Richard,
I would use:
>0&". each 6{"1 dat
In pieces:
6{"1 dat NB. read as 6 from rank 1 dat
NB. column 6 of table dat, this is a boxed list
0&". each NB. convert to numeric, while still boxed
> NB. convert the boxed list to a list
Tom Arneson
http://home.comcast.net/~tomarneson/
-----Original Message-----
From: [EMAIL PROTECTED]
[mailto:[EMAIL PROTECTED] On Behalf Of Richard Cunningham
Sent: Sunday, April 02, 2006 14:58
To: [email protected]
Subject: [Jprogramming] Neophyte wishes critique, please
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.
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