st: Calculating average value using STATA program language

[Suryadipta Roy <sroy2138@gmail.com>]

Hello everyone,
I am a newbie in terms of writing STATA codes, and intend to get that
corrected pretty soon. Meanwhile, I was wondering if any one could
help me with the following. I have been trying use foreach/forvalues
without any success for some time. Any help/suggestion/hint is greatly
appreciated.

importer	exporter	year	isic     imports   sum_import   avg_import
AUS	FRA	1979	311	8	24	12
AUS	FRA	1980	311	16	24	12
AUS	FRA	1979	313	0	0	0
AUS	FRA	1980	313	0	0	0
AUS	GAB	1979	369	0	0	0
AUS	GBR	1976	311	10	30	10
AUS	GBR	1977	311	20	30	10
AUS	GBR	1978	311	0	30	10
AUT	CAN	1977	312	10	10	10
AUT	CAN	1978	390	20	20	20
FRA	AUS	1976	311	5	10	5
FRA	AUS	1977	311	5	10	5
FRA	AUT	1980	369	4	 4	4

In the hypothetical dataset above, I am trying to create the last two
columns (sum_import, and avg_import) from the "imports" column. For
the first column, the idea is to calculate the sum of imports e.g.
from FRA to AUS for each industry category (isic). Thus for the isic
category 311, the sum of imports in 1979 and 1980 is 8 + 16 = 24, and
the average in the two years is 12. Similarly, there were no trade
between AUS and GAB in 1979 for the industry category 369, and hence
the sum and the average value of imports = 0. The actual dataset has
over 200 countries covering data from 1976 - 1980 and I am trying to
calculate the average of the imports between 1976- 1980 in each
industry category. There are 28 such industry categories.

I remain grateful for any help.

Sincerely,
Suryadipta.
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