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Introduced by the B programming language, which used the symbol "=" for variable assignment. The reasoning given by the creators of the language is that assignment is more frequent than equality testing, and thus deserves the shorter operator. The syntax was inherited by its successor C, and later adopted more widely. See wikipedia:Relational operator#Confusion with assignment operators.

==

1. (programming) In some programming languages, an operator that compares if two values are equal.

   if x==1 then print "success!" end (prints "success!" if x equals 1)

2. (mathematics) This term needs a definition. Please help out and add a definition, then remove the text {{rfdef}}.
   • 2012, John Browne, Grassmann Algebra Volume 1: Foundations: Exploring extended vector algebra with Mathematica (in English), John M Browne, →ISBN, page 373:

     The result of taking the regressive product of an m-element A and its complement ${\displaystyle {\bar {A}}}$ in the reverse order introduces a potential change of sign.

     ${\displaystyle {\bar {A}}}$∨${\displaystyle A}$ ⩵ ${\displaystyle (-1)^{m(n-m)}A}$∨${\displaystyle {\bar {A}}}$

• !=

• ++
• --

From the programming sense.

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1. (Internet slang) The same as; equal to.
   • 2000 October 8, Jan Schaumann, “Re: Comparing Perl and Java”, in comp.lang.perl.misc‎ (Usenet):

     As soon as I posted the article I knew that screwdriver and hammer would be abused in this example. I even did get to the "screwdriver == drink" part myself, only the "getting hammered" escaped me. ;-)

• =

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1. Alternative form of = =