5. Basic Data Structures #

Created Thursday 09 July 2020

Array #

Array is dynamic.
Syntax

let list = [1, "two" , [3,4] ]; // mixing and nesting is OK

Arrays are objects, and are assigned as references.
Array may be pooled. However, this doesn’t affect the code.
Array API

Action	Code	Return Value	Comment
Access Element	A[0], or A.at(0) (supports negative indexes)	element	Same as C++
Get length	A.length	length(number)	This property is readable/writable: it can change array size.
Check if element in array	`A.includes(x)`	Boolean
Append	A.push(newVal)	new length
Remove tail	A.pop()	removed element	returns removed element. Error if array is empty.
splice(changes memory)	A.splice(goal_index, #deletions, val1, val2…)	deleted array	Delete #deletion elements and add val1, val2 etc at goal index(inclusive).
Remove at index	A.splice(destination_index, 1)	element	Removes ith element
Remove head	A.splice(0, 1)	element
Resize Array	A.length = new_length;	length(number)
slice(extract subarray)	A.slice(start, length=A.length)	extracted array	No change in array, just like substr in C++
Clear(current reference only)	A = []	void	No change at memory location.
Deep copy	`structuredClone(A)`	array	Shipped to browsers 2022
Fake deep copy (change shell)	B = […A]		New memory allocated, same behavior for array literal rvalue.
reverse array	A.reverse()		Inplace and reference as return value.
Expand/Decrease array size	A.length = new_length		Change is in-place.
Linear Search	A.indexOf(key)	number (integer)	Returns -1 if key not found. Returns leftmost key index (if duplicates).
Reverse Linear Search	A.lastIndexOf(key)	Extended ops
sort	A.sort(criteriaFunction=LexCompare)	sorted array	function (a, b) { return numericDifference}; Return negative (swap yes), 0 or positive (no swap).
join array of strings	A.join(sep=’’)	string

Array can have empty values. Workaround: Array(10).fill(null)
The spread operator is indispensible. It is used almost everywhere.

Object #

Objects - It is a set of key-value pairs.
Object is a keyword.
Syntax:

var user = {
  name: "John",
  age:34,
  hobby: "Socccer",
  isMarried: false, // , does not matter
  shout: function(){ console.log(this.name);}, // method, can use 'this'.
  };

  user.favoriteFood = "spinach"; // add a member to an object
  user['favoriteFood'] = "spinach"; // same as above

undefined - unitialized value, or accessed non-existent key in object. Example:
```
const x = { name: 'Alice' }; // x['Bob']
```
null - Represents the state of being empty(consider like a null pointer). null and {}(empty object) are different.
Properties can be accessed by objectA.property_name or objectA[property_name].
Check key membership - x.hasOwnProperty('fieldName')
this is not accessible if the method is run after isolation.
Objects are not index sub-scriptable. Unless the key used is deliberately a number.

String #

string is a primitive type.
Strings are dynamically sized, immutable in JS.
String API - no in-place ops.

Action	Code	Return Value	Comment
Access Element	s.charAt(index)	string	[] works too. But fails for edge cases - ‘hello’[true] is an error
string exists within	`s.includes(x)`	Boolean
Get length	s.length	length(number)	This property is readable/writable: it can change array size.
Concatenation	a+b	string	Works with non-string
Char --> ASCII number	s.charCodeAt(index)	number
ASCII --> character	String.fromCharCode(65)	string	Need to use String class
Substring	s.substr(start, len=A.length)	removed element	start, length based extraction
Slice	s.slice(start, end=end)	extracted array	[start, end) based extraction
UpperCase	s.toLocaleUpperCase(lang=‘en’)		same as toUpperCase() for EN. Similar syntax for lowercase.
Reverse string	s.split(`).reverse().join(`)		Inplace and reference as return value.
Expand/Decrease array size	A.length = new_length		Change is in-place.

		Extended ops
Prefix membership	s.startsWith(key_string)	boolean	Same as Python3
Suffic membership	s.endsWith	boolean	Same as Python3
Split	s.spit(delim=undefined)	array of strings	No splitting without delimiter
Trim whitespace	s.trim()	array	No arguments. trimLeft, trimRight are available.

Resources: #

TIme Complexity of popular ops