Title stata.com
while — Looping
Syntax Description Remarks and examples Also see
Syntax
while exp {
stata commands
}
Braces must be specified with while, and
1. the open brace must appear on the same line as while;
2. nothing may follow the open brace, except, of course, comments; the first command to be
executed must appear on a new line;
3. the close brace must appear on a line by itself.
Description
while evaluates exp and, if it is true (nonzero), executes the stata commands enclosed in the
braces. It then repeats the process until exp evaluates to false (zero). whiles may be nested within
whiles. If the exp refers to any variables, their values in the first observation are used unless explicit
subscripts are specified; see [U] 13.7 Explicit subscripting.
Also see [P] foreach and [P] forvalues for alternatives to while.
Remarks and examples stata.com
while may be used interactively, but it is most often used in programs. See [U] 18 Programming
Stata for a description of programs.
The stata commands enclosed in the braces may be executed once, many times, or not at all. For
instance,
program demo
local i = ‘1’
while ‘i’>0 {
display "i is now ‘i’"
local i = ‘i’ - 1
}
display "done"
end
. demo 2
i is now 2
i is now 1
done
. demo 0
done
1
2 while — Looping
The above example is a bit contrived in that the best way to count down to one would be
program demo
forvalues i = ‘1’(-1)1 {
display "i is now ‘i’"
}
display "done"
end
while is used mostly in parsing contexts
program . . .
. . .
gettoken tok 0 : 0
while "‘tok’" != "" {
. . .
gettoken tok 0 : 0
}
. . .
end
or in mathematical contexts where we are iterating
program . . .
. . .
scalar ‘curval’ = .
scalar ‘lastval’ = .
while abs(‘lastval’ - ‘curval’) > ‘epsilon’ {
scalar ‘lastval’ = ‘curval’
scalar ‘curval’ = . . .
}
. . .
end
or in any context in which loop termination is based on calculation (whether it be numeric or string).
You can also create endless loops by using while,
program . . .
. . .
while 1 {
. . .
}
end
which is not really an endless loop if the code reads
program . . .
. . .
while 1 {
if (. . . ) exit
. . .
}
// this line is never reached
end
Should you make a mistake and really create an endless loop, you can stop program execution by
pressing the Break key.
