Wednesday, October 31, 2012

Local Variables, Function Parameters, and Blocks In Maxima

One of the things that was a real hang-up for me when I started programming in Maxima was its seeming lack of variable scoping rules.  It made some irritating and non-intuitive things happen.  But, because I was always in “get-it-done” mode, I didn’t stop to figure out if there was a better way.  I would resort to adding letters to the name that I wanted to use.  If my functions were going to use a, I would instead name those aa, so that my variables in the “true” global realm wouldn’t be overwritten after the next function call.  My convention would then be, “nice names in the global realm, ugly names in the local realm.”  This is far from ideal.  The fact is, Maxima defaults to global variables, but makes it easy to declare local variables.

Maxima’s primary scoping mechanism is the “block.”  A block starts with block( and ends with ).  Each statement within a block is separated by a comma (,).  The first statement in your block should normally be [v_1, v_2,…,v_n], where v_1, v_2, etc., are variables that you wish to be local (don’t literally type v_1, etc.).  If you don’t want any local variables, then omit the local statement.

What Maxima actually does is not quite the same as in most programming languages, although the apparent runtime behaviour will be the same, if the [v_1, v_2,…,v_n] statement is used as above.  Maxima saves the current values of the variables in the [v_1, v_2,…,v_n] statement and essentially “zeros out” (removes their properties) those variables.  They become blank slates.  When Maxima exits the block() in which the variable was declared as local, its current properties are removed and the saved values/properties are restored.  As far as I know, Maxima does not support multi-threading.  This is probably a good thing, as temporarily preventing a variable from “existing” (as itself, that is; a variable with the same name is not the same variable itself, philosophically) is problematic if that variable could be accessed at any time by a separate thread.

This behaviour is different from most other programming languages which use “dynamic scoping” in that there are no save and restore steps of the kind we are talking about here (in most other languages).  In Maxima, variables have names at run-time and are referenced according to their name.  In most languages, the name of a variable is solely a convenience for programmers.  At run-time, values will move to various parts of the program memory space and the memory address values will fly.  Just because the same name is used in the source code for two variable references, doesn’t mean the compiled code will have references to the same memory location.  For example, t declared in function1() has nothing to do with t declared in function2().  These memory locations will be on the stack and exist only while each function is being executed.  This, again, is what is true in most programming languages, not in Maxima.

When it comes to function parameters, however, Maxima`s behaviour is not unlike most other languages.  That is, the parameters are formal.  They are not considered to constitute a variable declaration.  When I write

f(x) := x^2;

x is not declared as a variable.
Even if I write

RotationalEquivalenceSet(n, bits) := block(
    local(res, size),
    res: {n},
    size: 2^bits,
    if n >= size then return({}),
    n: 2*n,
    if n >= size then n: n - size + 1,
    do (
        if elementp(n, res) then return(res),
        res: union(res, {n}),
        n: 2 * n,
        if  n >= size then n: n - size + 1
    )   
);


n is not declared as a variable.  This is a very helpful thing.  If n was assigned some value before the function call, nothing that I do inside this function call (which uses n as the name of a parameter) affects that value.  The value of n will remain the same after this function has been invoked.  Here is an input/output example of the behaviour:

/* [wxMaxima: input   start ] */
n: x^2;
RotationalEquivalenceSet(3,5);
n;
RotationalEquivalenceSet(5,5);
n;
/* [wxMaxima: input   end   ] */

(%o21) x^2
(%o22) {3,6,12,17,24}
(%o23) x^2
(%o24) {5,9,10,18,20}
(%o25) x^2


Note that I am using := to declare these functions.  You may end up with different (and undesired) behaviour if you don’t use the “delayed evaluation” assignment statement.  If you want the usual programming paradigm with your function declaration, use := as per my sample code above.
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