CAR and CDR - WikiHQ

In computer programming, CAR (<code>car</code>) and CDR (<code>cdr</code>) ( or ) are primitive operations on cons cells (or "non-atomic S-expressions") 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 <code>(car (cons x y))</code> evaluates to <code>x</code>, and <code>(cdr (cons x y))</code> evaluates to <code>y</code>.

When cons cells are used to implement singly linked lists (rather than trees and other more complicated structures), the car operation returns the first element 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.

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:

<code>car</code> ("contents of the address part of register number"),
<code>cdr</code> ("contents of the decrement part of register number"),
<code>cpr</code> ("contents of the prefix part of register number"), and
<code>ctr</code> ("contents of the tag part of register number"),

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 <code>car</code> was:

LXD JLOC 4 # C( Decrement of JLOC ) → C( C ) # Loads the Decrement of location JLOC into Index Register C

CLA 0,4 # C( 0 - C( C ) ) → C( AC ) # The AC register receives the start address of the list

PDX 0,4 # C( Decrement of AC ) → C( C ) # Loads the Decrement of AC into Index Register C

PXD 0,4 # C( C ) → C( Decrement of AC ) # Clears AC and loads Index Register C into the Decrement of AC </syntaxhighlight>

A machine word could be reassembled by cons, which took four arguments (a,d,p,t).

The prefix and tag parts were dropped in the early stages of Lisp's design, leaving CAR, CDR, and a two-argument CONS.