# The Science of Programming/And Now for Something Completely Different

Enough with the differentiation, already! Time for
something new.
New, but not all that different.
Once you have learned about differentiation in your
Math classes, you will surely start to learn about
*integration*. As stated very early on,
integration is the summing up
of all the tiny (infinitesimal) pieces of a curve.
So if *dy* represents one of the (infinitely) many
bits of the curve *y*, then:

```
```

Indeed, if you sum up all the bits of a thing, you
get the thing itself.
^{[1]}

## Summing a series Edit

As with differentiation, there are two kinds of integration, numeric and symbolic. Like differentiation, numeric integration gives an approximate result while symbolic integration gives an exact result. Since numeric integration involves the actual summing up of a bunch of little bits, let's get some practice summing.

What is the sum of:

```
```

with an infinite number of fractions following the pattern. We have a two issues to solve before we can write code to give us the answer. The first is that the sum seems to be composed of two different things, a whole number (1) and a bunch of fractions.

Life is so much easier when all things are the same; if
this is so, we
have no need of cluttering up our code with *if* expresssions to
decide on what kind of thing we are working on.
^{[2]}
In general, if we have two kinds of things we wish to treat the same,
we need to either make the first look like the second, the
second look like the first, or both look like some other thing.
In the specific case of this summation, we need to either:

- make the fractions look like whole numbers,
- make the whole number look like a fraction, or
- make the whole number and the fractions look like a third kind of thing

Of the three strategies, the second seems to be the easiest route since a a whole number is easily represented as a fraction by placing a one in the denominator:

```
```

At this point, you should familiarize yourself with iterative loops and recursive loops.

The remaining issue we need to solve is how to structure our code. It is clear that we will need some kind of loop, either recursive or iterative, to add up as many fractions as desired. In order to use a loop, we usually need to make the items we are looping over look the same or at least be a function of the loop counter. In our case, how do we make each fraction look alike?

If we look at the denominator, we see that each can be described as a power of two:

```
```

It seems that we can use a loop counter (starting at zero) as the exponent in the denominator of each fraction. With these two issues out of the way, we can begin to write some code. Let's try the recursive route first.

### The recursive way Edit

We will use *n* as our
loop counter:

function total(n) { 1 / (2 ^ n) + total(n + 1); }

If we call *total* with an initial value of zero for *n*,
we should get:

1 / (2 ^ 0) + total(1)

Upon the next recursive call, we should get:

1 / (2 ^ 0) + [1 / (2 ^ 1) + total(2)]

And then:

1 / (2 ^ 0) + [1 / (2 ^ 1) + [1 / (2 ^ 2) + total(3)]]

This is exactly what we want, but we are going to end up
summing an infinite number of terms.
We have neglected to have a base case for stopping
the recursion. Let's pass in the number of fractions we wish to
sum up as *max*:

function total(n,max) { if (n == max) { 0; } else { 1 / (2 ^ n) + total(n + 1,max); } }

The recursion stops when *n* reaches *max*.

What do we get if we total up five fractions?

sway> total(0,5); REAL_NUMBER: 1.9375000000

Is this correct? Let's add up five fractions explicitly:

sway> 1.0 / 1 + (1.0 / 2) + (1.0 / 4) + (1.0 / 8) + (1.0 / 16); REAL_NUMBER: 1.9375000000

Seems to be. How about ten fractions?

sway> total(0,10); REAL_NUMBER: 1.9980468750

Fifty fractions?

sway> total(0,50); REAL_NUMBER: 2.0000000000

One hundred fractions?

sway> total(0,100); REAL_NUMBER: 2.0000000000

These results lead us to believe that if we were to add up these fractions out to infinity, we would get 2 as a total. Of course, since our result is a real number, we need to be wary of trusting it absolutely, but in this case, I'd be willing to bet the result is correct.

How about we add some visualization to this function? We probably should have done this first to make sure our code was behaving as expected:

function total(n,max) { if (n == max) { println("0"); 0; } else { var denom = 2 ^ n; print("1/",integer(denom)," + "); 1 / denom + total(n + 1,max); } }

Note how we 'precomputed' the denominator since we need it twice,
once for the visualization and one for the actual computation.
We also use the *integer* function to convert the real number
that exponentiation produces back to an integer.

sway>total(0,5); 1/1 + 1/2 + 1/4 + 1/8 + 1/16 + 0 REAL_NUMBER: 1.9375000000

Looking good! Of course, our visualization does not produce a valid Sway expression (whitespace and precedence problems), but that is rarely necessary for a visualization.

### An iterative approach Edit

Let's try using an iterative loop instead.
There is a standard methodology for converting
a recursive loop to an iterative loop.
The first step is to initialize a local variable,
say *result*, to the recursive loop's base case
return value (and then return it).
In this case, the return value is
zero:

function total(n,max) { var result = 0;

return result; }

Now we add a while loop with the opposite
of the test found in the recursive loop's *if* expression:

function total(n,max) { var result = 0;

while (not(n == max)) //better is n != max) { }

return result; }

In the body of the loop, we place the recursive case calculation:

function total(n,max) { var result = 0;

while (n != max)) { var denom = 2 ^ n; 1 / denom + total(n + 1,max); }

return result; }

replacing the recursive call with *result*:

function total(n,max) { var result = 0;

while (n != max)) { var denom = 2 ^ n; 1 / denom + result; //was total(n + 1,max) } }

and assigning the whole expression back to *result*:

function total(n,max) { var result = 0;

while (n != max)) { var denom = 2 ^ n; result = 1 / denom + result; //was total(n + 1,max) }

return result; }

Finally, if any variable was updated in the original recursive call, update the variable at the bottom of the loop using assignment:

function total(n,max) { var result = 0;

while (n != max)) { var denom = 2 ^ n; result = 1 / denom + result; n = n + 1; }

return result; }

Voila! We're done. This technique generally works well and will even work if there are multiple base cases and mulitple recursive cases. Sometimes, however, things go a little bit wrong in the transformation. Here's and example; let's convert the recursive version of the greatest common divisor function:

function gcd(n,d) { if (d == 0) { n; } else { gcd(d,n % d); } }

We start with the base case return value:

function gcd(n,d) { var result = n;

return result; }

Next, we add a while loop:

function gcd(n,d) { var result = n;

while (d != 0) { }

return result; }

Then we copy over the recursive case calculation:

function gcd(n,d) { var result = n;

while (d != 0) { gcd(d,n % d); }

return result; }

We replace the recursive call in the calculation by *result*:

function gcd(n,d) { var result = n;

while (d != 0) { result; //was gcd(d,n % d); }

return result; }

Now we update the variables that change in the recursive call:

function gcd(n,d) { var result = n;

while (d != 0) { result; //was gcd(d,n % d); n = d; d = n % d; }

return result; }

We should be done at this point, but we still have a couple of problems. The first is that the statement:

result;

in the body of the while loop isn't doing anything useful. This is our first clue that something is amiss. For now, let's delete it:

function gcd(n,d) { var result = n;

while (d != 0) { n = d; d = n % d; }

return result; }

The second problem is that in the recursive call, the update to
variable *d* used the old value of *n* but in our transformation,
the update to *d* uses the *new* value of *n*. We need
to save the old value and we use a temporary variable to do so:

function gcd(n,d) { var result = n;

while (d != 0) { var temp = n; n = d; d = temp % d; }

return result; }

The final problem is that result never changes. That's because
we initially set result to the original value of *n*, not
the last value of *n*. Note that the recursive version uses
the last value of *n* as desired. Thus, we update *result*
after the while loop terminates to get the last value of *n*.

function gcd(n,d) { var result = n;

while (d != 0) { var temp = n; n = d; d = temp % d; }

result = n;

return result; }

Of course, we can clean up our function up a bit by realizing that
*result* isn't doing anything at all; we can just return
the last value of *n*:

function gcd(n,d) { while (d != 0) { var temp = n; n = d; d = temp % d; }

return n; }

This technique, at best, gets you the right answer. At worst, it gets you well on the way towards the right answer.

## Summing as an abstraction Edit

Note that in both our recursive and iterative solutions to the original
summation, we have hard-wired the function that computes the
next element in the sequence (namely the inverted exponentiation).
An important design strategy for writing computer programs is
called *separation of concerns*.
With this strategy, we try to write functions, or sets of
functions, so that each function performs a single task.
In our solutions, both the generation of the next fraction
(one concern) and the summing of those fractions (another
concern),
are found in a single function,
We can separate those concerns into individual functions.
One function generates the fractions to be summed; the
other performs the actual summing, given the previously
generated collection
of fractions.
Our first task, then, is to generate
the collection of fractions.
We will examine to ways of gathering a collection together,
*arrays* and *lists*.

At this point, you should familiarize yourself with arrays and lists.

Here is the code for generating a list of the desired fractions:

function invPowOfTwo(n,max) { if (n == max) { :null; } else { 1 / (2 ^ n) join invPowOfTwo(n + 1,max); } }

If I wanted an array instead of a list, I might write the function
as:^{[3]}

function invPowOfTwo(n,max) { var a = allocate(max);

while (n < max) { a[n] = 1 / (2 ^ n); n = n + 1; }

a; }

Either way, once I have my collection of fractions, I can now sum then with a general purpose summer:

function sum(items) { if (items == :null) { 0; } else { head(items) + sum(tail(items); } }

Whether I pass an array or list to *sum*, I get the total of
all the items in the collection.
.^{[4]}

I could also have written *sum* iteratively:

include("basics"); function sum(items) { var i; var total = 0;

for-each(i,items) { total = total + i; }

total; }

This version will also sum up the collection of items in either list or array form, but for one of these forms, the process is rather slow (see the exercises).

The advantage of separating the generation of elements from the process of summing is this. Once we have our list of elements, we can do more things than just sum with them. We can visualize them, invert them, (after inverting) find the ones that are Mersenne primes plus 1, and so on. Likeise, we can use our summing function to sum up other kinds of collections.

## Pseudo-infinite sequences Edit

What if we go through the trouble a generating a set of elements to be summed or processed in some fashion and neglected to produce a large enough collection? We are stuck with regenerating the collection again, throwing away all the work we did previously.

The problem is we had to specify the max before we perform the summing:

function invPowOfTwo(n,max) { if (n == max) { :null; } else { 1 / (2 ^ n) join invPowOfTwo(n + 1,max); } }

It sure would be nice if we didn't have to specify in advance
how large the collection should be. Not requiring a *max*
also would simplify the
code:

function invPowOfTwo(n) { 1 / (2 ^ n) join invPowOfTwo(n + 1); }

The problem is, without a base case,
we will fall into an infinite recursive
loop. However, we can avoid this pitfall through *delayed* or
*lazy evaluation*.

At this point, you should familiarize yourself with lazy evaluation.

Let's write a version of *invPowOfTwo* that delays evaluation
of the recursive call:

function invPowOfTwo(n) { 1 / (2 ^ n) join delay(invPowOfTwo(n + 1)); }

Note the use of the *delay* function wrapped around the
recursive call.
What *delay* does is save everything needed to compute
the recursive call without actually making the call. The
actual call can be made later using the information
saved by *delay*.

Let's look at the result of calling our new function:

sway> var items = invPowOfTwo(0); LIST: (1.0000000000 # <THUNK 11167>)

We see the first element of our list is 1.0, as expected.
Rather than seeing the rest of our list, however,
we see that a *thunk* has been glued on as the
tail of the list (the sharp sign signifies that the
tail of the list is not a proper list). We can examine
the thunk:

sway> var t = tail(items); THUNK: <THUNK 11167> sway> pp(t); <THUNK 11086>: context: <OBJECT 11071> code: invPowOfTwo(n + 1) THUNK: <THUNK 11086>

We see that the *code* field of the thunk object holds
the actual call. The *context* field holds an environment
containing the values
of *n* and *invPowOfTwo*, all that is
needed to make the actual call.

A list of this sort is known as a *delayed stream* or
*stream* for short. The stream metaphor is used since
as long as there is source of water (memory), the stream will never
run dry.

How do we get the elements other than the first element out
of the stream? With the *force* function. The *force*
function, when given a thunk as an argument, performs the
evaluation that was delayed in the first place:

sway> head(items); REAL_NUMBER: 1.0000000000 sway> tail(items): THUNK: <THUNK 11167>

sway> force(tail(items)); LIST: (0.5000000000 # <THUNK 11193>)

By forcing the tail of the original list, we end up with
a new list generated by the call `invPowOfTwo(n + 1)`.
When this call was delayed, *n* had a value of zero. Now
that the call is being made, *invPowOfTwo* is passed a
value of one. This generates a new list with
at the head and another delayed recursive call. This time, delay
will remember that *n* has a value of one instead of zero.

Let's define a function to obtain any element of a stream. We will pass in the index and then successively force the tail of the list until we get to the correct element:

function streamIndex(s,i) { if (index == 0) // the head of the stream is desired { head(s); } else { streamIndex(force(tail(s)),i - 1); } }

Note that if we want element *i* of a list with more than
*i* elements, that is
the same as wanting element *i* - 1 of the tail of the list.
When we take the tail of the list, the desired element appears
to move one step closer to the front of the list.
^{[5]}

Now we can sum a stream:

function sumStream(s,count) { if (count == 0) { 0; } else { head(s) + sumStream(force(tail(s)),count - 1); } }

without having to worry about how many elements were originally generated. Iteratively, summing a stream might look like:

function sumStream(s,count) { var total = 0;

while (count > 0) { total = total + head(s); s = force(tail(s)); count = count + 1; }

total; }

## Questions Edit

## Footnotes Edit

- ↑
Assuming you don't believe in that magical marketing term,
*synergy*. - ↑ We saw this previously when we made a constant look like a term.
- ↑ Lists lend themselves to recursive solutions, arrays to iterative solutions.
- ↑ Don't try this with other languages! In almost every other language, the operators/functions for decomposing a list are different than decomposing an array.
- ↑ This is very important. Convince yourself this is so.