# Why was the caret used for XOR instead of exponentiation?

Not that it's really a problem for anyone that has faced this syntactic issue before, but I see a wild amount of confusion stemming from the use of the caret (`^`) as the XOR operation in lieu of the widely accepted mathematical exponentiation operation.

Of course there are a lot of places where the (mis-)use of the caret is explained and corrected, but I haven't come across any definitive sources as to why the caret was given a different meaning.

Was it a matter of convenience? An accident? Obviously the reasoning could be different for the various languages, so information in any regard would be insightful.

• Commented Sep 22, 2016 at 22:29

Although there were older precursors, the influential French mathematician Rene Descartes is usually credited for introducing superscripted exponents (ab) into mathematical writing, in his work Geometrie which was published in 1637. This is the notation still universally used in mathematics today.

Fortran is the oldest programming language widely used for numerical computations that provides an exponentiation operator, it dates to 1954. The exponentiation operation is denoted by a double asterisk `**`. It should be noted that many computers at that time used 6-bit character encodings that did not provide a caret character `^`. The use of `**` was subsequently adopted by creators of various more recent programming languages that offer an exponentiation operation, such as Python.

The first widely adopted character set that contained the caret `^` was the 7-bit ASCII encoding which was first standardized in 1963. The oldest programming language I am aware of that used the caret to denote exponentiation is BASIC, which dates to 1964. Around the same time IBM adopted the EBCDIC character encoding, which also includes the caret `^`.

The C language came into existence in 1972. It does not provide an exponentiation operator, rather it supports exponentiation via library functions such as `pow()`. Therefore no symbol needs to be set aside for exponentiation in C, and other, later, languages in the C-family, such as C++ and CUDA.

On the other hand, and uncommon for programming languages up to that time, C provides symbols for bitwise operations. The number of special characters available in 7-bit ASCII was limited, and since there was a "natural affinity" of other operations to certain special characters, e.g. `&` for AND and `~` for NOT, there were not all that many choices for the symbol for XOR.

I am not aware of a published rationale provided by Ritchie or Kernighan as to why they chose `^` to denote XOR specifically; Ritchie's short history of C is silent on this issue. A look at the specification for the precursor to C, the language B, reveals that it did not have an XOR operator, but already used all special characters other than `^`, `\$`, `@`, `#`.

[Update] I sent email to Ken Thompson, creator of B and one of the co-creators of C, inquiring about the rationale for choosing `^` as C's XOR operator, and asking permission to share the answer here. His reply (slightly reformatted for readability):

From: Ken Thompson
Sent: Thursday, September 29, 2016 4:50 AM
To: Norbert Juffa
Subject: Re: Rationale behind choice of caret as XOR operator in C?

it was a random choice of the characters left.

if i had it to do over again (which i did) i would use the same operator for xor (^) and bit complement (~).

since ^ is now the better known operator, in go, ^ is xor and also complement.

The use of `^` for exponentiation in "mathematics" that you refer to is actually usage established at a much later date for typesetting systems such as Knuth's TeX which dates to 1978, command line interfaces for algebra systems such as Mathematica which dates to 1988, and graphing calculators in the early 1990s.

Why did these products adopt the use of `^` for exponentiation? In the case of calculators I suspect the influence of BASIC. Throughout the 1980s it was a very popular first programming language and was also embedded into other software products. The notation therefore would have been familiar to many buyers of the calculators. My memory is vague, but I believe there were even calculators that actually ran simple BASIC interpreters.

• That is some nice history lesson. So the real question would be "Why would Tex's software (and other from that period) had decided to use the XOR symbol has exponential?" ;) Commented Sep 19, 2016 at 10:53
• @AxelH Actually, it does not. TeX uses `^` for superscript. And it makes sense to me to use an "up" character for superscript and a "down" character (`_`) for subscript. Commented Sep 19, 2016 at 13:08
• @AxelH I agree with svick that the `^` suggests an up arrow, which in turn hints at the traditional position of a superscripted exponent, so it is a bit of a natural fit. I am sure Donald Knuth would respond to an email inquiring about his decision to use `^` for exponentiation in TeX, but given that he is busy working on more important things, such as completing TAOCP, I did not want to take that step (I did consider it). Commented Sep 19, 2016 at 13:46
• According to en.wikipedia.org/wiki/Caret, Algol 60 used an upwards pointing arrow for exponentiation, and notes that originally ASCII also had an up arrow at the same code point where it currently has the caret symbol, so the two were clearly equated to each other around the same time. Commented Sep 23, 2016 at 4:32
• Really nice historic answer. I would just like to add a likely reason for C implementing bit operators instead of an exponentiation: The goal of C is to be a low level language that allows the programmer easy access to a large part of the stuff that CPUs support natively. And CPUs always provide bit manipulation instructions, yet I do not know of a single CPU that has special instructions for exponentiation. As such, exponentiation always requires a loop (at least with integer arithmetic), and there is not a single feature in C that hides a loop behind a language construct. Commented Sep 24, 2016 at 20:31