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Stata FAQ
What is the difference between = and == in Stata?

Single equal: Set equal

The single equal, =, is used as a set equal operator. Old time Pascal programmers will recognize it as, :=. It is used in the generate, replace and recode commands. Examples:

generate tot = read + write + math
replace mean = total/freq
recode x 5=1 4=2 2=4 1=5

The single equal is also used in some of the multivariate commands, such as, mvreg and manova. For example,

mvreg read write math science = gpa gender honors
manova read write math science = gender program gender*program

Double equal: Test for equality

The double equals, ==, is used to test for equality.  It is sometimes called logical equals because it is part of a logical test that returns either a one (true) or a zero (false). Here are some examples:

regress write read female if ses==2
list read write math if prog==1
assert gender==1 

The double equals can also be used in an if command in a do-file or ado-file.  For example,

if stage==2 {
       local s2 "stage 2 analysis"
       display `s2'
       regress yvar `xvar1' `xvar2'
}

You can create some interesting commands that have both a single equal and double equal signs.  For example,

generate hises = ses==2

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