Write a function that returns a list of the format [1,2,..,N-1,N].

Write a function that returns a list of the format [N, N-1,..,2,1]

I've created 4 functions, create/1, create/2, reverse_create/1, and reverse_create/2. The /1 functions set up the arguments for the /2 worker functions to loop on.

[erlang]
-module(threetwo).
-export([create/1, reverse_create/1]).

create(1, [1|_T]=List) ->
List;
create(Num, List) ->
Next = Num-1,
create(Next, [Next|List]).

create(Num) when Num > 0 ->
create(Num, [Num]).

reverse_create_list(Num, [Num|_T]=List) ->
List;
reverse_create_list(Num, [H|_T]=List) ->
reverse_create_list(Num, [H+1|List]).

reverse_create(Num) when Num > 0 ->
reverse_create_list(Num, [1]).
[/erlang]

An example run:
[shell]
4> threetwo:create(10).
[1,2,3,4,5,6,7,8,9,10]
5> threetwo:reverse_create(10).
[10,9,8,7,6,5,4,3,2,1]
[/shell]

The key in is to pass your target number along and to build the list from right to left. In each iteration we peak at the head of the list, modify that value and then prepend the result to the head of the full list.
[erlang]
[H|_T]=List
[/erlang]
H is the first item in the list.
List is the full List.

Cheers,
Ben

Write a function sum/1 which, given a positive integer N, will return the sum of all the integers between 1 and N.

And the second part:

Write a function sum/2 which, given two integers N and M, where N =< M, will return the sum of the interval between N and M. If N > M, you want your process to terminate abnormally.

Here is my solution: threeone.erl

[erlang]
-module(threeone).
-export([sum/1, sum/2]).

sum(1) ->
1;
sum(Number) ->
Number + sum(Number - 1).

sum(Lower, Lower) ->
Lower;
sum(Lower, Upper) when Lower =< Upper ->
Upper + sum(Lower, Upper-1).
[/erlang]

If you give sum/1 a number less than 1, you'll infinite loop.

The above solution doesn't use Erlang's tail call optimizations. If you give them a very large set of numbers to add up, they keep eating up memory. I wanted to see what would happen when it ran out of memory or stack space, so I told it to sum up a billion. After my system started swapping and started to crawl I killed the process. Maybe next time.

Here is the reworked version that takes advantage of tail call optimization and can sum up numbers from 1 to a billion without making my system swap. The beam.smp process stays around 10mb of memory.

[erlang]
-module(threeone_tco).
-export([sum/1, sum/2]).

sum_range(Sum, Lower, Lower) ->
Sum + Lower;
sum_range(Sum, Lower, Upper) ->
sum_range(Sum + Upper, Lower, Upper-1).

sum(Number) when Number > 0 ->
sum_range(0, 1, Number).

sum(Lower, Upper) when Lower =< Upper ->
sum_range(0, Lower, Upper).
[/erlang]

The big change comes from passing the Sum as an argument to the next function call. To use TCO (Tail Call Optimization) you want to leave no work to be done in your function, and the last thing your function does, should be calling itself. Previously, the function would still have to add two numbers after it's recursion finished.

Cheers,
-Halzy

I started by reading Programming Erlang by Joe Armstrong and now I'm reading through Erlang Programming by Francesco Cesarini and Simon Thompson. I'm enjoying the second book more than the first one, but this could be a side effect of being more comfortable with Erlang because I read the first book. I'm so taken with this bizarre language that I've decided to work through all of the exercises in Erlang Programming. I'll be posting my solutions and hopefully, by the time I get through all of them I'll feel comfortable enough with Erlang to build something with it.

Cheers,
- Halzy

Open your ‘SVN configuration area’ configuration file in a text editor. The default location for this file is ~/.subversion.

Uncomment the line that specifies the diff-cmd and set its value to the path of the Merge araxissvndiff executable that you have installed on your machine:

diff-cmd = /Users/<userid>/bin/araxissvndiff

Note: in the above path, replace /Users/<userid>/bin/araxissvndiff with the full path to the Merge Merge araxissvndiff command-line utility that you have installed on your machine.

Also uncomment the line that specifies the diff-cmd3 and set its value to the path of the Merge araxissvndiff3 executable that you have installed on your machine:

diff3-cmd = /Users/<userid>/bin/araxissvndiff3

Note: in the above path, replace /Users/<userid>/bin/araxissvndiff3 with the full path to the Merge Merge araxissvndiff3 command-line utility that you have installed on your machine.

By doing what they say, you tell svn to use Araxis Merge every time it needs to merge a file. The problem with this is that svn will open Araxis Merge to make every merge. When you have a couple hundred files, you want svn to merge everything it can without opening Araxis Merge, and to only open it when there is a conflict. There is a way to do this, but it is not obvious.

These instructions are specific to OSX, but should be enough of a hint for getting set up on any platform.

We need to create a shell script that we can use as a merge tool. It needs to be somewhere in your execution PATH. I've added ~/bin to my execution PATH and have put the script there, but you could put it in a directory that is already in your path, such as /usr/local/bin.

Here is the script, it is named araxishelper.sh:

#!/bin/sh

araxissvndiff3 $2 $1 $3 > $4
exit $?

Make sure it's executable bit is set:

chmod +x araxishelper.sh

Now we need to configure subversion to use this new merge script. Edit your subversion configuration file ~/.subversion/config, and add the line:

merge-tool-cmd = araxishelper.sh

Thats it! Well, almost. If you try doing a merge now, nothing different happens. What gives? You have to tell subversion to use the merge-tool-cmd setting by adding '--accept launch' to your svn merge command.

svn merge svn+ssh://mrhalzy@foobar.com/code/branch/awesome --accept launch

It is important to know that we are using the default merge tool (probably diff3) for the bulk of the merging and only using araxismerge when diff3 can't handle it. Because araxis does a bit more than diff3, you might not have any conflicts to resolve when it pops up. This means that diff3 couldn't handle it, but araxis could. Just save, close and continue.

Cheers,
Halzy

"Wait!", screams the pseudo-technical guy in the back of the room. "What is it that the script is doing?"

Well, it's remapping the argument order. Subversion isn't consistent in it's argument order for diff3-cmd and merge-tool-cmd. So we have to use this proxy script to make it work. The arguments are: $1 = base, $2 = theirs, $3 = mine, $4=merge result location.

2009.10.7 Google Homepage

Today Googles homepage has a nice look to it, so much different that I was shocked when I arrived. The navigation and footer links faded in when you move your mouse. Very nice Google, I applaud you.

I know how much it can negatively affect life at home when I'm unable to arrive at a reasonable hour. Please understand my regrets that I will be late this evening..

A) There's an issue at work and I must save the company from certain doom
B) I'm staying late to help someone else with such an issue
C) There's been a terrible accident that
   i) I was involved in, but will contact you as soon as I can
   ii) I was witness to, and will contact you as soon as I can
D) I'm playing a game that
   i) won't be too long, I should be home sometime tonight
   ii) will be continued until I realize that strange light in the corner of my eye is the rising sun
E) I'm dead.

Thank you for your understanding and patience. I hate to worry you so.
Much love,
Your Devoted Spouse.