Stata Nested foreach loop substring comparison

问题

I have just started learning Stata and I'm having a hard time. My problem is this: I have two different variables, ATC and A, where A is potentially a substring of ATC. Now I want to mark all the observations in which A is a substring of ATC with OK = 1.

I tried this using a simple nested loop:

foreach x in ATC {
foreach j in A {
        replace OK = 1 if strpos(`x',`j')!=0
    }
}

However, whenever I run this loop no changes are being made even though there should be plenty. I feel like I should probably give an index specifying which OK is being changed (the one belonging to the ATC/x), but I have no idea how to do this. This is probably really simple but I've been struggling with it for some time.

I should have clarified: my A list is separate from the main list (simply appended to it) and only contains unique keys which I use to identify the ATCs which I want. So I have ~120 A-keys and a couple million ATC keys. What I wanted to do was iterate over every ATC key for every single A-key and mark those ATC-keys with A that qualify.

That means I don't have complete tuples of (ATC,A,OK) but instead separate lists of different sizes. For example: I have

ATC    OK  A 
ABCD   0   .
EFGH   0   .
...   ...  ...
.     .    AB
.     .    ET

and want the result that "ABCD" having OK is marked as 1 while "EFGH" remains at 0.

回答1:

We can separate your question into two parts. Your title implies a problem with loops, but your loops are just equivalent to

  replace OK = 1 if strpos(ATC, A)!=0

so the use of looping appears irrelevant. That leaves the substring comparison.

Let's supply an example:

. set obs 3 
obs was 0, now 3

. gen OK = 0 

. gen A = cond(_n == 1, "42", "something else")  

. gen ATC = "answer is 42"

. replace OK = 1 if strpos(ATC, A) != 0 
(1 real change made)

. list 

     +------------------------------------+
    | OK                A            ATC |
    |------------------------------------|
 1. |  1               42   answer is 42 |
 2. |  0   something else   answer is 42 |
 3. |  0   something else   answer is 42 |
    +------------------------------------+

So it works fine; and you really need to give a reproducible example if you think you have something different.

As for specifying where the variable should be changed: your code does precisely that, as again the example above shows.

The update makes the problem clear. Stata will only look in the same observation for a matching substring when you specify the syntax you gave. A variable in Stata is a field in a dataset. To cycle over a set of values, something like this should suffice

 gen byte OK = 0 
 levelsof A, local(Avals) 

 quietly foreach A of local Avals { 
     replace OK = 1 if strpos(ATC, `"`A'"') > 0 
 }

Notes:

Specifying byte cuts down storage.
You may need an if or in restriction on levelsof.
quietly cuts out messages about changed values. When debugging, it is often better left out.
> 0 could be omitted as a positive result from strpos() is automatically treated as true in logical comparisons. See this FAQ.

来源：https://stackoverflow.com/questions/27337523/stata-nested-foreach-loop-substring-comparison

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loops

foreach

nested

substring

stata