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Scratching the Itch
Jeff Parker, Merrimack College
Scratch@MIT
The primary purpose of the Data statement is to give names to constants;
instead of referring to pi as 3.141592653589793 at every appearance, the
variable Pi can be given that value with a Data statement and used instead
of the longer form of the constant.
This also simplifies modifying the program, should the value of pi change.
Fortran manual for Xerox Computers
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Outline
Setting:
Teaching CS1 to students that didn't apply to MIT
Problem:
Hard for students to follow what a program is doing
Harder for students to write programs
It takes a long time to get to the good stuff
Course Goals:
Computers can do interesting things
Computers do only what you program them to do
Good News:
Scratch can help us teach these points and many other things
Bad news:
They need to move from Scratch to C++ or Java at some point
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What do we use Scratch to teach?
We start CS1 with 2 weeks of Scratch
We cover
If Statements
If touching sprite2
Loops
Simple loops w/ observable events
Objects
Sprites have state (such as position) and behaviors
Plugability
Recursive nesting (see next slide)
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What do we use Scratch to teach?
Plugability
An if statement has a condition
A condition is a Boolean expression
An expression can compare two integer expressions
Integer expressions can be a number or a function
Sample function: RNG, which uses expressions
…
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Difficulties
Equations are a difficult concept to grasp
Students do not understand equations
Heck, most adults don't understand equations
"Each equation in the book would halve the sales."
Stephen Hawking
Students do not understand assignment
Students have trouble with the notion of a variable
See the work of Dietmar Kuchemann and Zalman Usiskin
And variable in CS is different from a variable in Math
They really have trouble with notion of an indexed variable
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Simplicity of Model
There is (almost) no syntax in Scratch. Compare
Further, there is no loop index
for (int i = 0; i < 10; i++)
pop();
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Examples
Focus on one issue: reading or writing a simple loop.
for (int i = ...; i < size; i++)
dest[i] = source[i];
// copy
if (a[i] > max)
max = a[i];
// Find max
a[i-1] = a[i];
// Shift left
fib[i] = fib[i-1] + fib[i-2] // fib
y[i]= y[i-1] + delta*fprime(x[i], y[i])
if (s[i] != s[size - i - 1)
return NOT_PALINDROME
They have trouble
thinking about the index
This doesn’t mean that
they cannot understand
these algorithms: it is
expressing it formally
that is difficult
// Euler
// Palindrome
for (j = i; j < size - 1; j++) // Bubble Sort
if (a[j] > a[j+1])
swap(a, j, j+1);
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Simplicity of the Logo model
for (t = 0; t < …)
// Closed form expression
(x, y) = (x0 + D * t, y0 + 20 t – t2)
for (…)
x = x + D;
vy = vy – a;
y = y + vy;
// Integration Model
for (…)
move(D);
turn(f);
// Logo Model
// x = x + D * cos(q); y = y + D * sin(q)
// q = q + f
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Transition from Scratch to C++
Use the time with Scratch to cover algorithms
Introduce Single Stepping (Under Extras menu)
In the language of choice
Focus on programs with visible results
Relate new constructs to Scratch (see David Malan's talk)
They know the semantics: just need to learn syntax
Postpone subscripts (indexing) and equations
K & R – style Filters
LISP – style processing: car, cdr, cons
Still allows interesting projects such as Igpay Atinlay
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Transition from Scratch to C++
Preserve the community: let students learn from each other
Encourage students to work in pairs
Never say something a student can say
Foster a community of learners who listen to each other
Every student can do something right
Select their best work – you can still suggest changes
Make your examples ones you would share with colleagues
Take the time to explain a good algorithm, rather than
introducing dumb example because it is simple
Keep it fun!