https://susam.net/fd-100.html

FD 100

By Susam Pal on 28 Oct 2019

I learnt how to write computer programs in IBM/LCSI PC Logo. That was
back in the year 1992. Computers were much simpler in those days. The
ones in our school were IBM PC compatible computers with mostly
monochrome displays. The ability to boot a computer using a 51/4-inch
floppy disk containing MS-DOS, load a Logo interpreter and then write
some programs without any distraction had its own charm that I find
missing from modern day computing.

The First Line of Code 

The first line of code I ever wrote was:

FD 100

Here is how the output looks:

A vertical line drawn with Logo along with Logo source code for it
The first Logo program

That is the "hello, world" of turtle graphics in Logo. That simple
line of code changed my world. I could make stuff happen in an
otherwise mostly blank monochrome CRT display. Until then I had seen
CRTs in televisions where I had very little control on what I see on
the screen. But now, I had control! The turtle became my toy and I
could make it draw anything on a 320 x 250 canvas.

Polygons 

With a little knowledge of geometry, one could draw polygons. Often
the first polygon one would learn to draw was a square. It involves
making the turtle walk forward 100 steps, then turn right 90deg, and
repeat these two operations four times in a loop. Here is the code:

REPEAT 4 [FD 100 RT 90]

Here is the output:

A square drawn with Logo along with source code A square drawn with
IBM Personal Computer Logo

Similarly, one could draw other polygons. The only thing my
nine-year-old self then needed to understand was that after drawing
an entire polygon, the turtle is back to its original position having
completed one full turn. Therefore to draw a polygon with \( n \)
sides, the turtle needs to turn by \( 360 / n \) degrees after
drawing each side. Drawing a pentagon is as simple as:

REPEAT 5 [FD 80 RT 72]

Here is the output:

A pentagon drawn with Logo along with source code A pentagon drawn
with IBM Personal Computer Logo

The same approach works for drawing a regular five-pointed star too.
The only new thing we need to consider here is that as the turtle
draws the shape, it makes two full turns. Therefore, it must turn by
\( 720 / 5 \) degrees after drawing each side. Here is the code:

REPEAT 5 [FD 100 RT 144]

Here is the output:

A pentagram drawn with Logo along with source code A pentagram drawn
with IBM Personal Computer Logo

I remember feeling uneasy about the lopsided appearance of the
polygons above and then trying to please my sense of aesthetics by
centring these polygons horizontally on the screen and having them
stand firmly on an imaginary horizontal line so that they look
balanced. I won't include the code and output for that on this page
for the sake of brevity of this post but here are links to some
screenshots I have kept that show a few of several ways to do it:
logo-square-centre.png, logo-pentagon-centre.png, and
logo-star-centre.png.

Circles 

Going from polygons to circles was especially fun. Here is the first
piece of code one would normally write to learn to draw a circle:

REPEAT 360 [FD 1 RT 1]

Here is the output:

A circle drawn with Logo along with source code A circle drawn with
IBM Personal Computer Logo

Now precisely speaking, this is not exactly a circle. This is a
triacosiahexeacontagon, i.e., a 360-gon. It is an approximation of a
circle with 360 very short line segments. Nevertheless it was enough
to get a young child who had just begun to learn using the computer
excited about programming. It showed me how control flow could be
used elegantly to express complex ideas in a simple expression. By
the way, here is one way to centre that circle horizontally on the
screen: logo-circle-centre.png.

Soon after learning to draw a circle, I learnt to write this:

REPEAT 20 [REPEAT 180 [FD 1 RT 2] RT 18]

This code draws 20 overlapping circles. The output looks like this:

A grid made with 20 circles along with Logo source code for it Grid
of circles drawn with IBM Personal Computer Logo

A Lasting Effect 

There is a lot more to Logo than turtle graphics. Logo gave me a
brief taste of functional programming even though back then I did not
know the term "functional programming". I discovered the same
simplicity and elegance later in Lisp about 15 years later. After
all, Logo can be thought of as a dialect of Lisp without parentheses
that controls a turtle.

At an impressionable age of nine, reading and writing code like this,
and using simple arithmetic, geometry, logic, and code to manipulate
a two-dimensional world had a lasting effect on me. Back in those
days, I used to find joy in sharing some of my interesting Logo
programs with my teachers and friends. I like to believe that my
passion for software engineering as well as my love for writing code,
sharing code, and open source development are a result of coming
across these beautiful code examples early in my life.

FD 100--it is a tiny piece of code, but it changed my world!

Comments | #programming | #dos | #technology
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