Learn How The Exclamation Mark is Used in Mathematics & Computers
Mathematics:
In mathematics the symbol represents the factorial operation. The expression n! means
“the product of the integers from 1 to n”. For example, 4! (read four factorial)
is 4 x 3 x 2 x 1 = 24. (0! is defined as 1, which is a neutral element in multiplication, not multiplied by
anything.)
Computers:
In computer programming, the exclamation mark corresponds to ASCII character 33 (21 in hexadecimal).
It is therefore found in Unicode at U+0021. The inverted exclamation mark is found in ISO-8859-1,
and at position 161 (A1HEX) and therefore in unicode at U+00A1.
Several computer languages use “!” for various meanings, most importantly for logical
negation; e.g. A != B means “A is not equal to B”, and !A means “the
logical negation of A” (also called “not A”). In this context, the exclamation
is named the bang character; other programmers call it a shriek or screech.
Invented in the US, it is claimed that bang is from Unix and shriek from Stanford
or MIT; however, shriek is known to have been in use in the 1860s. In the BBC BASIC programming
language (and BCPL) it is called a pling and is used to reference a 32-bit word.
Plings are also used in Acorn RISC OS to denote an “appfolder”: a folder that when
double clicked executes a program file inside called !Run. Other files in the appfolder
generally contain resources the application needs to run. The appfolder can be viewed as a normal
folder by double-clicking with the shift key held down. In addition, other special resource
files such as !Boot (executed the first time the application containing it comes into
view of the filer) and !Sprites (an icon file containing icon definitions loaded if
!Boot cannot be found) also start with a pling.
Early e-mail systems also used the exclamation point as a separator character between hostnames
for routing information, usually referred to as “bang path” notation.
In the IRC protocol, a user's nickname and ident are separated by an exclamation point in the hostmask
assigned to him or her by the server.
In the Geek Code, “!” is used before a letter to denote that the geek refuses to participate
in the topic at hand.
When computer programs display messages that alert the user, an exclamation mark may be shown alongside
it to indicate that the message is important and should be read. This often happens when an error is
made, or to obtain user consent for hazardous operations such as deleting data.
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