JavaScript basics

In this exercise we will practice programming with (JavaScript)[https://developer.mozilla.org/en-US/docs/Web/JavaScript], which we will need to create interactive visualizations in the browser. Today we will focus on processing data in JS, the functions used to plot the data are provided.
We would like to practice the functional style of programming, because it fits well with the d3.js library which we are going to use for the visualizations. This means that instead of for or while loops to iterate over arrays, we would use forEach, map, reduce and filter - if you haven’t seen them before, please take a look at the linked documentation which contains intuitive examples.
Please write your solutions in exercise/exercise.js and open the site exercise/index.html to execute them. Reload the page to re-run your script after applying changes. Remember that you can use the developer tools to interactively call your functions or see the values of variables.
Functions and iteration
isEven
Let’s try a basic function isEven that will check if an integer is divisible by 2.
Then apply it over an array of integers to see the results:
    • Apply isEven to each element of [1, 2, 3, 4, 5] using map.
    • Choose even numbers from 1...5 by using filter.
multiply
Now, imagine, you do not know how many numbers in your array. In JS, there is a way to create functions with arbitrary number of parameters. This is also called Spread syntax. Let’s implement a function multiply that computes a product of all numbers specified as parameters. For example, multiply(1,2,3,4,5) should return 120.
Closures
In JS, functions are treated as objects. When a function is created, it has access to all currently visible variables - the newly created function and these variables form a closure. The details are in the documentation about closures.
divisibleBy
Let’s generalize the above example, and create the function divisibleBy which:
    • takes an argument divisor
    • returns a function f such that f(x) returns true when x is divisible by divisor
The arrow function syntax is a convenient way to construct functions.
Now we have an alternative solution to the previous task const isEvenNew = divisibleBy(2). Try filtering [0, 1, 2, 3, 4, 5, 6] by divisibleBy(3).
increment
Sometimes, in JS, you will have to deal with nested functions. Implement a function increment such that it can be called as increment(100)(2) with the first parameter as an initial value (0 as a default parameter) and second params as a step size (1 as a default parameter).
colorCycle
In plots, we often want to apply different colors, for example to distinguish between lines illustrating different quantities. When making a general mechanism, we can’t predict how many colored objects there are going to be, so we will make the colors repeat in a cycle.
Create a function colorCycle such that
    • it takes one argument: an array of colors,
    • the default value of the the color array is COLOR_CYCLE_DEFAULT
    • it returns a function which repeats the colors in the cycle, for example
cc = colorCycle(['red', 'green']); console.log([cc(), cc(), cc(), cc()]); // ['red', 'green', 'red', 'green']
Now create a cycle my_cc with your chosen colors and run showColorCycle(my_cc) to apply the colors to a demo plot on the website index.html.

Range
In the above task, we were writing the sequences [1, 2, ..., N] explicitly, now let’s automate it.
    • Create a function range(N) which constructs a range of integers [0, 1, …, N-1].
For an additional challenge, you can try using a functional approach, that is without a for or while loop.
We could try creating an empty array of size N: Array(N). However, the elements of the array are not created and forEach or map does nothing. Try it yourself: Array(10).forEach(console.log)
Array.from allows us to create an array from another sequence - for example convert a string to an array like we did with letter counting. Array.from(Array(10)) actually creates the elements and we could iterate over them (the elements are undefined).
Additionally, we can pass a function as the second argument - this function will be applied on every element of the newly created array. Array.from(seq, f) is equivalent to Array.from(seq).map(f) but more efficient because it does not create the intermediate array. Remember that the mapping function receives as arguments both the current element and the current index.
    • Let’s find the integers from 0 to 100 which are divisible by 13. Create a range [0, ..., 99] and filter it by divisibility by 13
    • Implement a function randomInRange(min_val=0, max_val=100) which returns a random float value between min_val and max_val.
    • Implement a function randomArray(N, min_val=0, max_val=100) which generates an array of N random values between min_val and max_val.
Counting
    • Create a function countOccurences(string) which counts the number of occurences of each letter in a string. For example countOccurences("hello") yields {'h': 1, 'e': 1, 'l': 2, 'o': 1 }. A string is not an array and it does not have the forEach or map methods, so to use them, convert a string to an array with Array.from, for example let a = Array.from('a string!');.
To store the counts, use an Object which is key-value container with strings as keys (in our case, the key will be the letter).
To perform functional-style iteration over Objects, use Object.keys, Object.values and Object.entries. Please remember that they have to be called Object.entries(obj) instead of obj.entries().
    • Create the function normalizeCounts which takes the character counts outputted by countOccurences, and calculates normalized counts - that is divided by the total sum. Please calculate the sum using reduce. For example: normalizeCounts({'h': 1, 'e': 1, 'l': 2, 'o': 1}) yields {'h': 0.2, 'e': 0.2, 'l': 0.4, 'o': 0.2}
    • Create countOccurencesNormalized - a function which given a string, first applies countOccurences and then normalizes the counts using normalizeCounts.
    • Visualize the results by calling setCharacterCountingFunction(countOccurencesNormalized); - look at index.html, now you should be able to count the distribution of characters in any text you input. You can pass a colorCycle with your colors as the second argument to color the bars.

Advanced exercise. Throwing balls (optional)
We will simulate a ball thrown at angle b with velocity v0. The initial velocity (vx, vy) is:

vx = v0cos(b)

vy = v0sin(b)
The position of the ball at time t is given by:

x(t)=vx * t

y(t)=vy * t + (a * t2 * 0.5)
where a is the acceleration caused by gravity, usually -9.81 m/s2.
Implement a function simulateBall(v0, angle, num_steps, dt, g) such that:
    • v0 is the magnitude of the initial velocity
    • ‘angle’ is the inclination angle in degrees, multiply by DEG_TO_RAD = Math.PI / 180. to get radians for the trigonometric functions,
    • num_steps is the number of steps of the simulation, the default value should be 256,
    • dt is the time that advances between steps, default value 0.05,
    • g is the acceleration, default value -9.81,
    • it returns an array of ball positions at each time step,
    • each position is given as a array [x, y],
Use the range function to create the array of time points, then map them to the [x, y] values given by the equations above.
    • We want to finish the plot when the ball hits the ground (y=0), so please filter the point array to remove points with y below 0.
    • Visualize the ball trajectories using plotBall (the 2nd optional argument is the line color):
const ball_cc = colorCycle(['hsl(160, 100%, 64%)', 'hsl(200, 100%, 64%)', 'hsl(240, 100%, 64%)', 'hsl(120, 100%, 64%)', 'hsl(80, 100%, 64%)']);
plotBall(simulateBall(40, 60), ball_cc());
plotBall(simulateBall(40, 30), ball_cc());
plotBall(simulateBall(40, 45), ball_cc());
    • Use randomArray to create 20 random angles between 0 deg and 90 deg, then plot the ball trajectories for each angle.