A phrase of this form:

steps var over all the elements of the list produced by evaluating list-form.

(loop for i in (list 10 20 30 40) collect i) ==> (10 20 30 40)

Occasionally this clause is supplemented with a by phrase, which specifies a function to use to move down the list. The default is **CDR** but can be any function that takes a list and returns a sublist. For instance, you could collect every other element of a list with a loop like this:

This phrase too can take a by preposition:

Looping over the elements of a vector (which includes strings and bit vectors) is similar to looping over the elements of a list except the preposition across is used instead of .3 For instance:

(loop for x across "abcd" collect x) ==> (#\a #\b #\c #\d)

Iterating over a hash table or package is slightly more complicated because hash tables and packages have different sets of values you might want to iterate over—the keys or values in a hash table and the different kinds of symbols in a package. Both kinds of iteration follow the same pattern. The basic pattern looks like this:

To iterate over a package, things can be symbols, present-symbols, and external-symbols, which cause var to be bound to each of the symbols accessible in a package, each of the symbols present in a package (in other words, interned or imported into that package), or each of the symbols that have been exported from the package. The hash-or-package form is evaluated to produce the name of a package, which is looked up as if by **FIND-PACKAGE** or a package object. Synonyms are also available for parts of the for clause. In place of , you can use each; you can use of instead of in; and you can write the things in the singular (for example, hash-key or symbol).

Finally, since you’ll often want both the keys and the values when iterating over a hash table, the hash table clauses support a using subclause at the end of the hash table clause.

(loop for v being the hash-values in h using (hash-key k) ...)

Both of these loops will bind k to each key in the hash table and to the corresponding value. Note that the first element of the using subclause must be in the singular form.4