How to replace array elements with another given array elements? - javascript

I want to make a function that completely override the original array.
function putDatainthis(arr) {
//....some ways to get data
arr = [data,data,data...]; //this just reassigns arr = to new array original that was passed as reference hasn't been modified.
}
//so right now only way i can think of is this:
function putDatainthis(arr) {
var data = [3,4,6,2,6,1];
arr.length=0;
data.forEach(function(e){
arr.push(e);
});
}
but i want to know can it be improved or is there more native way.

The Array.prototype.splice() function is what you are looking for, this is your way to go:
function putDataInThis(arr) {
var data = [3, 4, 6, 2, 6, 1];
arr.length = 0;
arr.splice(0, arr.length, data);
return arr;
}
alert(putDataInThis(["h", "g"]).join(","));
Explantation:
In the following arr.splice(0, arr.length, data) statement, we used splice() function with the following parameters:
0 the beginning index for the replacement.
arr.length as the number of elements to replace.
And data is the list of new values to put in our array.
Read Array.splice( ): insert, remove, or replace array elements for further information.

I think you could use jquery merge.
function putDatainthis(arr) {
var data = [3,4,6,2,6,1];
$.merge(arr, data);
}

Try this way.
function putDatainthis(arr) {
var data = [3,4,6,2,6,1];
var concat_array = data.concat(arr);
}

Related

How to achieve map functionality with the use of filter in JavaScript, as we can use reduce as map and filter?

Suppose I have an array var arr = [1,2,3] and if I do var result = arr.filter(callback) I want value of result would be [2,4,6] by the use of filter. I want to only define callback function in order to do so. It can be easily done with map but I want to use only filter.
Array.prototype.filter only filters existing values in an array, effectively creating a new array that can hold the same values, their subset, or an empty array, based on the filtering function.
You could do it using filter():
const arr = [1, 2, 3];
arr.filter((c, i) => {
arr[i] = +arr[i] * 2;
return true;
});
console.log(arr)
we are always returning true, so filter() makes no sense in this case.
As stated many times, there is no reason why you should do it.
It is impossible to do it like map because map returns a new array. You can either alter on the original array or you have to make a clone of the original array so you do not change it.
// this modifies the orginal array
var arr1 = [1,2,3]
arr1.filter((v,index,arr)=>{
arr[index] = v * 2;
return true
})
console.log(arr1)
// or you can clone it and do the same thing
// this modifies the cloned array
var arr2 = [1,2,3]
var arr3 = arr2.slice()
arr3.filter((v,index,arr)=>{
arr[index] = v * 2;
return true
})
console.log(arr2, arr3)
So no, you can not recreate map with filter since you HAVE to modify the original array or cheat and use a copy of the array.
So I'm not sure I understand the second part of your question, but as for the first part:
The callback for filter has three arguments, two of which are optional.
The first argument is the current element in the traversal, and the second and third arguments (the optional ones) are the 0-based index of the current element, and a reference to the original array.
This third parameter is useful for what you're trying to do.
let myArr = [1, 2, 3];
myArr.filter((el, ind, orig) => {
orig[ind] = orig[ind] + 1; // Or, whatever map logic you want to use
return true; // since a Boolean condition must be returned here
});
This way you can do it without even even having to break the scope of the function!
If you want to do it without necessarily having a variable to originally call filter on (you do have to pass an array), you can use the prototype and the call method:
Array.prototype.filter.call([1, 2, 3], (el, ind, orig) => {
orig[ind] = orig[ind] + 1;
return true;
});

Javascript array.filter() element being skipped

I am trying to delete out the common elements in arguments given and
create a final array with unique elements.
In the below example my final array is giving me [1,3,4,5] instead of [1,4,5].
I did the debug and the one of element (3) is not going through the loop.
After element (2) it's skipping to element (4). I am confused why is this happening? Anything wrong with my code? Any help appreciated.
Thank you
function arrayBreaker(arr) {
let test = [];
test.push(arguments[1]);
test.push(arguments[2]);
let target = arguments[0];
target.filter(val => {
if (test.indexOf(val) > -1) {
target.splice(target.indexOf(val), 1);
}
});
return target;
}
console.log(arrayBreaker([1,2,3,4,5], 2, 3));
You're using .filter() like a general purpose iterator, and mutating the array you're iterating, which causes problems because the iterator is unaware of the items you're removing.
If you need to mutate the original array, instead return the result of the .indexOf() operation from the .filter() callback, or better yet, use .includes() instead, and then copy the result into the original after first clearing it.
function arrayBreaker(target, ...test) {
const res = target.filter(val => !test.includes(val));
target.length = 0;
target.push(...res);
return target;
}
console.log(arrayBreaker([1, 2, 3, 4, 5], 2, 3));
If you don't need to mutate, but can return a copy, then it's simpler.
function arrayBreaker(target, ...test) {
return target.filter(val => !test.includes(val));
}
console.log(arrayBreaker([1, 2, 3, 4, 5], 2, 3));
Array is returning a new array based on filter criteria, in this case you are checking index of test. In the callback function is expecting a true if you want to keep the value, otherwise return false. In your case you shouldn't slice the target inside filter callback, but rather:
function arrayBreaker(arr) {
let test = [];
test.push(arguments[1]);
test.push(arguments[2]);
let target = arguments[0];
let newArray = target.filter(val => {
if (test.indexOf(val) > -1) {
// filter value out
return false;
}
// keep the val inside new array
return true;
});
return newArray;
}
console.log(arrayBreaker([1,2,3,4,5], 2, 3));
for more information about array filter, check documentation here:
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/filter
Depending on your use case there are few things that I think would be helpful, first I would declare the function parameters so it's clear that arrayBreaker is expecting multiple arguments. Secondly I would expect second param as an array since we are expecting multiple values. Something like below:
function arrayBreaker(arr, filterValues) {
return arr.filter(val => {
if (filterValues.indexOf(val) > -1) return false;
return true;
})
}
console.log(arrayBreaker([1,2,3,4,5], [2, 3]));

Javascript - Do something when an element in two arrays are the same?

I found a solution to where I get returned an array of elements without duplicates:
Array1 = Array1.filter(function(val) {
return Array2.indexOf(val) == -1;
});
However, I want to modify this code just a little bit. Instead of being returned an array without duplicates, I want to do something when there is a duplicate. The problem is, I'm not sure how exactly this code works. The thing is I'm not sure how val gets set, or what it even is.
for (var i = 0; i < json.length; i++) {
var item = json[i];
// if json.indexOf(val?), do something
}
Read the docs for the Array filter method then. The val parameter of the callback will be passed the single array items, i.e. json[i] or item in your case:
for (var i = 0; i < json.length; i++) {
var item = json[i];
if (json.indexOf(item) >= 0) {
// do something
}
}
var newArray = array1.filter(function(v, i) {
return array1.indexOf(v) == i;
});
This will return only unique itesm from array1;
array1.filter(function(v, i) {
// write your code here ('v' is individual value and 'i' is its index)
// don't return any anything if you don't want unique array to be returned.
// 'array1.indexOf(v) == i' checks if current value is duplicate from previous any values.
// try putting console.log on values you don't understand like (console.log(v,i) for values of 'v' and 'i')
return array1.indexOf(v) == i;
});
and off-curse you can loop an array with for loop as
for(i in array1){
// where i is index of array1, to get current value use array1[i]
if(array2.indexOf(array1[i]) >= 0){
// do something
}
console.log(i);
}
val is set by Array.prototype.filter, which calls the callback function on each element in the array. Since you don't want to filter you can use Array.prototype.forEach instead, which also calls the callback function once for each element in the array:
Array1.forEach(
// This function is called once per element in Array1
function(val){
if(Array2.indexOf(val) != -1){ // Check if that element is also in Array2
// `val` is in both arrays,
// Do something with it
}
}
);
You can utilize some modern libraries... like underscorejs.
Intersection is what you're looking for i guess: http://underscorejs.org/#intersection
_.intersection([1, 2, 3], [101, 2, 1, 10], [2, 1]);
=> [1, 2]
So your code may be something like
if(_.insersection(arr1, arr2)){
//since [] array is Falsy in JS this will work as a charm
}
From MDN: indexOf
Returns the first index at which a given element can be found in the array, or -1 if it is not present.
From MDN: filter
Creates a new array with all elements that pass the test implemented by the provided function.
The first function works by returning true when an item from array1 isn't found in array2 (== -1). i.e.: Iterate through A and add anything not found in B.
So, to change to return only duplicates return true for anything that is found in both:
Array1 = Array1.filter(function(val) {
return Array2.indexOf(val) >= 0;
});
Array1 now contains only items with duplicates.

What is the concept of Array.map?

I am having problems understanding the concept of Array.map. I did go to Mozilla and Tutorials Point, but they provided very limited info regarding this.
This is how I am using Array.map. It is a little complex (a bit of d3.js involved; just ignore it)
var mapCell = function (row) {
return columns.map(function(column) {
return { column : column, value : getColumnCell(row, column) }
})
}
//getColumnCell is a function defined in my code
//columns is array defined at the top of my code
I do not understand exactly what this code is doing. I know its returning a new array and stuff but this part is a little tricky!
If you want to go through my code: http://jsfiddle.net/ddfsb/2/
I am using console to actually understand what's happening inside the code. Looking at the answers provided, I have clearly understood the concept of array.map. Now the only part remaining is parameters rows and columns, but there is a difference between row and rows, and column and columns in the fiddle provided
var rows//completely ok
var columns//completely ok
funcion(row)//here,source of row is unknown.getColumncell function utilizes this parameter further making it more critical
function(column)//source of column is unknown..getColumncell function utilizes this parameter further making it more critical
Let's rewrite it a bit, and start working from inside out.
var mapCell = function (row) {
return columns.map(
function(column) {
return {
column : column,
value : getColumnCell(row, column)
}
}
)
}
The function(column) part is essentially a function that takes a column as a parameter, and returns a new object with two properties:
column, that is the original value of the parameter, and
value, that is the result of calling the getColumnCell function on the row (external variable) and column (parameter)
The columns.map() part calls the Array.map function, that takes an array and a function, and runs the function for every last item of it, and returns the results. i.e. if the input is the array [1, 2, 3, 4, 5] and the function is something like isEven, the result will be the array [false, true, false, true, false]. In your case, the input are the columns, and the output is a list of objects, each of which has a column and a value properties.
Lastly, the var mapCell = function (row) part declares that the variable mapCell will contain a function of one variable called row - and this is the same row that is used in the inner function.
In a single sentence, this line of code, declares a function that when run, will take a row and return values for all columns for that row.
map loops through your original array and calls the method for each value in the array. It collects the results of your function to create a new array with the results. You are "mapping" the array of values into a new array of mapped values. Your code is equivalent to:
var mapCell = function (row) {
var result = [];
for (var i = 0; i < columns.length; ++i) {
var mappedValue = {
column: columns[i],
value : getColumnCell(row, columns[i])
};
result.push(mappedValue);
}
return result;
};
Understanding the map function is only part of the solution here, there is also the function mapCell. It takes one parameter row and it returns something like:
[ {
"column": "parties",
"value": [cell value]
}, {
"column": "star-speak",
"value": [cell value]
} ]
Where the cell value depends on the row and the column (parties, stars-speak etc.)
A map function applies a transformation to a value, and returns that transformed value.
A simple example:
function square(x) { return x * x; }
[ 2, 3, 4 ].map(square); // gives: [ 4, 9, 16 ]
Similarly:
[ "parties", "starspeak" ].map(function (column) {
return {
column: column,
value: findTheValue(column)
}
});
Now since that map is nested with a function that gets a row parameter. You can use it in the map function, to get:
function (row) {
return [ "parties", "starspeak" ].map(function (column) {
return {
column: column,
value: findTheValue(row, column)
}
});
}
And this gets pretty close to your code.
Map function goes through each element of an array in ascending order and invokes function f on all of them.
It returns new array which is being computed after function is invoked on it.
Syntax:
array.map(f)
Example:
<!doctype html>
<html>
<head>
<script>
var arr = [4,5,6];
document.write(arr.map(function(x){return x*2;}));
</script>
</head>
</html>
Answer: 8,10,12
Summary
Array.map is a function which is located on Array.prototype.map. The function does the following:
Creates a new array with the same amount of entries/elements.
Executes a callback function, this function receives and current array element as an argument and returns the entry for the new array.
Returns the newly created array.
Example:
Basic usage:
const array = [1, 2, 3, 4];
// receive each element of array then multiply it times two
// map returns a new array
const map = array.map(x => x * 2);
console.log(map);
The callback function also exposes an index and the original array:
const array = [1, 2, 3, 4];
// the callback function can also receive the index and the
// original array on which map was called upon
const map = array.map((x, index, array) => {
console.log(index);
console.log(array);
return x + index;
});
console.log(map);
Probably most people coming here (like me) just want a basic array.map usage example:
myArray = [1,2,3]
mappedArray = [];
mappedArray = myArray.map(function(currentValue){
return currentValue *= 2;
})
//myArray is still [1,2,3]
//mappedArray is now [2,4,6]
This is it at it's most basic. For additional parameers, check out: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/map
IF you have an array of elements and you have to perform the same operation on the
each element of the array that time you can use the javascript map function for array
it helps to iterate throw the array then we can perform the operation of each element and
return it.
let NumberArray = [1,2,3,4,5,6,7,8];
let UpdatedArray = NumberArray.map( (Num , index )=>{
return Num*10;
})
console.log(UpdatedArray);
//UpdatedArray ==> [10, 20, 30, 40, 50, 60, 70, 80]
Javascript map() Syntax
arrayObj.map(callback[,context]);
arrayObj is a original array on which map() is invoked.
The map() takes 2 named arguments, First is a callback function and the second is a context object. callback function gets triggered on every element of an array.
In addition, callback function takes 3 arguments:
function callback(currentElement,index,array){
}
currentElement – This is a current elements of array which is passed to callback function
index – Index of the current element
array – complete array on which map() is applied
Out of these 3 elements, currentElement parameter is mandatory while the rest 2 parameters are optional.
However, map() does not change the original array, it creates a new array element which is generated by callback function.
You can read more on JavaScript map function
Array map() method returns a new array.
It does not change the original array.
let array = arr.map((c, i, arr) => { //return element to new array });
here,
array is the new array that is returned.
arr is the original array on which the map method is called.
c is the current value that is being processed.
i is the index of current value.
For example:-
const originalArr = [4, 3, 2]; let newArr = originalArr.map((val) => val + val);
result:-
newArr: [8, 6, 4]
originalArr: [4, 3, 2]
in simple words you can perform operations on array using map
Examples
1.Array
let arr = ["sam","tom"]
console.log("Before map :",arr)
arr.map((d,i)=>{
arr[i] = d+"yy"
})
console.log("After map :",arr)
Examples
2.Array Of Objects
// console.log(Date.now());
let arr = [
{name:"sam",roll_no:10},
{name:"tom",roll_no:12}
]
console.log("Before map :",arr)
arr.map(d=>{
if(d.name == "sam")
{
d.name = "sammy",
d.roll_no=100
}
})
console.log("After map :",arr)

Get first element of a sparse JavaScript array

I have an array of objects in javascript. I use jquery.
How do i get the first element in the array? I cant use the array index - as I assign each elements index when I am adding the objects to the array. So the indexes arent 0, 1, 2 etc.
Just need to get the first element of the array?
If you don't use sequentially numbered elements, you'll have to loop through until you hit the first one:
var firstIndex = 0;
while (firstIndex < myarray.length && myarray[firstIndex] === undefined) {
firstIndex++;
}
if (firstIndex < myarray.length) {
var firstElement = myarray[firstIndex];
} else {
// no elements.
}
or some equivalently silly construction. This gets you the first item's index, which you might or might not care about it.
If this is something you need to do often, you should keep a lookaside reference to the current first valid index, so this becomes an O(1) operation instead of O(n) every time. If you're frequently needing to iterate through a truly sparse array, consider another data structure, like keeping an object alongside it that back-maps ordinal results to indexes, or something that fits your data.
The filter method works with sparse arrays.
var first = array.filter(x => true)[0];
Have you considered:
function getFirstIndex(array){
var result;
if(array instanceof Array){
for(var i in array){
result = i;
break;
}
} else {
return null;
}
return result;
}
?
And as a way to get the last element in the array:
function getLastIndex(array){
var result;
if(array instanceof Array){
result = array.push("");
array.pop;
}
} else {
return null;
}
return result;
}
Neither of these uses jquery.
Object.keys(array)[0] returns the index (in String form) of the first element in the sparse array.
var array = [];
array[2] = true;
array[5] = undefined;
var keys = Object.keys(array); // => ["2", "5"]
var first = Number(keys[0]); // => 2
var last = Number(keys[keys.length - 1]); // => 5
I was also facing a similar problem and was surprised that no one has considered the following:
var testArray = [];
testArray [1245]= 31;
testArray[2045] = 45;
for(index in testArray){
console.log(index+','+testArray[index])
}
The above will produce
1245,31
2045,45
If needed you could exist after the first iteration if all that was required but generally we need to know where in the array to begin.
This is a proposal with ES5 method with Array#some.
The code gets the first nonsparse element and the index. The iteration stops immediately with returning true in the callback:
var a = [, , 22, 33],
value,
index;
a.some(function (v, i) {
value = v;
index = i;
return true;
});
console.log(index, value);
If you find yourself needing to do manipulation of arrays a lot, you might be interested in the Underscore library. It provides utility methods for manipulating arrays, for example compact:
var yourArray = [];
yourArray[10] = "foo";
var firstValue = _.compact(yourArray)[0];
However, it does sound like you are doing something strange when you are constructing your array. Perhaps Array.push would help you out?

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