can Hold In My Head At One Time.

Keeping track of where a variable\’s value is located takes up the bulk of a bf programmer\’s mental bookkeeping. Heck, a quarter of the language\’s syntax ( < and >) is devoted to updating what the current pointer is looking at. bf doesn\’t have any syntactic notion of variables, but it is impossible for me not to assign a cell\’s location some \”reason\” for existing in my program. Variables, in my head, are usually named along the lines of that reason.

Another note, cells in bf hold only numbers, much like every computer; it is simply how you choose to interpret those numbers that makes reasoning about computation simpler or harder. For the most part, I\’ve interpreted bf numbers as one of only three distinct types:

integers, which need no conversion and are usually 8 bits in size and signed
characters, in which the number actually corresponds to an ASCII value
booleans, where the number 0 represents false and any other integer, true

Lets look at some code. A succinct bit of bf is doubling a number. In bf, it might look like this:

++++++  initialize cell 0 to 6
[       while cell 0 is not 0
>++     move to cell 1 and add 2
<-]     move back to cell 0 and decrement

Notice that since bf code only consists of 8 distinct characters, any others including whitespace are considered comments and thrown away, so you can mix plain text comments anywhere in a bf program that you\'d like.

Here cell 0 acts both as the value we are doubling and the index counter for the loop. We decrement it once each time through the loop until it is zero. The doubled result now resides in cell 1, NOT cell 0 where it began life. This is a universally common pattern in bf. Data is its own counter. Unlike the way that most programmers think about memory references — esp. array elements — data is always moving around in bf.

There are two ways to deal with this, accept that data moves around and update where your program will thenceforth look for that variable\'s value, or copy the doubled value back to its original cell. The second approach takes another seven characters of bf, minimum, to achieve, while the first approach takes no extra characters, but diligent and correct bookkeeping on the part of the programmer.

Or, if you are a programmer, then, inevitably, the program you\'ve created to do this for you.

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