In computer programming, CAR and CDR are primitive operations on cons cells introduced in the Lisp programming language. A cons cell is composed of two pointers; the car operation extracts the first pointer, and the cdr operation extracts the second. Thus, the expression evaluates to x, and evaluates to y. When cons cells are used to implement singly linked lists, the car operation returns the firstelement of the list, while cdr returns the rest of the list. For this reason, the operations are sometimes given the names first and rest or head and tail.
Etymology
Lisp was originally implemented on the IBM 704 computer, in the late 1950s. The popular explanation that CAR and CDR stand for "Contents of the Address Register" and "Contents of the Decrement Register" does not quite match the IBM 704 architecture; the IBM 704 does not have a programmer-accessible address register and the three address modification registers are called "index registers" by IBM. The 704 and its successors have a 36-bit word length and a 15-bit address space. These computers had two instruction formats, one of which, the Type A, had a short, 3-bit, operation code prefix and two 15-bit fields separated by a 3-bit tag. The first 15-bit field was the operand address and the second held a decrement or count. The tag specified one of three index registers. Indexing was a subtractive process on the 704, hence the value to be loaded into an index register was called a "decrement". The 704 hardware had special instructions for accessing the address and decrement fields in a word. As a result it was efficient to use those two fields to store within a single word the two pointers needed for a list. Thus, "CAR" is "Contents of the Address part of the Register". The term "register" in this context refers to "memory location". Precursors to Lisp included functions:
car,
cdr,
cpr, and
ctr,
each of which took a machine address as an argument, loaded the corresponding word from memory, and extracted the appropriate bits.
704 macros
The 704 assembler macro for car was: LXD JLOC 4 # C → C # Loads the Decrement of location JLOC into Index Register C CLA 0,4 # C → C # The AC register receives the start address of the list PAX 0,4 # C → C # Loads the Address of AC into Index Register C PXD 0,4 # C → C # Clears AC and loads Index Register C into the Decrement of AC The 704 assembler macro for cdr was: LXD JLOC 4 # C → C # Loads the Decrement of location JLOC into Index Register C CLA 0,4 # C → C # The AC register receives the start address of the list PDX 0,4 # C → C # Loads the Decrement of AC into Index Register C PXD 0,4 # C → C # Clears AC and loads Index Register C into the Decrement of AC A machine word could be reassembled by cons, which took four arguments. The prefix and tag parts were dropped in the early stages of Lisp's design, leaving CAR, CDR, and a two-argument CONS.
Compositions
of car and cdr can be given short and more or less pronounceable names of the same form. In Lisp, is the equivalent of ; its value is 2. Similarly, )) is the same as ))); its value is 1. Most Lisps, for example Common Lisp and Scheme, systematically define all variations of two to four compositions of car and cdr.
Many languages use a singly linked list as a basic data structure, and provide primitives or functions similar to car and cdr. These are named variously first and rest, head and tail, etc. In Lisp, however, the cons cell is not used only to build linked lists but also to build pair and nested pair structures, i.e. the cdr of a cons cell need not be a list. In this case, most other languages provide different primitives as they typically distinguish pair structures from list structures either typefully or semantically. Particularly in typed languages, lists, pairs, and trees will all have different accessor functions with different type signatures: in Haskell, for example, car and cdr become fst and snd when dealing with a pair type. Exact analogues of car and cdr are thus rare in other languages.
