Hello all -
I've recently started learning J, and need to get some practice. I'm learning
at home, and I have no colleagues to bounce ideas off. Therefore, please tell
me what you make of my efforts. I run on Win XP. I've installed MySQL at
home, and have much experience of using SQL on Oracle 7 databases. However, J
is my first non von-Neuman language. I've lots of experience of 'C' and PL/M.
For an exercise I thought I'd attempt to reproduce finding sums and averages of
numeric columns. I started with the J studio for simple ODBC, and successfully
loaded the MySQL ODBC handler. From the studio I got all the dd lines, and
could retreive rows from the table and display them in boxed format. Here is
what I had...
load 'dd'
(<'MySQL') e. {."1 ddsrc ''
ch =: ddcon 'dsn=MySQL;modifysql=0;uid=scott;pwd=tiger'
sh =: 'select purch_ref, from_whom, description, category, ir_category,
date_format(date_purch,''%Y-%m-%d''), amount, pay_method from purchases2' ddsel
ch
dat =. ddfet sh,5
dat NB. Display the first 5 rows, fetched by ddfet
ddend sh
dddis ch
The table is a table of business purchases. For interest, MySQL dates need
formating, as simple date columns cause an error: ISI09 Unsupported data type.
The column called "amount" contain the money paid, in GBP. I wanted to
manipulate that column, and by a process of trial and error (much error!) ended
up with this...
". > {. 6 }. |: dat NB. Display the contents of column 6 (which
contains numbers) as numbers
+/ ". > {. 6 }. |: dat NB. Sum of column 6
(+/ % #) ". > {. 6 }. |: dat NB. Average of column 6
NB. To unpack the sentence (+/ % #) ". > {. 6 }. |: dat
NB. we explain as follows:
NB. dat The boxed result of the select statement
NB. |: Transpose it so that the 1st column now lies along the 1st row
NB. 6 {. Cut off the top 6 rows so that the 1st row is now the amounts
NB. {. Cut off the bottom row which contains nulls - and is so in the
original database table
NB. > Unpack to a column of strings, which look like numbers
NB. ". Convert string to numbers
It seems that MySQL and J504 on Windows XP deliver strings of characters that
"look like" numbers.
So, is my sentence ".>{.6}.|: dat good J, or can it be improved?
I really struggled for a while to develop that sentence. I see from another
post in the J programming forum that someone else would like precedence,
pretty-printing or other help from the J interpreter. What I find difficult is
the dimensions of the lists/tables and the rank of the operators. Also, the
names of the verbs (eg }. for cutting of the head of a list) makes learning
awkward. Any suggestions for these neophyte problems?
Thanks for your replies.
Richard Cunningham, Reading, England.
----------------------------------------------------------------------
For information about J forums see http://www.jsoftware.com/forums.htm
