Monday, October 20, 2014

GSP and LOGO (for MITx: 11.132x)

Note: This post is an assignment for the Edx MOOC MITx: 11.132x Design and Development of Educational Technology. The assignment had to be posted online, and since it relates somewhat to the themes of this blog, I put it here.

Educational Technology Then and Now: Geometer's Sketchpad and LOGO

Geomter's Sketchpad (GSP) is an example of current educational technology that is based on design and educational principles that can generally be described as constructionist. Widely used in contemporary classrooms, GSP is based on ideas about computer-human interaction that date back to the 1960s, and bears interesting points of comparison with LOGO, an educational technology which approaches a similar subject from a distinctly different direction.
A classic GSP construction: Pythagoras Tree

GSP is an environment for dynamic geometry exploration - it allows uses to start from some simple elements (point, line segments, circles) and construct sophisticated geometric forms. Transformations, iterations, and animation can take these forms well beyond what can be done with ruler and compass. The dynamic element resides in how the initial inputs (the free points) can be dragged, mapped, or animated while preserving the structures that were constructed with them. GSP is commonly used in middle school and high school, but is also used in elementary school settings, and at the university level - its simple, yet powerful design make it useable in many settings. 

In the 1980s, the original developers of GSP based their ideas on earlier earlier work from the 1960s on human-computer interaction (the SKETCHPAD program, 1967), and ultimately on the concepts of Euclidian geometry, which date back over two millennia.  Since the time of its release in the early 1990s, GSP has gone through several iterations, maturing and inspiring other dynamic geometry software, such as GeoGebra.

The way that GSP embodies the concepts of Euclidian geometry make it educational without seeming to explicitly trying to teach. It allows students to act as practitioners, creating their own works by providing them the means of discovering and applying geometric concepts. This is one of its sources of charm and power - presenting itself as a tool that is useful both to learners, to general geometry enthusiasts, and mathematicians.

One difficulty that teachers and students can initially encounter with GSP is its complete openness: it presents a blank canvass, with no prompts, few tool icons, and a set of menu items that seem to be disabled. Only when the correct inputs are highlighted do constructions become possible: when two points are selected, you can opt to construct a circle, line, segment or ray. When three points are selected you can choose to make a triangle, or an arc. As you learn what elements are required to construct new elements, you can begin to use geometric constructions to create something interesting. The pitfall that some students fall into is to treat GSP as a "drawing" program instead of a "constructing" program. Teachers find it difficult sometimes to lead students through the many steps required to build up a sketch. A solution to both dilemmas is to have students add to and explore pre-existing sketches, a GSP technique sometimes called the "SWIMMM" approach (Start with Immediately Meaningful Mathematical Models).

LOGO, developed by Seymore Papert and others around 1967, introduced a new language and perspective for creating geometric forms and learning programming concepts: turtle graphics. Allowing students to draw on a canvas by providing simple procedural commands to move a point (sometimes conceptualized as a turtle with a pen tied to its tail), LOGO was an environment and programming language of surprising power and expressiveness.

Educational technology is sometimes evaluated by the "low floor, high ceiling, wide wall" rubric, meaning that it should be easy to get started with, be empowering, and admit a wide range of expressiveness. Classical implementations of LOGO present difficulties to young learners because of the failure-prone nature of typing in the sometimes tricky syntax into command-line style interpreters. More contemporary implementations of LOGO help overcome this problem: The "pen" and "movement" operations in SCRATCH provide a LOGO-like environment that makes writing programs much easier. The SNAP extended implementation of SCRATCH features a logo-like turtle-cursor, making it easy to get started with turtle graphics.   

LOGO-style turtle graphics in SNAP

LOGO and GSP both greet their user with a blank canvas and an invitation to create geometric shapes and patterns through interaction and experimentation, but each provides a very different interactive experience. LOGO presents a "turtle's eye view" of geometry - a procedural exploration of analytic (Cartesian) geometry where a kinesthetic approach that engages learners' proprioception is used to navigate the Cartesian plane. GSP provides a more traditional synthetic (Euclidian) approach, where the geometer has a more global view (as opposed to the turtle's necessarily local one). 

In addition to offering different perspectives on geometry, GSP and LOGO also present different views of human-computer interaction: two different approaches to computer programming. Learners engaging with LOGO learn the fundamentals of computer programming in a rather direct way: the LOGO language's procedural flow and its constructs make it look very much like many other programming languages. GSP also introduces programming ideas, but more abstractly and less recognizably. The Pythagoras tree sketch, shown at the top of this post uses fundamental programming ideas of iteration (to create the diminishing repeated elements of the sketch) and parameterization (the colour of the squares is a function of their area). Both LOGO and GSP help their users learn how to provide input and instructions to computers (i.e. the basics of computer programming), but in very different ways.

Viewed as educational technologies, LOGO and GSP both follow a constructive approach, providing open environments that encourage an affective and aesthetic engagement with learning mathematics. The differences in approach taken by these tools to the same learning goals show how varied educational technology can be.

Thursday, October 2, 2014

Monday, September 29, 2014

A year of tinkering

You really should take advantage of the free until August 2015 license that is currently being offered with a fresh download TinkerPlots. Would that it was freely available in perpetuity without condition, but a year of tinkering is nice.

If you are a middle school teacher, then this is designed for you and yours. If, like me, you are not, you may find it fun to play with anyway.  Here is something I was playing with recently:

An elementary school number sense activity

In the JUMP math curriculum for grades 3 and 4, there are lessons where students investigate the patterns formed when multiples of a number are written in a 3 or 4 column grid. After some number talk students play a game where some entries in the table are covered up or erased, and students have to identify the blanked out value and state the corresponding multiplication fact.

An interactive game like this is easy to make in TP - in the above screen shot multiples of 3 are arranged in a 3 column table. When a student provides an answer, clicking on the square will cause the answer to be highlighted in the number line. For fun, I added an additional plot that shows where the number would lie in a 10 point circle diagram (which students may have already been using to help remember and identify patterns in multiples). The multiple used and the number of columns displayed can be adjusted using sliders, and the "blanks" can be re-randomized by clicking Ctrl-Y. The file is here.

Update: John Mighton gives an overview of the kind of lesson that uses charts like these in JUMPs guided discovery method here.

Wednesday, September 17, 2014

modular tables

No, not a post about IKEA furniture. A while  ago I put up a post on colouring multiplication tables by assigning ranges of numbers a colour value. You end up with something that looks like a rainbow.

This image was made in Tinkerplots, so it was easy to go from a 10 x 10 table to a 50 x 50 table (removing the numbers and just keeping the colours, and shrinking each cell down a bit):

Inspired by the "Zn Multiplication visualizer" found here and mentioned here, and thinking about modular arithmetic from the last post, I decided to make a few more images.

If you take the values in this 50 x 50 table mod 9, you'll get this nice quilt - repeating in 9 x 9 blocks.

If you choose your mod value to line up with the width of the table (take the values mod 50) you get this:

The light blue along the bottom and the right side are all zeros - anything that is a multiple of 50, as the bottom and the far right side are, will have a remainder of 0 when you divide them by 50.

If you take the values mod 2500, then you get something that looks like the rainbow we started with - except for one square (maybe you can spot it, or figure out where it will be).

Here's a 75x75 table, mod 75, in shades of blue.

Monday, September 15, 2014

squashing multiples

An elementary school exercise leads to writing a simple program, a little proof by contradiction, and learning about some mostly-forgotten calculation tricks: just some of the fun that can be had when playing with simple math. Sound good? It all starts with squashing numbers...

No doubt you've noticed some patterns in the non-zero multiples of 9: 9, 18, 27, 36, 45,... One thing to notice is that if you (repeatedly) add up all the digits of a multiple of 9, you always get 9 as your answer.

This works immediately for many multiples of 9, like 9*14 = 126 (1 + 2 + 6 = 9), for others you need to keep squashing - if the first digit sum itself has more than one digit, sum those digits and repeat until you get a single digit answer. For example 9 * 42 = 378 (3 + 7 + 8 = 18, which has two digits, so keep squashing: 1 + 8 = 9). 

All multiples of 9 squash down to 9, which is neat. More importantly this also works in the other direction: any positive integer that squashes to 9 is a multiple of 9. This makes squashing an easy divisibility test for 9 and makes it easy to find multiples of 9 (is 359 a multiple of 9? No, but 369 is, and so is 459).  Also: Any number made by rearranging the digits of a multiple of 9 will also be a multiple of 9 (re-arranging the digits won't change the squash value). So, since I know that 882 is a multiple of 9, I also know that 288 and 828 are also multiples of 9.

Can you always squash a number? Could there ever be a number that gets bigger when you sum its digits?

One (wordy) argument goes like this: To obtain a contradiction, assume there are some positive integers greater than 9, whose digit sums are equal to or greater than the original number. Such a number would be a problem for us: we need to be sure that any number with two or more digits will always have a digit sum less than itself (so we can keep doing digit sums until the number is squashed down to a single digit).  Choose n to be the smallest such troublesome number. Suppose the digit sum of n is another number k such that n <= k. Now we are going to build a new number m by taking the digits of n and changing one of them: chose a non-zero digit in a position bigger than the ones place and decrease it by 1. For example, if our number n was 567 (which it isn't) our new number m could be 557. Now m is at least 10 less than n, but its digit sum is only one less than k (since only one digit was decreased by 1). Now m <= n -10 < - 1 <= k-1, and so m is also less than its digit sum (k - 1). But n was chosen to be the smallest number with this property, and m is definitely smaller than n. So we have a contradiction: it cannot happen that the digit sum of a number is greater than the number itself. This means that it is safe to squash: you will always get smaller and smaller numbers until you get down to the single digits.

Squashing multiples of 9 was interesting: What about patterns in other multiples? Consider positive multiples of 3: 3, 6, 9, 12, 15, 18, 21, 27, ... they don't squash down to the same value, but if you try it out you'll notice a pattern: 3, 6, 9, 3, 6, 9, 3, 6, 9, .... It's easy to see this if you write the multiples in a 3 column table.

In the chart above, the first column entries all squash to 3, the second to 6, and the third to 9. Some other multiples produces similar charts, for example 6 and 12. (I noticed these squashing patterns when looking at material from the JUMP math program for grades 3 and 4, where tables like these are used to explore patterns for learning multiplication facts.)

Not all integers will have their multiples fit into this pattern. For example, 4 needs a 9 column table to show a squash pattern.

You might want to write a little program to do your squashing for you - whether you do or squash by hand, you'll see a clear pattern.

Just a brief note on the Java utility below: the while(true) statement in the squash() method relies on our assumption that digit sums get smaller - otherwise we'd potentially have an infinite loop.  The digits() method is simple example of recursion.

package squash;
import java.util.ArrayList;
import java.util.List;

public class SquashCalculator {

public List digits(Integer start) {
if (start == 0) {
return new ArrayList();
int val = start % 10;
List recur = digits(start/10);
return recur;

public Integer sum(List list) {
Integer sum = 0;
for (Integer i: list) {
sum += i;
return sum;

public Integer digitSum(Integer i) {
return sum(digits(i));

public Integer squash(Integer n) {
if (n == 0) {
return 0;
Integer current = n;
while(true) {
current = digitSum(current);
if (digits(current).size() == 1) break;
return current;

Here's what you'll observe squashing the numbers 1 - 40:

Maybe you knew that this would happen, but I didn't. I was surprised at first that squashing numbers formed this regular pattern, but it really isn't surprising if you think about it. If you consider the squash of n  and then wonder what squash of n + 1 should be, it should just be one more, unless the squash of n was already 9, in which case adding one more will give you squash of 10, which is 1.

So, the squash function maps the integers onto a structure like the one on the right below, that is very close to taking the number "mod 9".

Putting the integers from 1 to 36 in a 9 column chart like the "multiples of 4" above shows this also.

This suggests that there is a direct formula for the squash of a number close to its mod 9 value. It turns out this can be expressed as:

One aspect of this is that n and squash (n) are congruent modulo 9 (which means that if you divide n by 9, or the squash of n by 9, you will get the same remainder).

This is the important relationship that makes squashing useful. It's kind of amazing that you can take a number and completely re-arrange its digits, or sum up all its digits, and still retain something about the original number (the remainder after dividing by 9). When you do something this violent to a number it's surprising that some information about the original number remains.

This is why every number that can be squashed to 9 is divisible by 9 (both are equal to 0 mod 9). This also helps to explain the number of columns in the tables above. For a number m whose multiples we are playing with, if m is divisible by 3 (like 3, 6, and 12), its multiples will have the same squash with a period of 3 (and will repeat in a 3 column table). If m is not divisible by 3 (the only factor 9 other than 1 and itself), then the squash of the multiples of m will have a period of 9 (and will repeat in a 9 column table). If m is a multiple of 9, then its multiples will be multiples of 9 also, and their squash will always be 9.

We also get a divisibility test for 3: If a number's squash value is 3, 6, or 9, then that number is divisible by 3. For example, suppose n squashes to 6. That means that n is congruent to 6 mod 9, which means that there is some positive integer k such that n = 9k + 6. Since the right hand side of that equation is divisible by 3 (dividing the right  by 3 gives 3k +2), so is the  left hand side.

But wait, there is more. Calculating squash values mod 9 has a short-cut: when you are adding up all the digits, you can throw out any multiples of 9, since they will always end up contributing zero to the final answer (because multiples of 9 have a remainder of 0 when divided by 9). This calculation is part of an error-checking technique called "casting out nines" which can be used to check arithmetic. When casting out nines, you essentially squash (ignoring 9s and multiples of 9s) the inputs and outputs of your calculation, and if they are different then you know you made a mistake.

If you want to learn more about this (and there is a lot more), you should Google "digital root" which is the standard name for what I've been calling "squash."