Don't LeetCode Yet - 6 - DIY Built-In Functions

Understand and try to build the built-in functions/methods by our selves.

Built-in Array/String Methods

Array.map()

“The map() method creates a new array populated with the results of calling a provided function on every element in the calling array.”

Need some time for NOOBS to get used to the usage of CALLBACK functions.

The .map() can create a new array.
example:

let arr = [1,2,3,4,5]

function map(arr, callback){
    let outputArr = []
    for (let i=0; i<arr.length; i++){
        outputArr.push(callback(arr[i]))
    }
    return outputArr
}

function double(n){
    return n*2
}

console.log(map(arr, double))
// output : [2, 4, 6, 8, 10]

This is the same as:

arr.map(double)

String.repeat

REPEAT a string!

let str = "ABCDE"

function repeat(str, times){
    let outputStr = ""
    for (let i=0; i<times; i++){
        outputStr += str
    }
    return outputStr
}

console.log(repeat(str, 3))
// output : "ABCDEABCDEABCDE"

Same as:

"ABCDE".repeat(3)

Array.lastIndexOf

Related to .indexOf method.

console.log([1,2,3,1].indexOf(4))
// -1
console.log([1,2,3,1].indexOf(2))
// 1
console.log([1,2,3,1].indexOf(1))
// 0 (it returns only the first found)

So the .lastIndexOf finds the element starting from the last one.

let arr = [1,2,3,1]

function lIF(arr, num){
    let indexNum = -1
    for (let i=arr.length-1; i>0; i--){
        if(arr[i] === num){
            indexNum = i
            break
        }
    }
    return indexNum
}

console.log(lIF(arr, 1))
// 3 (Returns the last one in the array.)

Same as:

console.log(arr.lastIndexOf(1)

Project Unit 6

LIOJ 1036 Array.reverse

function reverse(arr)
    let revArr = []
    for (let i = arr.length-1; i > 0; i--){
        revArr.push(arr[i])
    }
    return r evArr
}
console.log(reverse([1,2,3,4,5])
// [5,4,3,2,1]

LIOJ 1037 Array.filter

I accidentally used the built-in .filter method when I try to solve the problem the first time.

let newArr = orgArr.filter(findthenum)

This is actually a good practice to get familiar with the basic of how to use the callback function.

• Main function: myFilter(arr, findTarget), no need to pass in any argument when calling the callback(findTarget in this case)
• Function used as callback in the main function: findTarget
• Check if callback function returns anything, if yes push it into the new array.

Some improvements:

• return n != target is the same as if (n != target){ return n}

The instructor talked about the “arrow function” in ES6. And how to use the filter method.
An example of the usage of .filter() is arr.filter(x => x > 2).
The x => x > 2 is an arrow function.
So the findTarget() function can also be written as:
let findTarget = n => n !== target
Or we can also directly put it in the myFilter() function: function myFilter(arr, n => n !== target)

LIOJ 1038 Array.indexOf

EASY! If you can solve problem 1037. Don’t even need a callback function, can directly give the target as argument.

LIOJ 1039, 1040, 1042

Array.fill, Array.join, String.toLowerCase

[1,2,3,4].join(“/“) => “1/2/3/4”

LIOJ 1041 Array.trim

I tried to count the white space before the words and after the words. Then make a new string out of those index number.

for (i = preCounter; i < orgString.length-postCounter; i++){
    newString += orgString[i]
}

Instructor trim the white space before the words first then trim the white space after the words. Test if we MEET a letter!

LIOJ 1043 String.endsWith

Instructor’s solution:

......
console.log(endsWith(lines[0], lines[1]) ? "true" : "false")

let strInd = str.length-1
let sStrInd = searchString.length-1
while (sStrInd >= 0){
    .........
    strIndex --
    sStrInd --
    ......... 
}

LIOJ 1044, 1045

String.padEnd, String.slice

• Algorithm
• JavaScript
• Lidemy