Title stata.com
Programming functions
Contents Functions References Also see
Contents
autocode(x,n,x
0
,x
1
) partitions the interval from x
0
to x
1
into n equal-length intervals
and returns the upper bound of the interval that contains x or
the upper bound of the first or last interval if x < x
0
or x > x
1
,
respectively
byteorder() 1 if your computer stores numbers by using a hilo byte order and
evaluates to 2 if your computer stores numbers by using a lohi
byte order
c(name) the value of the system or constant result c(name) (see [P] creturn)
caller() version of the program or session that invoked the currently running
program; see [P] version
chop(x, ) round(x) if abs(x − round(x)) < ; otherwise, x; or x if x is
missing
clip(x,a,b) x if a < x < b, b if x ≥ b, a if x ≤ a, or missing if x is missing
or if a > b; x if x is missing
cond(x,a,b
,c
) a if x is true and nonmissing, b if x is false, and c if x is missing;
a if c is not specified and x evaluates to missing
e(name) the value of stored result e(name); see [U] 18.8 Accessing results
calculated by other programs
e(sample) 1 if the observation is in the estimation sample and 0 otherwise
epsdouble() the machine precision of a double-precision number
epsfloat() the machine precision of a floating-point number
fileexists(f) 1 if the file specified by f exists; otherwise, 0
fileread(f) the contents of the file specified by f
filereaderror(s) 0 or positive integer, said value having the interpretation of a return
code
filewrite(f,s
,r
) writes the string specified by s to the file specified by f and returns
the number of bytes in the resulting file
float(x) the value of x rounded to float precision
fmtwidth(fmtstr) the output length of the %fmt contained in fmtstr; missing if fmtstr
does not contain a valid %fmt
frval() returns values of variables stored in other frames
frval() programmer’s version of frval()
has eprop(name) 1 if name appears as a word in e(properties); otherwise, 0
inlist(z,a,b,. . .) 1 if z is a member of the remaining arguments; otherwise, 0
inrange(z,a,b) 1 if it is known that a ≤ z ≤ b; otherwise, 0
irecode(x,x
1
,. . .,x
n
) missing if x is missing or x
1
, . . . , x
n
is not weakly increasing; 0
if x ≤ x
1
; 1 if x
1
< x ≤ x
2
; 2 if x
2
< x ≤ x
3
; . . . ; n if
x > x
n
1
2 Programming functions
matrix(exp) restricts name interpretation to scalars and matrices; see scalar()
maxbyte() the largest value that can be stored in storage type byte
maxdouble() the largest value that can be stored in storage type double
maxfloat() the largest value that can be stored in storage type float
maxint() the largest value that can be stored in storage type int
maxlong() the largest value that can be stored in storage type long
mi(x
1
,x
2
,. . .,x
n
) a synonym for missing(x
1
,x
2
,. . .,x
n
)
minbyte() the smallest value that can be stored in storage type byte
mindouble() the smallest value that can be stored in storage type double
minfloat() the smallest value that can be stored in storage type float
minint() the smallest value that can be stored in storage type int
minlong() the smallest value that can be stored in storage type long
missing(x
1
,x
2
,. . .,x
n
) 1 if any x
i
evaluates to missing; otherwise, 0
r(name) the value of the stored result r(name); see [U] 18.8 Accessing
results calculated by other programs
recode(x,x
1
,. . .,x
n
) missing if x
1
, x
2
, . . . , x
n
is not weakly increasing; x if x is missing;
x
1
if x ≤ x
1
; x
2
if x ≤ x
2
, . . .; otherwise, x
n
if x > x
1
, x
2
,
. . . , x
n−1
. x
i
≥ . is interpreted as x
i
= +∞
replay() 1 if the first nonblank character of local macro ‘0’ is a comma, or
if ‘0’ is empty
return(name) the value of the to-be-stored result r(name); see [P] return
s(name) the value of stored result s(name); see [U] 18.8 Accessing results
calculated by other programs
scalar(exp) restricts name interpretation to scalars and matrices
smallestdouble() the smallest double-precision number greater than zero
Programming functions 3
Functions
autocode(x,n,x
0
,x
1
)
Description: partitions the interval from x
0
to x
1
into n equal-length intervals and returns the
upper bound of the interval that contains x or the upper bound of the first or last
interval if x < x
0
or x > x
1
, respectively
This function is an automated version of recode(). See [U] 26 Working with
categorical data and factor variables for an example.
The algorithm for autocode() is
if (n ≥ . | x
0
≥ . | x
1
≥ . | n ≤ 0 | x
0
≥ x
1
)
then return missing
if x ≥ ., then return x
otherwise
for i = 1 to n − 1
xmap = x
0
+ i ∗ (x
1
− x
0
)/n
if x ≤ xmap then return xmap
end
otherwise
return x
1
Domain x: −8e+307 to 8e+307
Domain n: integers 1 to 10,000
Domain x
0
: −8e+307 to 8e+307
Domain x
1
: x
0
to 8e+307
Range: x
0
to x
1
byteorder()
Description: 1 if your computer stores numbers by using a hilo byte order and evaluates to 2
if your computer stores numbers by using a lohi byte order
Consider the number 1 written as a 2-byte integer. On some computers (called
hilo), it is written as “00 01”, and on other computers (called lohi), it is written
as “01 00” (with the least significant byte written first). There are similar issues
for 4-byte integers, 4-byte floats, and 8-byte floats. Stata automatically handles
byte-order differences for Stata-created files. Users need not be concerned about
this issue. Programmers producing custom binary files can use byteorder() to
determine the native byte ordering; see [P] file.
Range: 1 and 2
c(name)
Description: the value of the system or constant result c(name) (see [P] creturn)
Referencing c(name) will return an error if the result does not exist.
Domain: names
Range: real values, strings, or missing
4 Programming functions
caller()
Description: version of the program or session that invoked the currently running program;
see [P] version
This is a function for use by programmers.
Range: 1 to 18.0 (or 1 to 18.5 for StataNow)
chop(x, )
Description: round(x) if abs(x − round(x)) < ; otherwise, x; or x if x is missing
Domain x: −8e+307 to 8e+307
Domain : −8e+307 to 8e+307
Range: −8e+307 to 8e+307
clip(x,a,b)
Description: x if a < x < b, b if x ≥ b, a if x ≤ a, or missing if x is missing or if a > b; x
if x is missing
If a or b is missing, this is interpreted as a = −∞ or b = +∞, respectively.
Domain x: −8e+307 to 8e+307
Domain a: −8e+307 to 8e+307
Domain b: −8e+307 to 8e+307
Range: −8e+307 to 8e+307
cond(x,a,b
,c
)
Description: a if x is true and nonmissing, b if x is false, and c if x is missing; a if c is not
specified and x evaluates to missing
Note that expressions such as x > 2 will never evaluate to missing.
cond(x>2,50,70) returns 50 if x > 2 (includes x ≥ .)
cond(x>2,50,70) returns 70 if x ≤ 2
If you need a case for missing values in the above examples, try
cond(missing(x), ., cond(x>2,50,70)) returns . if x is missing,
returns 50 if x > 2, and returns 70 if x ≤ 2
If the first argument is a scalar that may contain a missing value or a variable
containing missing values, the fourth argument has an effect.
cond(wage,1,0,.) returns 1 if wage is not zero and not missing
cond(wage,1,0,.) returns 0 if wage is zero
cond(wage,1,0,.) returns . if wage is missing
Caution: If the first argument to cond() is a logical expression, that is,
cond(x>2,50,70,.), the fourth argument is never reached.
Domain x: −8e+307 to 8e+307 or missing; 0 ⇒ false, otherwise interpreted as true
Domain a: numbers and strings
Domain b: numbers if a is a number; strings if a is a string
Domain c: numbers if a is a number; strings if a is a string
Range: a, b, and c
Programming functions 5
e(name)
Description: the value of stored result e(name); see [U] 18.8 Accessing results calculated by
other programs
e(name) = scalar missing if the stored result does not exist
e(name) = specified matrix if the stored result is a matrix
e(name) = scalar numeric value if the stored result is a scalar
Domain: names
Range: strings, scalars, matrices, or missing
e(sample)
Description: 1 if the observation is in the estimation sample and 0 otherwise
Range: 0 and 1
epsdouble()
Description: the machine precision of a double-precision number
If d < epsdouble() and (double) x = 1, then x + d = (double) 1. This function
takes no arguments, but the parentheses must be included.
Range: a double-precision number close to 0
epsfloat()
Description: the machine precision of a floating-point number
If d < epsfloat() and (float) x = 1, then x + d = (float) 1. This function takes
no arguments, but the parentheses must be included.
Range: a floating-point number close to 0
fileexists(f)
Description: 1 if the file specified by f exists; otherwise, 0
If the file exists but is not readable, fileexists() will still return 1, because it
does exist. If the “file” is a directory, fileexists() will return 0.
Domain: filenames
Range: 0 and 1
fileread(f)
Description: the contents of the file specified by f
If the file does not exist or an I/O error occurs while reading the file, then
“fileread() error #” is returned, where # is a standard Stata error return code.
Domain: filenames
Range: strings
6 Programming functions
filereaderror(s)
Description: 0 or positive integer, said value having the interpretation of a return code
It is used like this
. generate strL s = fileread(filename) if fileexists(filename)
. assert filereaderror(s)==0
or this
. generate strL s = fileread(filename) if fileexists(filename)
. generate rc = filereaderror(s)
That is, filereaderror(s) is used on the result returned by fileread(filename)
to determine whether an I/O error occurred.
In the example, we only fileread() files that fileexists(). That is not
required. If the file does not exist, that will be detected by filereaderror() as
an error. The way we showed the example, we did not want to read missing files
as errors. If we wanted to treat missing files as errors, we would have coded
. generate strL s = fileread(filename)
. assert filereaderror(s)==0
or
. generate strL s = fileread(filename)
. generate rc = filereaderror(s)
Domain: strings
Range: integers
filewrite(f,s
,r
)
Description: writes the string specified by s to the file specified by f and returns the number
of bytes in the resulting file
If the optional argument r is specified as 1, the file specified by f will be replaced
if it exists. If r is specified as 2, the file specified by f will be appended to if it
exists. Any other values of r are treated as if r were not specified; that is, f will
only be written to if it does not already exist.
When the file f is freshly created or is replaced, the value returned by filewrite()
is the number of bytes written to the file, strlen(s). If r is specified as 2, and
thus filewrite() is appending to an existing file, the value returned is the total
number of bytes in the resulting file; that is, the value is the sum of the number of
the bytes in the file as it existed before filewrite() was called and the number
of bytes newly written to it, strlen(s).
If the file exists and r is not specified as 1 or 2, or an error occurs while writing
to the file, then a negative number (#) is returned, where abs(#) is a standard
Stata error return code.
Domain f : filenames
Domain s: strings
Domain r: integers 1 or 2
Range: integers
Programming functions 7
float(x)
Description: the value of x rounded to float precision
Although you may store your numeric variables as byte, int, long, float, or
double, Stata converts all numbers to double before performing any calculations.
Consequently, difficulties can arise in comparing numbers that have no finite binary
representation.
For example, if the variable x is stored as a float and contains the value 1.1
(a repeating “decimal” in binary), the expression x==1.1 will evaluate to false
because the literal 1.1 is the double representation of 1.1, which is different
from the float representation stored in x. (They differ by 2.384 × 10
−8
.) The
expression x==float(1.1) will evaluate to true because the float() function
converts the literal 1.1 to its float representation before it is compared with x.
(See [U] 13.12 Precision and problems therein for more information.)
Domain: −1e+38 to 1e+38
Range: −1e+38 to 1e+38
fmtwidth(fmtstr)
Description: the output length of the %fmt contained in fmtstr; missing if fmtstr does not contain
a valid %fmt
For example, fmtwidth("%9.2f") returns 9 and fmtwidth("%tc") returns 18.
Range: strings
frval(lvar,var)
Description: returns values of variables stored in other frames
The frame functions frval() and frval() access values of variables in frames
outside the current frame. If you do not know what a frame is, see [D] frames
intro.
The two functions do the same thing, but frval() is easier to use, and it is safer.
frval() is a programmer’s function.
lvar is the name of a variable created by frlink that links the current frame to
another frame.
var is the name of a variable in the other frame.
Returned is the value of var from the observation in the other frame that matches
the observation in the current frame.
Example 1: The current frame contains data on persons. Among the variables in the current
frame is countyid containing the county in which each person lives.
Frame frcounty contains data on counties. In these data, variable countyid also
records the county’s ID, and the other variables record county characteristics.
In the current frame, you have previously created variable linkcnty that links
the current frame to frcounty. You did this by typing
. frlink m:1 countyid, frame(frcounty) generate(linkcnty)
Thus, you can now type
. generate rel income = income / frval(linkcnty, median income)
8 Programming functions
income is an existing variable in the current frame. median income is an existing
variable in frcounty. rel income will be a new variable in the current frame,
containing the income of each person divided by the median income of the county
in which they live.
Example 2: It is usual to name frames after dataset names and to name link variables after
frame names. Here is an example of this, following the names used above:
. use persons, clear
. frame create county
. frame county: use county
. frlink m:1 countyid, frame(county)
. generate rel income = income / frval(county, median income)
Domain lvar: the name of a variable created by frlink that links the current frame to another
frame
Domain var: any variable (string or numeric) in the frame to which lvar links; varname
abbreviation is allowed
Range: range of var, plus missing value (missing value is defined as . when var contains
numeric data and "" when var contains string data; missing value is returned for
observations in the current frame that are unmatched in the other frame)
frval(lvar,var,unm)
Description: the frval() function described above but with a third argument unm
frval() returns the value of var from the observation in the frame linked using
lvar that matches the observation in the current frame and the value in unm if
there is no matching observation.
For example, type
. generate median inc = frval(county, median income, .a)
to create new variable median inc in the current frame, containing me-
dian income from the other frame, or .a when there is no matched observation
in the other frame.
Domain lvar: the name of a variable created by frlink that links the current frame to another
frame
Domain var: any variable (string or numeric) in the frame to which lvar links; varname
abbreviation is allowed
Domain unm: any numeric value if var is numeric; any string value when var is string
Range: range of var, plus unm
frval(frm,var,i)
Description: programmer’s version of frval()
It is useful for those wishing to write their own frlink and create special (or at
least different) effects.
frval() returns values of variables stored in other frames. It returns var’s ith
observation (var
i
) from the frame frm; see [D] frames intro.
If i is outside the valid range of observations for the frame, frval() returns
missing.
Programming functions 9
For example, you have two datasets in memory. The current frame is named
default and contains 57 observations. The other dataset, we will assume, is stored
in frame xdata. It contains different variables but on the same 57 observations.
The two datasets are in the same order so that observation 1 in default corresponds
to observation 1 in xdata, observation 2 to observation 2, and so on. You can
type
. generate hrlywage = income / frval(xdata, hrswrked, n)
This will divide values of income stored in default by values of hrswrked
stored in xdata.
The first thing to notice is that frval()’s first two arguments are not expressions.
You just type the name of the frame and the name of the variable without embedding
them in quotes. We specified xdata for the frame name and and hrswrked for
the variable name.
The second thing to notice is that the third argument is an expression. To emphasize
that, let’s change the example. Assume that xdata contains 58 instead of 57
observations. Assume that observation 1 in default corresponds to observation
2 in xdata, observation 2 corresponds to observation 3, and so on. There is no
observation in default that corresponds to observation 1 in xdata. In this case,
you type
. generate hrlywage = income / frval(xdata, hrswrked, n+1)
These examples are artificial. You will normally use frval() by creating a
variable in default that contains the corresponding observation numbers in
xdata. If the variable were called xobsno, then in the first example, xobsno
would contain 1, 2, . . . , 57.
In the second example, xobsno would contain 2, 3, . . . , 58.
In another example, xobsno might contain 9, 6, . . . , 32, which is to say, the
numbers 2, 3, . . . , 58, but permuted to reflect the datasets’ jumbled order.
In yet another example, xobsno might contain 9, 6, 9, . . . , 32, which is to
say, observation 1 and 3 in default both correspond to observation 9 in xdata.
xdata in this example might record geographic location and in default, persons
in observations 1 and 3 live in the same locale.
And in a final example, xobsno might contain all the above and missing values
(.). The missing values would indicate observations in default that have no
corresponding observation in xdata. If observations 7 and 11 contained missing,
that means there would be no observations in xdata corresponding to observations
7 and 11 in default. ( frval() has a second syntax that allows you to specify
the value returned when there are no corresponding observations; see below.)
Regardless of the complexity of the example, the value of xobsno in observation
j is the corresponding observation number i in xdata. Regardless of complexity,
to create new variable hrlywage in default, you would type
. generate hrlywage = income / frval(xdata, hrswrked, xobsno)
That leaves only the question of how to generate xobsno in all the above situations,
and it is easy to do. See [D] frlink.
10 Programming functions
There are two more things to know.
First, variables across frames are distinct. If the variable we have been calling
income in default were named x, and the variable hrswrked in xdata were
also named x, you would type
. generate hrlywage = x / frval(xdata, x, xobsno)
Second, although we have demonstrated the use of frval() with numeric
variables, it works with string variables too. If var is a string variable name,
frval() returns a string result.
Domain frm: any existing framename
Domain var: any existing variable name in frm; varname abbreviation is allowed
Domain i: any numeric values including missing values even though the nonmissing values
should be integers in the range 1 to frm’s N; nonintegers will be interpreted as
the corresponding integer obtained by truncation, and values outside the range will
be treated as if they were missing value
Range: range of var in frm plus missing value; numeric missing value (.) when var is
numeric, and string missing value ("") when var is string
frval(frm,var,i, v)
Description: the frval() function described above but with a fourth argument v
frval() returns values of variables stored in other frames. It returns var’s ith
observation (var
i
) from the frame frm.
When v is specified, frval() returns v if var
i
is missing or if i is outside
the valid range of observations.
. generate hwage = income / frval(xdata, hrswrked, xobsno, .z)
. generate hwage = income / frval(xdata, hrswrked, xobsno, avg)
In the first case, .z is returned for observations in which xobsno contains values
that are out of range. In the second case, the value recorded in variable avg is
returned.
Domain frm: any existing framename
Domain var: any existing variable name in frm; varname abbreviation is allowed
Domain i: any numeric values including missing values even though the nonmissing values
should be integers in the range 1 to frm’s N; nonintegers will be interpreted as
the corresponding integer obtained by truncation, and values outside the range will
be treated as if they were missing value
Domain v: any numeric value when var is numeric; any string value when var is string (can
be a constant or vary observation by observation)
Range: range of var in frm plus v
has eprop(name)
Description: 1 if name appears as a word in e(properties); otherwise, 0
Domain: names
Range: 0 or 1
Programming functions 11
inlist(z,a,b,. . .)
Description: 1 if z is a member of the remaining arguments; otherwise, 0
All arguments must be reals or all must be strings. The number of arguments is
between 2 and 250 for reals and between 2 and 10 for strings.
Domain: all reals or all strings
Range: 0 or 1
inrange(z,a,b)
Description: 1 if it is known that a ≤ z ≤ b; otherwise, 0
The following ordered rules apply:
z ≥ . returns 0.
a ≥ . and b = . returns 1.
a ≥ . returns 1 if z ≤ b; otherwise, it returns 0.
b ≥ . returns 1 if a ≤ z; otherwise, it returns 0.
Otherwise, 1 is returned if a ≤ z ≤ b.
If the arguments are strings, “.” is interpreted as "".
Domain: all reals or all strings
Range: 0 or 1
irecode(x,x
1
,x
2
,x
3
,. . .,x
n
)
Description: missing if x is missing or x
1
, . . . , x
n
is not weakly increasing; 0 if x ≤ x
1
; 1 if
x
1
< x ≤ x
2
; 2 if x
2
< x ≤ x
3
; . . . ; n if x > x
n
Also see autocode() and recode() for other styles of recode functions.
irecode(3, -10, -5, -3, -3, 0, 15, .) = 5
Domain x: −8e+307 to 8e+307
Domain x
i
: −8e+307 to 8e+307
Range: nonnegative integers
matrix(exp)
Description: restricts name interpretation to scalars and matrices; see scalar()
Domain: any valid expression
Range: evaluation of exp
maxbyte()
Description: the largest value that can be stored in storage type byte
This function takes no arguments, but the parentheses must be included.
Range: one integer number
maxdouble()
Description: the largest value that can be stored in storage type double
This function takes no arguments, but the parentheses must be included.
Range: one double-precision number
maxfloat()
Description: the largest value that can be stored in storage type float
This function takes no arguments, but the parentheses must be included.
Range: one floating-point number
12 Programming functions
maxint()
Description: the largest value that can be stored in storage type int
This function takes no arguments, but the parentheses must be included.
Range: one integer number
maxlong()
Description: the largest value that can be stored in storage type long
This function takes no arguments, but the parentheses must be included.
Range: one integer number
mi(x
1
,x
2
,. . .,x
n
)
Description: a synonym for missing(x
1
,x
2
,. . .,x
n
)
minbyte()
Description: the smallest value that can be stored in storage type byte
This function takes no arguments, but the parentheses must be included.
Range: one integer number
mindouble()
Description: the smallest value that can be stored in storage type double
This function takes no arguments, but the parentheses must be included.
Range: one double-precision number
minfloat()
Description: the smallest value that can be stored in storage type float
This function takes no arguments, but the parentheses must be included.
Range: one floating-point number
minint()
Description: the smallest value that can be stored in storage type int
This function takes no arguments, but the parentheses must be included.
Range: one integer number
minlong()
Description: the smallest value that can be stored in storage type long
This function takes no arguments, but the parentheses must be included.
Range: one integer number
Programming functions 13
missing(x
1
,x
2
,. . .,x
n
)
Description: 1 if any x
i
evaluates to missing; otherwise, 0
Stata has two concepts of missing values: a numeric missing value (., .a, .b, . . . ,
.z) and a string missing value (""). missing() returns 1 (meaning true) if any
expression x
i
evaluates to missing. If x is numeric, missing(x) is equivalent to
x ≥ .. If x is string, missing(x) is equivalent to x=="".
Domain x
i
: any string or numeric expression
Range: 0 and 1
r(name)
Description: the value of the stored result r(name); see [U] 18.8 Accessing results calculated
by other programs
r(name) = scalar missing if the stored result does not exist
r(name) = specified matrix if the stored result is a matrix
r(name) = scalar numeric value if the stored result is a scalar that can be
interpreted as a number
Domain: names
Range: strings, scalars, matrices, or missing
recode(x,x
1
,x
2
,. . .,x
n
)
Description: missing if x
1
, x
2
, . . . , x
n
is not weakly increasing; x if x is missing; x
1
if x ≤ x
1
;
x
2
if x ≤ x
2
, . . .; otherwise, x
n
if x > x
1
, x
2
, . . . , x
n−1
. x
i
≥ . is interpreted
as x
i
= +∞
Also see autocode() and irecode() for other styles of recode functions.
Domain x: −8e+307 to 8e+307 or missing
Domain x
1
: −8e+307 to 8e+307
Domain x
2
: x
1
to 8e+307
. . .
Domain x
n
: x
n−1
to 8e+307
Range: x
1
, x
2
, . . ., x
n
or missing
replay()
Description: 1 if the first nonblank character of local macro ‘0’ is a comma, or if ‘0’ is empty
This is a function for use by programmers writing estimation commands; see
[P] ereturn.
Range: integers 0 and 1, meaning false and true, respectively
return(name)
Description: the value of the to-be-stored result r(name); see [P] return
return(name) = scalar missing if the stored result does not exist
return(name) = specified matrix if the stored result is a matrix
return(name) = scalar numeric value if the stored result is a scalar
Domain: names
Range: strings, scalars, matrices, or missing
14 Programming functions
s(name)
Description: the value of stored result s(name); see [U] 18.8 Accessing results calculated by
other programs
s(name) = . if the stored result does not exist
Domain: names
Range: strings or missing
scalar(exp)
Description: restricts name interpretation to scalars and matrices
Names in expressions can refer to names of variables in the dataset, names of
matrices, or names of scalars. Matrices and scalars can have the same names as
variables in the dataset. If names conflict, Stata assumes that you are referring to
the name of the variable in the dataset.
matrix() and scalar() explicitly state that you are referring to matrices and
scalars. matrix() and scalar() are the same function; scalars and matrices may
not have the same names and so cannot be confused. Typing scalar(x) makes
it clear that you are referring to the scalar or matrix named x and not the variable
named x, should there happen to be a variable of that name.
Domain: any valid expression
Range: evaluation of exp
smallestdouble()
Description: the smallest double-precision number greater than zero
If 0 < d < smallestdouble(), then d does not have full double precision; these
are called the denormalized numbers. This function takes no arguments, but the
parentheses must be included.
Range: a double-precision number close to 0
References
Kantor, D., and N. J. Cox. 2005. Depending on conditions: A tutorial on the cond() function. Stata Journal 5:
413–420.
Rising, W. R. 2010. Stata tip 86: The missing() function. Stata Journal 10: 303–304.
Also see
[FN] Functions by category
[D] egen — Extensions to generate
[D] generate — Create or change contents of variable
[M-4] Programming — Programming functions
[U] 13.3 Functions
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