## Exercise 1

Here are three WebPPL programs:

flip() ? flip(.7) : flip(.1)

flip(flip() ? .7 : .1)

flip(.4)


a) Show that the marginal distribution on return values for these three programs is the same by directly computing the probability using the rules of probability (hint: write down each possible history of random choices for each program). Check your answers by sampling from the programs.

b) Explain why these different-looking programs can give the same results.

## Exercise 2

Explain why (in terms of the evaluation process) these two programs give different answers (i.e. have different distributions on return values):

var foo = flip();
display([foo, foo, foo]);

var foo = function() {return flip()};
display([foo(), foo(), foo()]);


How could you use mem to make the second program have the same distribution as the first?

## Exercise 3

In the simple medical diagnosis example we imagined a generative process for the diseases and symptoms of a single patient. If we wanted to represent the diseases of many patients we might have tried to make each disease and symptom into a function from a person to whether they have that disease, like this:

var lungCancer = function(person) {return flip(.01)};
var cold = function(person) {return flip(.2)}

var cough = function(person) {return cold(person) || lungCancer(person)}

display([cough('bob'), cough('alice')])


Why doesn’t this capture our intuitions correctly if we try to do the same thing for the more complex medical diagnosis example? How could we fix it?

## Exercise 4

Work through the evaluation process for the bend higher-order function in this example:

var makeCoin = function(weight) {
return function() {
return flip(weight) ? 'h' : 't'
}
}
var bend = function(coin) {
return function() {
return coin() == 'h' ? makeCoin(.7)() : makeCoin(.1)()
}
}

var fairCoin = makeCoin(.5)
var bentCoin = bend(fairCoin)


Directly compute the probability distribution of the bent coin in the example. Check your answer by using Infer.

## Exercise 5

Use the rules of probability to compute the probability that the geometric distribution defined by the following stochastic recursion returns the number 5. Check your answer by using Infer.

var geometric = function(p) {
return flip(p) ? 0 : 1 + geometric(p)
};


## Exercise 6

Convert the following probability table to a compact WebPPL program:

A B P(A,B)
F F 0.14
F T 0.06
T F 0.4
T T 0.4

Hint: fix the probability of A and then define the probability of B to depend on whether A is true or not.

var a = ...
var b = ...
[a, b]


Run your WebPPL program and use Infer to check that you get the correct distribution.

## Exercise 7

In Example: Intuitive physics we modeled stability of a tower as the probability that the tower falls when perturbed, and we modeled “falling” as getting shorter. It would be reasonable to instead measure how much shorter the tower gets.

a) Below, modify the stability model by writing a continuous measure, towerFallDegree. Make sure that your continuous measure is in some way numerically comparable to the discrete measure, doesTowerFall (defined here as either 0 or 1). Mathematically, what is your continuous measure?

///fold:
var listMin = function(xs) {
if (xs.length == 1) {
return xs[0]
} else {
return Math.min(xs[0], listMin(rest(xs)))
}
}

var highestY = function (w) { listMin(map(function(obj) { return obj.y }, w)) }
var ground = {shape: 'rect', static: true, dims: [worldWidth, 10],
x: worldWidth/2, y: worldHeight+6};

var almostUnstableWorld = [
ground,
{shape: 'rect', static: false, dims: [24, 22], x: 175, y: 473},
{shape: 'rect', static: false, dims: [15, 38], x: 159.97995044874122, y: 413},
{shape: 'rect', static: false, dims: [11, 35], x: 166.91912737427202, y: 340},
{shape: 'rect', static: false, dims: [11, 29], x: 177.26195677111082, y: 276},
{shape: 'rect', static: false, dims: [11, 17], x: 168.51354470809122, y: 230}
]

var noisify = function (world) {
var perturbX = function (obj) {
var noiseWidth = 10
return obj.static ? obj : _.extend({}, obj, {x: uniform(obj.x - noiseWidth, obj.x + noiseWidth) })
}
map(perturbX, world)
}

///

// Returns height of tower
var getTowerHeight = function(world) {
return worldHeight - highestY(world);
};

var doesTowerFall = function (initialW, finalW) {
var approxEqual = function (a, b) { Math.abs(a - b) < 1.0 }
return !approxEqual(highestY(initialW), highestY(finalW))
}

var towerFallDegree = function(initialW, finalW) {
// FILL THIS PART IN
return -999;
};

var visualizeStabilityMeasure = function(measureFunction) {
var initialWorld = noisify(almostUnstableWorld)
var finalWorld = physics.run(1000, initialWorld)
var measureValue = measureFunction(initialWorld, finalWorld);
print("Stability measure: " + measureValue)
print("Initial height: " + getTowerHeight(initialWorld))
print("Final height: " + getTowerHeight(finalWorld))
physics.animate(1000, initialWorld)
};

// Test binary doesTowerFall measure
// visualizeStabilityMeasure(doesTowerFall);

// Test custom towerFallDegree measure
visualizeStabilityMeasure(towerFallDegree);


b) Are there worlds where your new model makes very different predictions about stability from the original model? Which best captures the meaning of “stable”? (it might be useful to actually code up your worlds and test them).