How do I update a variable inputted to a function? - javascript

I am trying to make a function in my js file that will remove an item from an array and then save the new array to the variable. But here's the thing: I don't want it to only save ONE variable, I want it to save any array variable that I input. What I mean is something like this:
const list = [1,2,3];
function removeItem(array,index)
{
let newArray = [];
for(var i = 0 ; i < array.length ; i++)
{
if(i != index)
{
newArray.push(array[i]);
}
}
array = newArray; // where it saves the variable
}
removeItem(list,0);

You can create a prototype function, see this
Array.prototype.removeItem = function(what) {
if (this.indexOf(what) >= 0) this.splice(this.indexOf(what), 1);
}
var foo = [1, 2, 3];
foo.removeItem(2);
console.log(foo);

You just want arr.splice(i, 1);

The simplest way is probably to return the new array and let the calling code decide where to save it:
function removeItem(array, index) {
let newArray = [];
for(var i = 0 ; i < array.length ; i++) {
if(i != index) {
newArray.push(array[i]);
}
}
return newArray;
}
And:
list = removeItem(list, 0);
This is probably ideal because modifying objects passed to a function as arguments is generally unexpected. It's more common to return the new value.
If you want to modify the array in place then you'd need to only make edits to the array reference directly in the function and not use any kind of newArray copy. Which could be as simple as:
function removeItem(array, index) {
array.splice(index, 1);
}
And:
removeItem(list, 0);
But this could be very unintuitive since list is an argument to the function and you're not calling anything on list itself. Contrast that to the syntax used in splice which is called on the array itself and implies that it may modify that array:
list.splice(0, 1);
(Though some array methods in JavaScript return a new array while others modify the array in place. It's always best to refer to the documentation to make sure.)

Related

Javascript how do i get my function array to give me more then the first value

function countUniqueItems(arr) {
nums = [];
for (i = 0; i < arguments.length; i++) {
const item = arr[i];
console.log(i);
//console.log(item);
if (nums.includes(arr) === true) {
//console.log('8 is in the array');
//nums.push(arr)
} else {
nums.push(arr);
//console.log('8 is NOT in the array');
//nums.push(item)
}
}
return nums;
}
countUniqueItems(1, 2);
So it will give back the first argument which is 1 but i want it to be able to say argument 2 and 3 and so on
So you need to pass an array into the function, in place of 1,2 pass [1,2].
Then inside your function, you should use arr.length in place of arguments.length.
Then you look at your logic for the loop, you are pushing atm arr into nums, but if you pass and array that isn't really want you want, you should be pushing item as that is the variable which represents your current element from the array.
It looks from you comments like you're trying to make a unique list of inputs. Perhaps something like this would do the trick.
EDIT: Updated to use arguments
function uniqueNumbers() {
let arrayOfArguments = [...arguments]
let uniqueNums = [];
arrayOfArguments.map(i => !uniqueNums.includes(i) ? uniqueNums.push(i) : null);
return uniqueNums;
};
console.log(uniqueNumbers(1,2,3,3));
you should either pass an array to countUniqueItems or use the arguments keyword in the for-loop.
Your code is only seeing 1 (as arr inside the function).
basic implementation to find unique items
function countUniqueItems(...arr) {
let nums = [];
for (let num of arr) {
if (nums.indexOf(num) === -1) nums.push(num);
}
return nums;
}
console.log(countUniqueItems(1, 2, 1));
Using Set you can remove the duplicate values, you dont need to do logic run the loop to find the unique values from array.
const a = [1,2,3,3];
const uniqueSet = new Set(a);
uniqueSet.size
let uniqueFinder = arr => { const uniqueSet = new Set(arr); return uniqueSet.size}
const arrywithduplicates = [1,2,3,3,4,4];
uniqueFinder(arrywithduplicates) // return 4
Read more about Set : https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Set

Student trying to understand callback functions

Hi I'm trying to learn how to implement callback functions. My teacher has helped me out multiple times but I still can't pass data through the following equation below. I'm trying to get certain elements of array to get pushed into a new function if only they pass a test within the function. Please have a look and thank you for your input. An explanation as to why I get an empty array and resources to further my understanding would be appreciated.
// EACH DEFINITION
function each (collection, callback) {
for(var i = 0; i < collection.length; i ++){
callback(collection[i]);
}
}
// VARIABLE DECLARATION
var myArray = [1,2,3,4,5,6];
var isEven = function (num) {
return num % 2 === 0;
};
// IMPLEMENT DEFINITION
function implement(array, test){ // array = myArray, test = isEven
var arr = [];
each(array, function(item){
test(item);
});
if(test(array)){
arr.push(array);
}
return arr;
}
// IMPLEMENT INVOCATION
implement(myArray, isEven);
You are building arr outside the each() loop.
I would think your code would be like this:
// IMPLEMENT DEFINITION
function implement(array, test){ // array = myArray, test = isEven
var arr = [];
each(array, function(item){
if(test(item)) {
arr.push(item);
}
});
return arr;
}
Though in this case there is no reason for your implement() filtering function at all, since javascript Array prototype already has a filter method. You could simplify your call to this:
var filteredArray = myArray.filter(isEven);
Though you might also then want to change your isEven definition to be more correct as:
var isEven = function (num, index, array) {
In your case you don't need to work with the last two parameters.
// EACH DEFINITION
function each (collection, callback, results) {
for(var i = 0; i < collection.length; i ++){
callback(collection[i]);
}
console.log(results);
}
// VARIABLE DECLARATION
var myArray = [1,2,3,4,5,6];
var isEven = function (num, array) {
return num % 2 === 0;
};
// IMPLEMENT DEFINITION
function implement(array, test){ // array = myArray, test = isEven
var arr = [];
function filter (item) {
if (test(item)) {
arr.push(item);
}
}
each(array, filter, arr);
// If you return arr here, it will still be empty. You must pass it to functions it is being operated on.
}
// IMPLEMENT INVOCATION
implement(myArray, isEven);
Not only are you trying to push to arr outside of your loop, but you're trying to return arr before it has gained any values.
Two points:
First, your implementation of callback functions is correct. As far as the concept of callbacks goes, you are calling and passing the functions correctly.
However, your implement() function probably has a bug. You are not pushing to arr until after each() has already been called:
function implement(array, test) { // array = myArray, test = isEven
var arr = [];
each(array, function(item) {
result = test(item);
});
// This block should be in the loop itself
// It should also refer to item, not array
if (test(array)) {
arr.push(array);
}
return arr;
}
Try this fix based on the code you provided:
// EACH DEFINITION
function each(collection, callback) {
for (var i = 0; i < collection.length; i++) {
callback(collection[i]);
}
}
// VARIABLE DECLARATION
var myArray = [1, 2, 3, 4, 5, 6];
var isEven = function(num) {
return num % 2 === 0;
};
// IMPLEMENT DEFINITION
function implement(array, test) { // array = myArray, test = isEven
var arr = [];
each(array, function(item) {
if (test(item)) {
arr.push(item)
}
});
if (test(array)) {
arr.push(array);
}
return arr;
}
// IMPLEMENT INVOCATION
var result = implement(myArray, isEven);
console.log(result); // For snippet results
Your callback, as you defined it, is
function(item){
test(item);
}
this will only call test on each item and that's it. Since you want to take it further and add item to arr if test returns true, you should put that checking code inside the callback as well, making it
function(item){
if (test(item)) {
arr.push(item);
}
}
so that this function will be called for each of the item.
Also, this part
if(test(array)){
arr.push(array);
}
is incorrect because you are passing a whole array into isEven when isEven is expecting a number. test(array) will always return false; that's why your arr is empty.
Modifying your code to work as you wanted, it would be
// IMPLEMENT DEFINITION
function implement(array, test){ // array = myArray, test = isEven
var arr = [];
each(array, function(item){
if (test(item)) {
arr.push(item);
}
});
return arr;
}
Resources wise, there are callbacks tutorial widely available online, as well as best practices. You can easily find one that suits you best by googling.
It looks to me like the entire issue here is in the implementation section you denote. All of the other code looks adequate.
each(array, function(item){
test(item);
});
Alright, first let's examine this piece of code. You are making a call to your each function, which will use the callback anonymous function defined here as shown.
However, if you were to look at the each function itself, there is no return (which means it returns undefined by default). There is also no modification being done in each. As a result, this set of code has no effect on the execution of the code, and from certain advanced compilation technique may actually be removed by the V8 engine in chrome if that was being used.
This means the only aspect of your code which is executing is
var arr = [];
if(test(array)){
arr.push(array);
}
return arr;
At this point, test is still the isEven function, so you are basically asking this
if(array % 2 === 0) arr.push(array);
Arrays in JavaScript behave interestingly when used in conditional statements, and in this situation the array essentially has toString called on it (more in depth here: https://stackoverflow.com/a/10556035/1026459 , but basically when you have object === number then it will attempt to use toPrimitive on the object which results in a string), which makes it
if("1,2,3" % 2 === 0)
which is false. As a result arr is unchanged, and returned in its original state of [].

How to get index of an array element which has more than one sub elements in javascript

I am working in javascript and I would like to find the index of an element in an array .
var task = [{taskno:'1',todo:'cooking'},{taskno:'2',todo:'play'}];
now i want to find the index of task which has todo as cooking and i dont know its task no .... is there any inbuilt function ?
I have tried this code but It's not working:
var index = task.indexOf(todo:'cooking');
Now I'm using a for loop is there any other way?
Thanks in advance :)
indexOf will give you the index of an object in an array, but you must already have a reference to that object.
e.g.
var o1 = { val: 'o1' },
o2 = { val: 'o2' };
[o2, o1].indexOf(o1); //1
If you do not have a reference to the object, you will have to create your own search algorithm.
var tasks = [{taskno:'1',todo:'cooking'}, {taskno:'2',todo:'play'}];
var index = indexOfFirstMatch(tasks, function (task) {
return task.todo === 'cooking';
});
console.log(index); //0
function indexOfFirstMatch(arr, matcher) {
var i = 0, len = arr.length;
for (; i < len; i++) {
if (matcher(arr[i])) return i;
}
return -1;
}
If you need the actual object, vs. just it's index. You could use the underscore library and it's findWhere function:
var task_wanted = _findWhere(task, {todo: "Cooking"});
If you actually need the index, then use task_wanted to search the array with indexOf().

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.

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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