We Keep Coding

How to get the sum of the last index from multiple arrays in javascript

I know this is a very basic question, but I am not able to write the correct code for this. Let's say I have 3 arrays:
arr1 = [1,2,3,4]
arr2 = [5,6,7,8]
arr3 = [7,3,2,1]
I want the sum of the last index, i.e. 4+ 8 + 1 = 13
I am not able to write the correct for loop for the same.
EDIT :
My array is inside an object as shown below:
So, I will have more arrays inside the object. So how do I proceed with this?

let ree = {
arr1: [1, 2, 3, 4],
arr2: [5, 6, 7, 8],
arr3: [7, 3, 2, 1]
};
let total = 0;
for (var key in ree) {
if (Array.isArray(ree[key])) {
total += ree[key][ree[key].length - 1];
}
}
console.log(total);

try this short and easy solution
let arr1 = [1,2,3,4]
let arr2 = [5,6,7,8]
let arr3 = [7,3,2,1]
arrList = [arr1,arr2,arr3]
finalSum = arrList.reduce((acc, cur) => acc+[...cur].pop(), 0)
i created a copy of arrays in reduce as pop modifies original array.
if your array is inside object as key value pair you can get it as array like this
let yourObject = {1:arr1, 2:arr2, 3:arr3}
let arrList = Object.values(yourObject);
an array assigned to a key in an object can be accessed like this
let yourObject = {'somekey':[1,2,3]};
arrList = yourObject.somekey;
console.log(arrList);

You can do something like this using array.length property,
sum = arr1[arr1.length - 1] + arr2[arr2.length - 1] + arr3[arr3.length - 1];
If they are inside another array you can do something like this,
let parentArr = [[1,2,3,4],[5,6,7,8],[7,3,2,1]];
let sum = 0;
for(let i =0;i<parentArr.length; i++) {
sum = sum + parentArr[i][parentArr[i].length - 1];
}
console.log(sum);
If the arrays are inside an object you can do the following,
let parentObj = {'arr1': [1,2,3,4], 'arr2': [5,6,7,8], 'arr3': [7,3,2,1]};
let sum = 0;
let keys = Object.keys(parentObj);
for(let i =0;i<keys.length; i++) {
sum = sum + parentObj[keys[i]][parentObj[keys[i]].length - 1];
}
console.log(sum);

Basic thing to know before we continue :
how many arrays you have ?
can array be empty ?
if number of arrays is static i.e. 3 and they can't be empty then use sabir.alam's
else please define your requirements.

Shorter answer:
const parentArr = [[1,2,3,4],[5,6,7,8],[7,3,2,1]];
const answer = parentArr.map(e=>e[e.length-1]).reduce((x,y)=>x+y);
console.log(answer);

how to add 3 different number to the i, i+1 and i+2 of my array

I want to know how can I add 3 different number to the 3n, 3n+1 and 3n+2 indices. I mean for example I have following array :
var arr = [1,1,1,2,2,2,3,3,3]
and then I want add the (3n)th to 5 and then I want add (3n+1)th of an array to 2 and (3n+2) to 3,
I mean the final array I want to be like following result array:
var result = [6,3,4,7,4,5,8,5,6]
and I try to do it as following code:
// arr = [1,1,1,2,2,2,3,3,3]
let res = [];
for (let i = 0; i < arr.length; i++) {
res.push([arr[i*3] * 5,
arr[(i*3)+1] *2,
arr[(i*3)+2] *3])
}

This should do the trick:
var arr = [1,1,1,2,2,2,3,3,3],
add = [5,2,3], res=[];
// result = [6,3,4,7,4,5,8,5,6]
for (let i=0;i<arr.length;i+=add.length) add.forEach((v,j)=>res[i+j]=arr[i+j]+v);
console.log(JSON.stringify(res))
An alternative and even shorter solution (similar to #Robin's answer) would be:
var arr = [1,1,1,2,2,2,3,3,3],
add = [5,2,3], res=[];
res=arr.map((v,i)=>v+add[i%add.length]);
console.log(JSON.stringify(res))
( I noticed #Nina came up with a very similar answer ...)

You can simply use map, making use of the fact that its function argument takes the current index an optional second argument:
var arr = [1,1,1,2,2,2,3,3,3];
var result = arr.map((num, idx) => {
switch (idx % 3) {
case 0:
return num + 5;
case 1:
return num + 2;
case 2:
return num + 3;
}
});
console.log(result);

You could mapp the array directly by taking a closure over an index for the values array for adding.
var array = [1, 1, 1, 2, 2, 2, 3, 3, 3],
add = [5, 2, 3],
result = array.map((i => v => v + add[i++ % add.length])(0));
console.log(...result);

Create an array alternating between 2 different values

I am trying to create an array alternating between 2 different values with a predetermined length.
Example:
conts value1 = 1;
const value2 = 2;
cont length = 6;
//desired output
const array1 = [1, 2, 1, 2, 1, 2];

You can create the array using Array.from with the desired length and map it to have the desired values :
const value1 = 1;
const value2 = 2;
const length = 6;
const result = Array.from({ length }).map((e, ndx) => ndx % 2 ? value2 : value1);
console.log(result);

Try with:
var repeated = new Array(3).fill([1, 2]).flat();
Or more general:
function repeat(n, value){
return new Array(n).fill(value).flat();
}
result = repeat(3, [1, 2]);
Credits: https://stackoverflow.com/a/54935305/4628597

An easy beginner solution would be something like this:
function createArray(value1, value2, length){
var array = new Array()
for(var i=0; i<length;i++){
if(i%2 == 0){
array.push(value1);
}else{
array.push(value2);
}
return array;
}
}

You could take an array of values in the wanted order and use a closure over the index of the values array adjusted by taking the remainder with the length of the values array.
const values = [1, 2],
length = 6,
result = Array.from({ length }, (i => _ => values[i++ % values.length])(0));
console.log(result);

Is there any way to modify criteria of what values to allow in an Array?

I have an array of values. I want to make a second array based on the first one with stricter criteria. For example, what I want specifically is:
arrayOne[1,1,1,1,1,2,2,2,3,3,3,3,4,4,4,4,4,5,5,5]
How would I make it so in my new array, only the values that show up 5 times are a part of the array and only show up once. Example: arrayTwo [1,4]
I'm fairly new to JavaScript and have been given an opportunity to code a decision making system for one of my business courses instead of doing the final exam. Any help you can give would be much appreciated. Thank You.

You could use a hash table, which counts each found element and then use the count for filtering and get only the fifth element as result set in a single loop.
var array = [1, 1, 1, 1, 1, 2, 2, 2, 3, 3, 3, 3, 4, 4, 4, 4, 4, 5, 5, 5],
count = Object.create(null),
result = array.filter(v => (count[v] = (count[v] || 0) + 1) === 5);
console.log(result);

I commented the code with the steps I took:
const arrayOne = [1, 1, 1, 1, 1, 2, 2, 2, 3, 3, 3, 3, 4, 4, 4, 4, 4, 5, 5, 5];
function method(arr, minimum) {
//Create an object containing the amount each number occurs
const occurrences = arr.reduce((o, n) => {
//If number is already in the object add 1
if (o[n]) o[n] = o[n] + 1;
//Else set its occurence to 1
else o[n] = 1;
//Return the object for the next iteration
return o;
}, {});
//Deduplicate the array be creating a Set(every elements can only occur once) and spread it back into an array
const deduplicate = [...new Set(arr)];
//Filter array to only contain elements which have the minimum of occurences
const filtered = deduplicate.filter(n => occurrences[n] >= minimum);
return filtered;
}
console.log(method(arrayOne, 5));

You can use a Map for this.
let arrayOne = [1, 1, 1, 1, 1, 2, 2, 2, 3, 3, 3, 3, 4, 4, 4, 4, 4, 5, 5, 5];
let counterMap = new Map();
arrayOne.forEach(value => {
let valueStr = value.toString();
counterMap.set(valueStr, counterMap.has(valueStr) ? counterMap.get(valueStr) + 1 : 1);
});
let arrayTwo = [];
counterMap.forEach((value, key, map) => {
if(value >= 5) {
arrayTwo.push(key);
}
});
console.log(arrayTwo);

Not the most elegant answer, but I assume you're looking just to find all values that appear at least 5 times.
const arrayOne = [1,1,1,1,1,2,2,2,3,3,3,3,4,4,4,4,4,5,5,5]
const arrayTwo = Object.entries(arrayOne.reduce((obj, num) => {
if(!obj[num]){
obj[num] = 1
} else {
obj[num] = obj[num] + 1
}
return obj
}, {})).filter(([key, value]) => {
return value >= 5
}).map((item) => {
return parseInt(item[0])
})
console.log(arrayTwo)

const a = [1,1,1,1,1,2,2,2,3,3,3,3,4,4,4,4,4,5,5,5,5,5,5,5,5,5];
Define a function that will take an array and numOfOccurrences
const filterByOcur = (arr, numOfOccurrences) => {
// create an object to act as a counter this will let us
// iterate over the array only once
const counter = {};
const res = new Set();
for (let num of arr) {
// if it's the first time we see the num set counter.num to 1;
if (!counter[num]) counter[num] = 1;
// if counter.num is greater or equal to numOfOccurrences
// and we don't have the num in the set add it to the set
else if (++counter[num] >= numOfOccurrences && !res.has(num)) res.add(num);
}
// spread the Set into an array
return [...res];
};
console.log(
filterByOcur(a, 5)
);

There is number of ways of doing this, I will try to explain this step by step:
Array declaration
const a = [1,1,1,1,1,2,2,2,3,3,3,3,4,4,4,4,4,5,5,5]
Method to count elements in an array, we are using reducer function that as a first argument takes object where key is our value from array and has a incremental number as a value. Remeber to start reducer with empty object
const counted = a.reduce((counter, value) => {
if (counter[value]) counter[value]++
else counter[value] = 1
return counter
}, {})
Make your array unique with Set constructor
const uniq = Array.from(new Set(a))
Fire filter functions on the uniq array with a help of counted array, look how we access it:
const onlyOne = uniq.filter(val => counted[val] === 1)
const onlyFive = uniq.filter(val => counted[val] === 5)
Merge all filtered arrays into one
const final = [].concat(onlyOne, onlyFive)

Splicing string values using functions and loops and storing as an array

I have a list of numbers that is a string value using a loop I want to split this string into different variables in an array, the first of length 3 and the 6 of length 7 and the last of length 3. How can this be done using functions and loops.

We could do something like this:
let str = '000111111122222223333333444444455555556666666mmmm';
// Defines the lengths we're using
let lengths = [3,7,7,7,7,7,7,3];
let index = 0;
let result = lengths.reduce((acc,n) => {
acc.push(str.slice(index, index += n));
return acc;
} , [])
console.log(result);

You could map the sub strings.
var str = '000111111122222223333333444444455555556666666mmmm',
lengths = [3, 7, 7, 7, 7, 7, 7, 3],
result = lengths.map((i => l => str.slice(i, i += l))(0));
console.log(result);

Here's one way to do that:
let theArray = document.getElementById('theArray');
let theVariable = document.getElementById('theVariable');
let targetString = "122333444455555666666";
let dataSizes = [1, 2, 3, 4, 5, 6];
var result = [];
var pos = 0;
dataSizes.forEach( (size) => {
result.push(targetString.substr(pos, size));
pos += size;
});
theArray.textContent = result.toString();
let [one, two, three, four, five, six] = result;
theVariables.textContent = `${one}-${two}-${three}-${four}-${five}-${six}`;

a generic way of doing this will be, if you want in a variable you can use subStringLengthMaps key, :-
let str="abcdefghijklmnopqrstu";
let subStringLengthMap={a:3, b:7, c:7 , d:3};
//making pure funciton
var getStrings = function(str, subStringLengthMap){
let result =[];
Object.keys(subStringLengthMap).forEach(function(key){
let temp = str.slice(0, subStringLengthMap[key]);
result.push(temp);
str = str.replace(temp,'');
})
return result;
}
//call the function
console.log(getStrings(str, subStringLengthMap))