1. Imperative Data Structures
“Get your data structures correct first, and the rest of the program will write itself.”
-Davids Johnson, Assen, The Netherlands

2. Why Does LISP Need Them?
Lists can represent any combination of data you could need
Strings and arrays are just lists of data
Association lists are like hash tables because you can look up values by keys
But they are inefficient
Can only access at the front
The simplest access operations are linear time on average

3. Strings
We already used strings in our IO operations, and we know how to return them using format
>(format nil "Blah Blah ~A" "Blah")
"Blah Blah Blah"
Strings are one instance of a sequence:
Lists, strings, arrays and vectors are sequences
But strings are still atomic
(atom "String") → T

4. String Functions
(char s i) returns the ith character of string s (zero-based index)
With setf, can be used to destructively modify character in a string
(string c) takes a character c and returns a string containing the single character represented by c

5. Make-String
You can create strings of a specified size using make-string
>(make-string 10)
" "
You can specify an initial element using the appropriate keyword
Default is #\Space (depends on implementation)
>(make-string 10 :initial-element #\h)
"hhhhhhhhhh"

6. Intern
(intern s) returns the corresponding symbol for a given string s
If the symbol already exists, a second value of :INTERNAL is returned
Otherwise a second value of NIL is returned
>(intern "A") A
NIL
:INTERNAL
>(intern "a")
|a|

7. String-Trim Removes leading and trailing characters from a string
First argument is characters to remove, second is string to remove from
>(string-trim "24" " ")
" 3 3 "
>(string-trim "24 " " ")
"3 3"
>(string-trim "24 3" " ")
"535"
string-left-trim and string-right-trim also exist

8. Arrays A single dimensional array is a vector
Strings are vectors of characters
Create arrays using make-array
(make-array n) makes a vector of size n
(make-array '(n1 n2 n3 …)) makes a multi-dimensional array with sizes n1, n2, n3, etc. along the corresponding dimensions
:initial-element allows specification of a uniform initial value (default nil;depends on implementation)
:initial-contents allows you to fully specify the initial contents as a list

9. Make-Array Examples >(make-array 9)
#(NIL NIL NIL NIL NIL NIL NIL NIL NIL)
>(make-array 9 :initial-element 5)
#( )
>(make-array '(3 2))
#2A((NIL NIL) (NIL NIL) (NIL NIL))
>(make-array 3 :initial-contents '(1 3 5))
#(1 3 5)
>(make-array '(2 2) :initial-contents '((1 3) (2 4)))
#2A((1 3) (2 4))
>(make-array '(2 2) :initial-contents '(1 2 3))
Error

10. Accessing Array Elements
aref is used to access array elements
An index is needed for each dimension of the array
>(setf x (make-array '(3 2)
:initial-contents
'((1 2) (3 4) (5 6))))
#2A((1 2) (3 4) (5 6))
>(aref x 0 0) 1
>(aref x 3 1)
Error
>(aref x 2 1) 6

11. Vectors One-dimensional arrays Can create and fill using vector
>(vector "a" 3 'foo)
#("a" 3 FOO)
Can access vectors using aref, but can also use the faster svref for vectors

12. Position Search for an element in any sequence
Keyword arguments include :key, :test, :from-end, :start and :end
>(position #\o "location") 1
>(position #\o "location" :start 2) 6
>(position #\o "location" :start 2 :end 4)
NIL
>(position #\o "location" :from-end t)
>(position 'a '((c d) (a b)) :key #'car)
>(position '(a b) '((c d) (a b)) :test #'equal)

13. Other Sequence Functions
We’ve already seen subseq, remove and remove-if
Others include position-if, find, find-if
reduce folds a sequence into a single value
(reduce #'fn '(a b c d))
is the same as
(fn (fn (fn 'a 'b) 'c) 'd)
>(reduce #'cons '(a b c d e))
((((A . B) . C) . D) . E)

14. Hash Tables More efficient than association lists
Create using make-hash-table
Access using get-hash
Add entries using setf on the get-hash
Remove elements using remhash

15. Make-Hash-Table >(make-hash-table) #<hash-table 083cae8c>
Can set initial size using :size
Can change the test used for looking up entries using :test
Default is eql

16. Hash Table Examples >(setf ht (make-hash-table))
#<hash-table 083caa2c>
>(gethash 'color ht)
NIL
>(setf (gethash 'color ht) 'red)
RED T
>(remhash 'color ht)

17. Maphash
Maphash takes a function of two arguments and a hash table and calls that function on every key/value pair in the table, in no particular order
>(setf (gethash 'shape ht) 'round)
ROUND
>(setf (gethash 'size ht) 30) 30
>(maphash
#'(lambda (k v) (format t "~A = ~A~%" k v)) ht)
SIZE = 30
SHAPE = ROUND
NIL