The Daily Parker

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H is for Human Factors

Blogging A to ZThe Blogging A-to-Z challenge enters its second week with a note about you, the human.

Last week I discussed several topics that you probably thought were about computers. They weren't. They were about how you interact with computers.

Computers don't need programming languages. This is a perfectly runnable program for the 6502 microprocessor:

0600: a9 01 8d 00 02 a9 05 8d 01 02 a9 08 8d 02 02

The human-readable version looks like this:

$0600    a9 01     LDA #$01
$0602    8d 00 02  STA $0200
$0605    a9 05     LDA #$05
$0607    8d 01 02  STA $0201
$060a    a9 08     LDA #$08
$060c    8d 02 02  STA $0202

Imagine looking at that all day and trying to debug it. You'd go insane. I know, because back in the day, I wrote that kind of code for 6502 chips and, well, you have evidence about my sanity in these pages.

This is why we use high-level languages instead of low-level ones. We try to abstract away all the crap. Instead of writing for the machine to read, as you do with assembly language (and even there you have a thin abstraction layer), you write software for a human to read.

And the languages continuously improve. Here, below, is some of the first code I ever plagiarized wrote that lived in a production application. You might understand what it does from reading it:

day=VAL(MID$(dtee$,4,2))
month=VAL(LEFT$(dtee$,2))
year=VAL(RIGHT$(dtee$,4))
hour=VAL(LEFT$(ttee$,2))
minute=VAL(MID$(ttee$,4,2))

MDYTOJULIAN:
   julian!=INT(365.2422# * year + 30.44 * (month-1) + day + 1)
   t1=month - 2 - 12 * (month<3)
   t2=year + (month<3)
   t3=INT(t2/100)
   t2=t2-100*t3
   weekday!=INT(2.61*t1-.2) + day + t2 + INT(t2/4)
   weekday! = (weekday! + INT(t3/4) - t3 - t3 + 77) MOD 7 + 1
   t4=julian! - 7 * INT(julian!/7)
   julian!=julian!-t4+weekday!+7*(t4<weekday!-1)+1721060#
   day$=dow$(weekday!)+","+STR$(day)+" "+month$(month)+STR$(year)
   h=hour-12
   IF h<0 THEN julian!=julian!-1:_
      h=h+24
   ju#=CDBL(julian!)
   ju#=ju#+(h/24)+(minute/1440)

That language is called QuickBASIC, which Microsoft published from 1985 until 1991, when Visual BASIC took over. It's a subroutine that converts a date into its Julian day number. I developed it because in QuickBASIC there was no way to do simple date calculations, but in astronomy, there was.

Here's the C# version:

public static double ToJulianDayNumber(this DateTimeOffset date)
{
   return date.ToOADate() + 2415018.5;
}

Forget that this is a naïve implementation, as there are a ton of special cases before the 19th century; and also forget that it uses a magic number that depends on understanding a bit of .NET and Windows internals (like, for example, that JD 2415018.5 is 30 December 1899, which is the epoch date for OLE automation).

The point is, I can abstract away all the calculations and create an extension method which allows me to get any Julian day number just by calling that method on any date-time struct:

var julian = DateTimeOffset.UtcNow.ToJulianDayNumber();

As a human, I'd bet you find that a lot easier to read and understand than the MDYTOJULIAN subroutine above.

But C# makes it even easier to calculate the difference between two dates, obviating the Julian day entirely, unless you're writing software specifically for astronomers:

var otherDay = new DateTimeOffset(2017, 01, 20, 17, 0, 0, TimeSpan.Zero)
var days = DateTimeOffset.Now.Subtract(otherDay).TotalDays;

Because really, you're trying to get the total days between two dates. The Julian day is a required abstraction when you don't have date arithmetic built in. But when you can just do date.Subtract(otherDate) to get a TimeSpan object, the code is a lot easier for humans to read.

Let the compiler worry about the machine. You worry about other humans.

Further reading:

Comments (1) -

  • David Harper

    4/9/2018 7:33:47 PM +00:00 |

    We software developers owe a tremendous debt to the late John Backus and his team at IBM who developed the first true high-level compiled language back in the mid 1950s.  Before their pioneering work, all programming was done in assembler (if you were lucky) or directly in machine code (if you weren't).  Backus and his team faced considerable skepticism from the programming community, who couldn't believe that compiled code could ever be as efficient as hand-crafted assembler.  One of the IBM team's major accomplishments was not only to create a high-level language and a compiler for it, but to make that compiler capable of generating machine code that was very nearly as good as anything the hand-crafters could write.

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