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From |
n j cox <n.j.cox@durham.ac.uk> |

To |
statalist@hsphsun2.harvard.edu |

Subject |
Re: -by:- is sweet [was: Re: Re: st: Creating a new variable withinformation from other observations] |

Date |
Mon, 19 May 2008 18:11:36 +0100 |

.

Sorry, my typo; that code should be

if is_capital[_N]

not

& is_capital[_N]

The extra code is just to cope with countries with no capital.

It seems you have none, so that's fine.

Nick

n.j.cox@durham.ac.uk

Davide Cantoni

Thank you very much, Nick. This is elegant indeed, and congratulations

for retrieving that piece of high poetry by Sta Ta.

I'm wondering, though, about your statement:

> A more cautious approach slaps an extra condition on the second statement

>

> & is_capital[_N]

why would you need this? To make sure that the -bysort countryid

(is_capital)- command works fine, and puts capital cities at the end

of the block indeed?

But if I do

bysort countryid (is_capital) : gen latitude_capital = latitude[_N] &

is_capital[_N]

I obtain latitude_capital equal to 1 for all observations, instead of

the desired result (which I get if I do not add "& is_capital[_N]").

Davide

2008/5/19 n j cox <n.j.cox@durham.ac.uk>:

> .

>

> A more mundane solution uses -by:-.

>

> gen is_capital = capitalid == cityid

> bysort countryid (is_capital) : gen latitude_capital = latitude[_N]

>

> The indicator (dummy) is 1 when a city is the capital and 0 otherwise.

>

> If you sort each capital city to the end of the block of observations for a

> country, then you can just pick up its value for the new variable.

>

> A more cautious approach slaps an extra condition on the second statement

>

> & is_capital[_N]

>

> So, no loops necessary at all. Or, more precisely, Stata does the loop

> required automatically as a consequence of -by:-.

>

> The following poem [by one Sta Ta?] fell into my hands recently.

>

> Something to repeat?

> Seek a method neat.

> Loops are lovely,

> -by:- is sweet.

>

> The style leaves much to be desired, but the content is good.

>

> Nick

> n.j.cox@durham.ac.uk

>

> Teresio Poggio

>

> from your dataset I'd build a just capitals dataset:

> - select just the capitals (drop if cityid !=capitalid)

> - in the new dataset keep just capitalid and latitude

> - rename latitude into latitude_capital

> - sort the data by capitalid and save it

>

> then open you original data set and sort it by capitalid,

> merge it with the new "just capital dataset" using capitalid as a key

> and the option uniqmaster

> (help merge for details)

>

> Davide Cantoni

>

>> I am having a rather intricate problem in creating a new variable in a

>> panel dataset, and I appreciate any help you could offer. I hope the

>> problem can potentially be of general interest.

>>

>> I have a panel dataset of cities and their characteristics in

>> different countries. I know the latitude of each one of these cities,

>> but now I want to create an additional variable reflecting the

>> latitude of the capital city of the country a given city lies in. So

>> for example: for the cities of New York, Chicago, etc., I want this

>> new variable to contain the latitude of Washington, DC.

>>

>> Here is a description of the dataset's structure: it is a panel in

>> long form, with cities in different countries, observed over different

>> years. Each city has a unique numeric identifier, "cityid". Then there

>> is a country identifier, called "countryid". Finally, there is a

>> variable that repeats the capital city's cityid for each city in a

>> given country, "capitalid". For instance, if the cityid of London was

>> 135, all cities in the dataset that are in the UK would get a value of

>> 135 in the variable "capitalid". Finally, there is a variable called

>> "latitude" that refelcts the latitude of each city.

>>

>> How would I now proceed to create this new variable, call it

>> "latitude_capital", by using the variables above?

>>

>> Basically, the problem I'm having is

>> - tell stata to look up for each city its capitalid

>> - browse the dataset until you find a city that has the cityid equal

>> to this capitalid

>> - find out the latitude of this capital city

>> - go back to the original city and replace "latitude_capital" with the

>> latitude you've just retrieved

>>

>> The additional problem I encounter while trying to construct something

>> with "foreach..." (that, at least, is what I was trying so far) is

>> that the values that the capitalid variable takes are of course not a

>> clean numlist (like "1(1)100"), but rather a sequence of numbers

>> without any regularity, such as 11 12 50 54 60 131... and so on.

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