----

endif::html_target[]

(((minimum (exercise))))If you have trouble putting braces and

parentheses in the right place to get a valid function definition,

(((return keyword)))A function may contain multiple `return`

statements.

=== Recursion ===

----

endif::html_target[]

(((isEven (exercise))))(((if keyword,chaining)))(((recursion)))Your

function will likely look somewhat similar to the inner `find`

thus getting further and further away from returning a result. It will

eventually run out of stack space and abort.

=== Bean counting ===

----

endif::html_target[]

(((bean counting (exercise))))(((length property,for

strings)))(((counter variable)))A ((loop)) in your function will have

(((local variable)))Take care to make all the variables used in the

function _local_ to the function by using the `var` keyword.

endif::tex_target[]

number of measurements where the first measurement (pizza) is false

(0) and the second measurement (squirrelness) is true (1). In this

sum of all measurements where the first variable is true, which is 10

refers to the sum of the measurements where the squirrel variable is false.

(((correlation)))(((phi coefficient)))So for the pizza table, the part

above the division line (the dividend) would be 1×76 - 9×4 = 40, and

the part below it (the divisor) would be the square root of

comes out to _ϕ_ ≈ 0.069, which is tiny. Eating ((pizza)) does not

appear to have influence on the transformations.

translation of the _ϕ_ formula into JavaScript. `Math.sqrt` is the

square root function, as provided by the `Math` object in a standard

JavaScript environment. We have to sum two fields from the table to

the sums of rows or columns are not stored directly in our data

structure.

(((JOURNAL data set)))Jacques kept his journal for three months. The

resulting ((data set)) is available in the coding sandbox for this

where it is stored in the `JOURNAL` variable, and in a downloadable

http://eloquentjavascript.net/code/jacques_journal.js[file].

----

endif::html_target[]

(((summing (exercise))))(((array,creation)))(((square

brackets)))Building up an array is most easily done by first

`range(5, 2)` returns something meaningful, rather than getting stuck

in an ((infinite loop)).

=== Reversing an array ===

----

endif::html_target[]

(((reversing (exercise))))There are two obvious ways to implement

`reverseArray`. The first is to simply go over the input array from

mirror image, and then put the value from the local variable in the

place where the mirror image used to be.

=== A list ===

----

endif::html_target[]

(((list (exercise))))(((linked list)))Building up a list is best done

back to front. So `arrayToList` could iterate over the array backward

head node. To get element _N_ + 1, you take the __N__th element of the

list that's in this list's `rest` property.

[[exercise_deep_compare]]

=== Deep comparison ===

----

endif::html_target[]

(((deep comparison (exercise))))(((typeof operator)))(((object)))(((=== operator)))Your test for whether you are dealing with a

real object will look something like `typeof x == "object" && x !=

best done by immediately returning false when a mismatch is noticed

and returning true at the end of the function.

(((ANCESTRY_FILE data set)))The variable `ANCESTRY_FILE`, available in

the ((sandbox)) for this chapter as well as in

http://eloquentjavascript.net/code/ancestry.js[a downloadable file] on

content of my ((JSON)) file as a string. Let's decode it and see how

many people it contains:

----

endif::html_target[]

(((age difference (exercise))))(((filter method)))(((map

method)))(((null)))(((average function)))Because not all elements in

or `null` if no mother is known. Then, you can call `filter` to remove

the `null` elements before passing the array to `average`.

=== Historical life expectancy ===

----

endif::html_target[]

(((life expectancy (exercise))))The essence of this example lies in

((grouping)) the elements of a collection by some aspect of

(((for/in loop)))(((average function)))Finally, a `for`/`in` loop can

be used to print the average ages for the individual centuries.

(((grouping)))(((map)))(((object,as map)))(((groupBy function)))For

bonus points, write a function `groupBy` that abstracts the grouping

----

endif::html_target[]

(((every and some (exercise))))(((short-circuit evaluation)))(((return

keyword)))The functions can follow a similar pattern to the

the function also returns the correct value when it reaches the end of

the array.

mountains that we are trying to build is available in the `MOUNTAINS`

variable in the ((sandbox)) and also

http://eloquentjavascript.net/code/mountains.js[downloadable] from the

(((table example)))We will want to highlight the top row, which

contains the column names, by underlining the cells with a series of

----

endif::html_target[]

(((vector (exercise))))Your solution can follow the pattern of the

`Rabbit` constructor from this chapter quite closely.

compute the distance from (0, 0) to (x, y), you can use the

Pythagorean theorem, which says that the square of the distance we are

looking for is equal to the square of the x coordinate plus the square

is the number you want, and `Math.sqrt` is the way you compute a square

root in JavaScript.

=== Another cell ===

endif::html_target[]

(((StretchCell (exercise))))You'll have to store all three constructor

arguments in the instance object. The `minWidth` and `minHeight`

Don't forget to add a `draw` method that simply forwards the call to

the inner cell.

=== Sequence interface ===

endif::html_target[]

(((sequence (exercise))))(((collection)))One way to solve this is to

give the sequence objects _((state))_, meaning their properties are

JavaScript because it involves creating a lot of objects during

iteration.

----

(((animation)))(((simulation)))Let's see what happens if we run this.

ifdef::html_target[]

endif::html_target[]

(((artificial stupidity (exercise))))(((artificial

intelligence)))(((behavior)))(((state)))The greediness problem can be

munching on a sea of plants and never reproducing, that makes for a

very stable ecosystem. But no one wants to watch that.

=== Predators ===

endif::html_target[]

(((predators (exercise))))(((reproduction)))(((starvation)))Many of

the same tricks that worked for the previous exercise also apply here.

some ((data structure)) kept on the predator objects, and have it base

its ((behavior)) on what it has seen recently.