From: Brian Candler <b.candler@...>
Date: 2010-08-18T22:58:27+09:00
Subject: Re: can i do this in ruby? a simulation process

> sorry I'm totally suck in ruby here is my code:

For a newbie, it's a fine start, and this is the best way to learn.

In addition to the previous comments, I make the following observations:

>  $depart = rand(35)

I don't see any need to process the first car any differently from the 
subsequent ones, which means you should be able to simplify your code a 
lot, by putting *all* the car processing inside the loop.

In my case I assumed the first car left at time 0, but you could equally 
have it start at time 5-35.

Both cases are easily handled. Just initialize depart to 0 outside the 
loop, and then add 5-35 either at the top of the loop, or at the end (if 
the first car departs at time 0).

>      if $delay1==0;
>         $delay1 = $delay1+(1);
>      elsif $delay1==1
>         $delay1 = $delay1+(1);
>      end

I'm not sure what the above is trying to do. Both branches of the 'if' 
are saying $delay1 = $delay1 + 1, so you could replace all that with 
just

    $delay1 = $delay1 + 1

Or there is an equivalent shortcut,

    $delay1 += 1

If you want an integer random number between 1 and 30, you can do 
rand(30)+1

In my case I decided to go with floating point random numbers - rand*30 
gives a value between 0 and 29.99999 ... - but both are reasonable 
intepretations of the original question.

If you've decided to use integers, you have to decide if 5..35 includes 
both ends of the range. To get a value between 0 and 20 *inclusive* you 
need rand(21).

>   sleep($delay1)
>  $EXT = Time.now
>   sleep($delay2)
>  $CEXT = Time.now

I'd take this out. Firstly it's very slow to run the simulation in real 
time; secondly, you can't wait until the first car has arrived before 
the second one sets off; and thirdly, you can't rely on Time.now because 
your computer could be very busy doing other things which could result 
the sleep taking longer than you think. That is, sleep(n) only 
guarantees to sleep for *at least* n seconds, not exactly n seconds.

I suggest you simply calculate arrival time as departure time plus 
transit time.

> $passenger2= 1;
>  $delay101 = rand(10)

Here you calculate $delay101, but then never use it.

$passenger2 and $passenger1 are fixed, so at some point you need to 
consider some logic for choosing the number of passengers at random. But 
this can wait for now; simplifying the problem so that all cars have 2 
passengers, for example, will get you going more quickly.

>   $i = 1+1;

That's the same as $i = 2

By the way, you don't need semicolons at the end of the line. You only 
need them if you're putting multiple statements on the same line.

>       if $delay1==0
>          $delay1+(2)
>       elsif $delay1==1
>          $delay1+(2)
>       end

Those lines don't do anything at all. Note that

     $delay1+(2)

is the same as

     $delay1 + 2

which calculates a value, and then discards it, because it's not 
assigned anywhere.

I can't see what the $ET logic in this loop is doing, so I think you 
should add some comments to describe it, and if you can't, then think 
about rewriting it :-)

>     $totalTravel = $timewait.to_i

You haven't initialised $timewait, so this is the same as nil.to_i, 
which always returns zero. As a result, you always see a Total Traveling 
Time of 0 for the second and subsequent cars.

This is another good reason for not using global variables. If you had 
written

    total_travel = timewait.to_i

then ruby could have given you a helpful runtime error. You have not 
assigned to timewait earlier, so it's not a local variable, and neither 
do you have a method called 'timewait'.

This is also another good reason for treating all the cars the same, so 
that there's only one set of logic.

A minor stylistic point: conventionally, foo_bar is preferred over 
fooBar as a local variable name.

Finally: running the program with ruby -w will also spring up a few more 
warnings. These are because

   puts ("hello")

is considered bad form. Either omit the space before the parentheses, or 
leave out the parentheses altogether. i.e.

   puts("hello")    # fine
   puts( "hello" )  # also fine
   puts "hello"     # also fine

I hope there are a few useful pointers in there. So in summary:
* use local variables (foo) not global variables ($foo)
* process all cars inside the loop, so the first car isn't special
* calculate the departure and arrival times, don't use sleep and 
Time.now

I think you should now be able to assemble a program which loops round, 
printing the start and end times of each journey. Then you can add the 
random choice of number of passengers. Feel free to post your revised 
code as you go.

Once you've done that, all we need to do is work out the earliest time 
in which 500 people have arrived.

HTH,

Brian.
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