Data Structure Study in JS (Array)

• Keywords

  simplest memory DS, same data type, modified objects

Creating and Initializing

1) use new keyword

var testArray = new Array();

2) []

var testArray = []; //declare
var testArray2 = ['test1', 'test2', 'test3'] //initialize

Adding and removing elements

1) push(): add to the end

2) unshift(): add to the front
// add code

3) pop(): remove from the back

4) shift(): remove from the front
//manually code

5) splice(): remove in between / add in between

// remove
array.splice(1,2);
// add
array.splice(2,0,1,2,3);

*note:
push & pop is an emulation of stack. shift & unshift is an emulation of queue

Two-dimensional and multi-dimensional arrays

• We might want to use matrix (Two-dimensional array) in some cases.
• JS does not support matrices.
• Instead, we use arrays of arrays.

2 x 2

// everyday meal
// 0 ~ 4 --> mon ~ fri
dailyMeal[0] = [];
dailyMeal[0][0] = 'milk and bread';
dailyMeal[0][1] = '김치찌개';
dailyMeal[0][1] = 'fried chicken';
dailyMeal[1] = [];
dailyMeal[1][0] = 'yogurt';
dailyMeal[1][1] = '제육볶음';
dailyMeal[1][2] = 'roasted fish';
...

// function to print this matrix
function printMeal(dailyMeal){
    for(var i = 0 ; i < dailyMeal.length < i++) {
        for(var j = 0 ; j < dailyMeal[i].length < j++) {
            console.log(dailyMeal[i][j]);
        }
    }
}

3 x 3

// function to create 3x3 matrix
var matrix = [];
for (var i = 0 ; i < 3 ; i++){
    matrix[i] = [];
    for (var j = 0 ; j < 3 ; j++){
        matrix[i][j] = [];
        for (var z = 0 ; z < 3 ; z++){
            matrix[i][j][z] = i+j+z;
        }
    }
}

console.log(matrix);
// 0,1,2    1,2,3   2,3,4
// 1,2,3    2,3,4   3,4,5
// 2,3,4    3,4,5   4,5,6

Joining multiple arrays

1) concat() : allows joining multiple arrays and objects into one aray.

*note : works regardless types

var test = 'test';
var testArray1 = [1,3,5,7];
var testArray2 = [2,4,6,8];
testArray1.concat(test,testArray2);
>> [1, 3, 5, 7, "test", 2, 4, 6, 8]

Iterator functions

//sample function
var isEight = function(x){
	console.log(x);
	return x == 8;
}

1) every() : iterates each element, passes it to a function, stops when result is false.

var numberArray = [1,2,3,4,5,6,7,8,9,10];
numberArray.some(isEight);
>>1
>>false

2) some() : iterates each element, passes it to a function, stops when result is true.

numberArray.some(isEight);
>>1
>>2
>>...
>>8
>>true

3) map() : Returns an array that stores result of every function run.

numberArray.map(isEight);
>> [false, false, false, false, false, false, false, true, false, false];

4) filter() : returns an array with elements of true value.

var lotsOfEight = [1,8,3,4,8,8,3,4,8];
>> [8, 8, 8, 8]

5) reduce(function(prevValue, currentValue, currentIndex, arr), initialValue) : returns a result that has been ran continuously with given function through each element.
*note: prevValue == 전 회차에서 반환된 값.
See here for more detail.

numberArray.reduce(function(prevValue, currentValue, currentIndex, arr), initialValue) {
	return prevValue + current;
});
>> 55;

Searching and Sorting

1) reverse() : literally reverses the order of elements.

2) sort() : sorts elements of an array. This sort is lexicographic, which means by string, not by numbers.

If we want our sort() to sort numerically, we need to pass another function to the sort() function. This is called compareFunction.

array.sort(compareFunction)

To sort with ascending order:

numberArray.sort(function(a,b){
  return a-b;
});
>> [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]

To sort with descending order:

numberArray.sort(function(a,b){
  return b-a;
});
>> [10, 9, 8, 7, 6, 5, 4, 3, 2, 1]

• Custom sorting

We can use compareFunction to compare any type of elements as we define.

var petAge = [
  {name: '치즈', weight : 2},
  {name: '봉구', weight : 1.8},
  {name: '루루', weight : 2.2},
];

function compareWeight(a,b) {
  return a.weight - b.weight;
}

console.log(petAge.sort(compareWeight));
>>봉구 - 치즈 - 루루

• Sorting string

sort() will sort strings according to ASCII value.

But sometimes, you would want them to be ordered with different ordering rule.

// ignore cases
function ignoreCase(a,b){
  a.toLowerCase() - b.toLowerCase;
}

• Searching

1) indexOf() : first match

2) lastIndexOf() : last match

Outputting the array into a string

1) toString() : convert array to string, with , separator.

2) join() : do the same thing with toString(), but with given separator.