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st: if expression involving a variable in a while loop


From   Daifeng He <dhe.statlist@gmail.com>
To   statalist@hsphsun2.harvard.edu
Subject   st: if expression involving a variable in a while loop
Date   Wed, 13 Jul 2011 14:00:41 -0400

Dear all,


I have a quesiton about loops involving a variable in the "if expression."

I have 100 variables named var1 var2... var100 which sums up to 1.
These are share variables and most of the weights are carried by the
first few varialbes. I would like to truncate at 0.95 because I don't
want to carry all this 100 variables around.

So specifically, I would like to generate two variables: a) the sum of
the first few of those 100 variables so that the sum is just over
0.95; b)the number of variables used in the sum.  E.g., for the first
obs, if var1 +var2 = 0.94 and var1+var2+var3 = 0.97, then my first
variable should be 0.97 and my second varible should be 3; Similary,
for another obs, if var1-var10 sum up to 0.85 and var1-var11 sums up
to 0.96, then my first variable would be 0.96 and second variable be
11.

Here are my codes:

gen sum=0
gen n_95=.
local i=1

while sum<=0.95 {

      replace sum=sum+var`i'

      local `i'=`i'+1

      replace n_95=`i'

}

These codes runs but don't give me the variables I want. When
explaining "if exp in a loop," Stata manual says "If the expression
refers to any variables, their values in the first observation are
used unless explicit subscripts are specified," so I think my problem
might be associated with my "while sum<=0.95" code line because "sum"
is a varable, not a macro. But I am not sure how to explicitly
subcript.  I've desperately tried replacing "sum" by "sum[_n],"
"sum[n]", "sum[`n']", or "sum[_`n']" in the while loop and none of
them worked. Furthermore, I am not sure whether my problem is entirely
the subscript issue, because som eof the sum varialbe I got are bigger
than 1! (which I have no idea what happens here -- my var1-var100
strictly sums up to 1)

. list sum n_95 in 1/5

     +-----------------+
     |      sum   n_95 |
     |-----------------|
  1. | 1.242837      1 |
  2. | 1.583333      1 |
  3. | .7551287      1 |
  4. | .9887668      1 |
  5. | .6393646      1 |
     +-----------------+



I'd greatly appreciate any help on this.

Regards,
Daifeng

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