Advanced Programming Guide

4 • Chapter 2: Procedures, Variables, and Extending Mapleenvironment, the map command is used to apply an operation to theelements of a structure. For example, you can divide each element of alist by a number, such as 8.> lst := [8, 4, 2, 16]:> map( x->x/8, lst);[1, 1 2 , 1 4 , 2]Consider a variation on the map command, which appears in the followingprocedure.Example This new procedure divides each element of a list by the firstelement of that list.> nest := proc(x::list)> local v;> v := x[1];> map( y -> y/v, x );> end proc:> nest(lst);[1, 1 2 , 1 4 , 2]The procedure nest contains a second procedure, map, which in thiscase is the Maple command map. Maple applies its lexical scoping rules,which declare the v within the call to map as the same v as in the outerprocedure, nest.Scoping RulesThis section explains Maple scoping rules. You will learn how Maple determineswhich variables are local to a procedure and which are global.You must have a basic understanding of Maple evaluation rules for parameters,and for local and global variables. For more information, seeChapter 6 of the Introductory Programming Guide.Local Versus Global VariablesIn general, when writing a procedure, you should explicitly declare whichvariables are global and which are local. Declaring the scope of the variablesmakes your procedure easier to read and debug. However, sometimesdeclaring the variables is not the best method. In the previous nest procedure,the variable in the map command is defined by the surrounding