Append Data Records to a Log File

How to store information to a log file.

Let’s say you want to keep a log of data or events in a file:

log-file: %log.txt

You can use this super-efficient method (and it’s worth remembering):

write/append log-file append mold/only data newline

What makes this efficient is that you are not reading the entire log file first, you are just appending a record (a line) to it.

Here the data can be a block of information or just a word or string. The mold converts anything to a REBOL-readable string with the /only removes the outermost brackets “[]“.  We then append a newline (line terminator) to it.

Many programmers create a function called log (as in the verb “to log”) that writes out log events this way. Here is one that also adds a time-stamp to the start of each line:

log: func [data] [
    write/append log-file rejoin [now " - " mold/only reduce data newline]
]

The reduce evaluates the data block in order to get the actual contents of variables.

You can now just write:

log "some event"

or:

log ["some event" 1234 http://example.com]

Here’s how log might appear in a program that logs user account actions:

;The variables:
user: "Bob"
pass: "password"
action: 'new-user
account: 1234
website:  http://www.rebol.com
...
log [user pass action account website]

If you open the log file using an editor, you’ll now see:

22-Sep-2010/9:32:54-4:00 - "Bob" "password" new-user 1234 http://www.rebol.com

In REBOL 3, if you want to shorten up the time-stamp a bit, you can use now/utc to just store the universal time. If you want to hide the password, there are a few ways to do so, but we’ll explain that later.

Reading and processing a log file is the subject of another blog, but here are a few hints, in case you’re interested:

log-lines: read/lines log-file  ; a block of lines (strings)
foreach line log-lines [probe load line]

And, here’s another:

log-data: load log-file ; a continues block
parse log-data [
    some [copy line [skip [to date! | to end]] (probe line)]
]
Categories: Storing Data Tags: , , ,

Reblets: Little REBOL Scripts

REBOL is an elegant little programming language that’s very powerful, and fairly easy to use and learn; however, it is unlike most other computer languages, so you need to think differently as you learn it.

The main concept of REBOL is that expressions consist of words and values put into blocks and arranged in any order. Yes, any order. That makes it possible to express not only code, but also data, and most important of all, data-driven code, which REBOL calls dialects.

The dialect concept is very powerful — even more so that that of object-oriented coding — because as humans we tend to think in “mini languages” that are related to the focus of our attention. You know, doctors speak their own lingo, so do mechanics, artists, chefs, engineers, journalists, fishermen, bikers… well, you name it.

Anyway, the purpose of this blog is to help you better understand how to write REBOL code by providing a range of useful examples. Along the way we’ll point out some of the special features of the language that make it so powerful, able to produce a range of useful results but without making your programs large and complex.

So, welcome again to the REBOL “How To” blog, Reblets!

Store Data Records in Files

How to save multiple data records to a file and load them back again.

Let’s say you have a block of name records sort of like this:

group: [
    [1 "Bob" "Smith" 12-Aug-1965 "Seattle"  WA member]
    [2 "Tom" "Able"  20-Feb-1955 "San Jose" CA author]
    [3 "Jen" "Jones" 16-Jun-1972 "New York" NY leader]
    [4 "Sal" "Baker" 25-Oct-1975 "Boston"   MA member]
]

To save all of these to a file:

save/all %group.r group

Take a look at the group.r file in an editor, and you’ll be able to see the same records as above.

To load back the records you saved above:

group: load/all %group.r

Don’t forget to use the /all refinement or you’ll have some problems at times.

Now you have an easy way to save and access data from your programs. This technique even works well for files of more than 100 thousand records!

Categories: Storing Data Tags: , , , ,

Store Simple Values in Files

This example shows how to save data out to a file and loading it back again.

For simple single values, you just save them out to a file.

To store an integer:

num: 123
save %num.r num

To store a string:

name: "Tom Smith"
save %name.r name

You can use any suffix for the file. We use .r here because the data is in REBOL format, but the suffix doesn’t matter.

Now, you use load to get back the values.

To load the integer:

num: load %num.r
print num
123

To load the string:

name: load %name.r
print name
Tom Smith
Categories: Storing Data Tags: , , , ,

How to run a script

This example shows how to run a script from the console or from another script.

‘To run any script, you can type:

do %script.r

The do word means load the file script.r and evaluate it. (Note that in REBOL, they evaluate code, not execute it.)

The % indicates that the string following it is a file name or file path.  It’s a unique datatype called file! That’s an important. REBOL has a several different string datatypes and you will use them for different purposes. More on that later, but for now, just don’t forget the % in front of the filename.

You can put do in scripts too. That’s how you include other files in your program.

For example:

do %dictionary.r
do %html-tags.r
do %html-forms.r
do %name-database.r

This would evaluate and include four other files in your program.

The filename can be a variable also. For example:

tagfile: %html-tags.r
do tagfile

So, you could even do something clever like:

foreach file [%dictionary.r %html-tags.r %html-forms.r] [
    do file
]
Categories: Evaluation Tags: , ,
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