Iteration Control (The Universe Known as LISP)

15.25.6 Iteration Control

Iteration control clauses allow you to direct loop iteration.

The loop keywords:

as
for
repeat

designate iteration control clauses. They must precede any other loop clauses except

initially,
with, and
named

since they establish variable bindings.

Iteration control clauses differ with respect to the specification of

termination conditions and
the initialization and
stepping

of loop variables. When iteration control clauses are used in a loop, termination tests in the loop body are evaluated before any other loop body code is executed.

Iteration Control Clause Summary

The for and as keywords are synonyms, and the clauses iterate by

using one or more local loop variables
that are initialized to some value (Destructuring can be used to assign initial values to variables during iteration); and
that can be modified or stepped each iteration.
For these clauses, iteration terminates when a local variable reaches some specified value or when some other loop clause terminates iteration.
At each iteration, variables can be stepped by an increment or a decrement or can be assigned a new value by the evaluation of an expression.

7 Syntactic Representations

There are seven syntactic representations for these constructs.

arithmetic
in list
on list
equals then
across
hash
package

Data Types

In each syntactic description, the data type of var can be specified by the optional type-spec argument.
The variable argument in iteration control clauses can be a destructuring list. A destructuring list is a tree whose non-null atoms are symbols that can be assigned a value.
If var is a destructuring list, the data type specified by the type-spec argument must appropriately match the elements of the list.

Initialization and Return Values

All variables are initialized in the loop prologue.

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.

Scope of the Variable Binding is Lexical by Default

The scope of the variable binding is lexical unless it is proclaimed special; thus, the variable can be accessed only by expressions that lie textually within the loop. Stepping assignments are made in the loop body before any other expressions are evaluated in the body.

Multiple Iteration Clauses and `and` Construct

Sequential Binding by Default

If you use multiple iteration clauses to control iteration,

variable initialization and
stepping

occur sequentially by default.

Parallel Binding using `and`

You can use the and construct to connect two or more iteration clauses when sequential binding and stepping are not necessary. The iteration behavior of clauses joined by and is analogous to the behavior of the Common Lisp macro do relative to do*.

Syntax of Iteration Control Clauses

for-as ::= {for | as} for-as-subclause {and for-as-subclause}*
for-as-subclause ::= for-as-arithmetic
                   | for-as-in-list
                   | for-as-on-list
                   | for-as-equals-then
                   | for-as-across
                   | for-as-hash
                   | for-as-package

for-as-arithmetic ::= var [type-spec]
                     [{from | downfrom | upfrom} expr1 ]
                     [{to | downto | upto | below | above} expr2]
                     [by expr3]

for-as-in-list ::= var [type-spec] in expr1 [by step-fun]

for-as-on-list ::= var [type-spec] on expr1 [by step-fun]

for-as-equals-then ::= var [type-spec] = expr1 [then step-fun]

for-as-across ::= var [type-spec] across vector

for-as-hash ::= var [type-spec]
                being {each | the}
                      {hash-key | hash-keys | hash-value | hash-values}
                      {in | of} hash-table
                      [using ({hash-value | hash-key} other-var)]

for-as-package ::= var [type-spec]
                   being {each | the}
                   for-as-package-keyword {in | of} package

for-as-package-keyword ::= symbol
                         | present-symbol
                         | external-symbol
                         | symbols
                         | present-symbols
                         | external-symbols