Common Lisp Cheat Sheet

Basics

• Comment:

  ; Is an inline comment, as part of a code line. It is almost never used.
  ;; Is a standard line comment.
  

• Variable Declaration:

  (defvar var-name value)
  (setf var-name value)
  

• Constant Declaration:

  (defconstant constant-name value)
  

• Function Declaration:

  (defun function-name (args)
    body)
  

Conditionals

• IF:

  (if condition
      then-clause
      else-clause)
  

• COND:

  (cond
   (condition1 result1)
   (condition2 result2)
   ...
   (t default-result))
  

• CASE:

  (case variable
    (value1 result1)
    (value2 result2)
    ...
    (t default-result))
  

Loops

• DOLIST (for each item in a list):

  (dolist (var list result)
    body)
  

• DOTIMES (repeat n times):

  (dotimes (var n result)
    body)
  

• LOOP:

  (loop for var from start to end
        do body)
  

Lists

• Creating a list:

  (list elem1 elem2 ...)
  

• Car & Cdr:

  (car list)   ; Returns the first element
  (cdr list)   ; Returns the list excluding the first element
  

Note: When dealing with lists it's better to use first and rest than car and cdr. The latter should communicate dealing with conses as a data structure rather than conses as lists.

• Append:

  (append list1 list2)
  

Strings

• String concatenation:

  (concatenate 'string "Hello, " "world!")
  

• String comparison:

  (string= str1 str2)
  

I/O

• Print:

  (print expression)
  

• Read:

  (read)
  

• Format (similar to printf):

  (format nil "Hello, ~A" name)
  

Higher Order Functions

• MAPCAR:

  (mapcar #'function-name list)
  

• REDUCE:

  (reduce #'function-name list)
  

• FILTER:

  (remove-if-not #'predicate list)
  

Error Handling

• Condition Handling:

  (handler-case
      expression-to-try
    (error-type (variable)
      recovery-expression))