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Allegro CL |
ANSI Common Lisp  6 Iteration 6.1 The LOOP Facility 6.1.2 Variable Initialization and Stepping Clauses
6.1.2.1 Iteration ControlIteration control clauses allow direction of loop iteration. The loop keywords for and as designate iteration control clauses. Iteration control clauses differ with respect to the specification of termination tests and to the initialization and stepping1 of loop variables. Iteration clauses by themselves do not cause the Loop Facility to return values, but they can be used in conjunction with value-accumulation clauses to return values.All variables are initialized in the loop prologue. A variable binding has lexical scope unless it is proclaimed special; thus, by default, the variable can be accessed only by forms that lie textually within the loop. Stepping assignments are made in the loop body before any other forms are evaluated in the body. The variable argument in iteration control clauses can be a destructuring list. A destructuring list is a tree whose non-nil atoms are variable names. See Section 6.1.1.7 Destructuring. The iteration control clauses for, as, and repeat must precede any other loop clauses, except initially, with, and named, since they establish variable bindings. When iteration control clauses are used in a loop, the corresponding termination tests in the loop body are evaluated before any other loop body code is executed. If multiple iteration clauses are used to control iteration, variable initialization and stepping1 occur sequentially by default. The and construct can be used to connect two or more iteration clauses when sequential binding and stepping1 are not necessary. The iteration behavior of clauses joined by and is analogous to the behavior of the macro do with respect to do*. The for and as clauses iterate by using one or more local loop variables that are initialized to some value and that can be modified or stepped1 after each iteration. For these clauses, iteration terminates when a local variable reaches some supplied value or when some other loop clause terminates iteration. At each iteration, variables can be stepped1 by an increment or a decrement or can be assigned a new value by the evaluation of a form). Destructuring can be used to assign values to variables during iteration. The for and as keywords are synonyms; they can be used interchangeably. There are seven syntactic formats for these constructs. In each syntactic format, the type of var can be supplied by the optional type-spec argument. If var is a destructuring list, the type supplied by the type-spec argument must appropriately match the elements of the list. By convention, for introduces new iterations and as introduces iterations that depend on a previous iteration specification. |