I'm sure I'm doing something dumb here, but I'm not sure what. I'm adding anonymous functions to an array for later execution, and creating them in a for loop with variables that change each iteration.
I'm expecting the logged values to be:
https:server.net/a
https:server.net/b
but instead I'm getting:
https:server.net/b
https:server.net/b
It looks like maybe when I redefine the function it overwrites the last version of it, but I'm not sure why. I would think that each anonymous function would be different.
Why? What am I doing wrong here?
Here's some sample code:
f = [];
items = ['a', 'b'];
for(var i = 0; i < items.length; i++){
var itemID = items[i];
var itemAccessUrl = `https://server.net/${itemID}`;
var func = function(){
console.log(itemAccessUrl);
};
f.push(func);
}
console.log(f.length);
for(var j = 0; j < f.length; j++){
func();
}
This is because of the nature of var scoping. var is not block scoped, it is "hoisted", as if you declared it at the top of your function.
You could declare itemAccessUrl using let instead of var, which is block scoped, but it depends on what browser support you require. (Having said that, you're using templated strings, so you should be fine)
What's happening here? When you invoke your array's functions, they use the current value of itemAccessUrl, i.e. the last assigned string, with char 'b'. This because their invocation happens after the completion of first for-loop.
You can use a closure:
f = [];
items = ['a', 'b'];
for(var i = 0; i < items.length; i++){
var itemID = items[i];
var itemAccessUrl = `https://server.net/${itemID}`;
var func = (function(param) {
return function () {
console.log(param);
};
})(itemAccessUrl);
f.push(func);
}
console.log(f.length);
for(var j = 0; j < f.length; j++){
f[j]();
}
or bind your function to current param:
f = [];
items = ['a', 'b'];
for(var i = 0; i < items.length; i++){
var itemID = items[i];
var itemAccessUrl = `https://server.net/${itemID}`;
var func = (function (param) {
console.log(param);
}).bind(null, itemAccessUrl);
f.push(func);
}
console.log(f.length);
for(var j = 0; j < f.length; j++){
f[j]();
}
Furthermore you have to change the second loop, invoking f[j]();
Related
I tried to get this to work, but the outer loop stops after second iteration, and everything that's after it does not execute(just like it was the end of the script). I want to fill two dimensional array with any character(here i used 'q' as an example)
var A=[[],[]];
for(var i=0;i<12;i++){
for(var j=0;j<81;j++){
A[i][j]='q';
}
}
It didn't work, so i put alert(i+' '+j); to see if it's even executing, and, as i wrote before, it stops after second iteration of outer loop, and then ignores rest of the script.
All I want is to have this array filled with same character in the given range(12 rows, 81 columns in this specific case), so if there's no hope in this method, i'll be glad to see one that works.
This does the job in one line.
var A = Array(12).fill(null).map(()=>Array(81).fill('q'))
This is an array of references and a bad idea as harunurhan commented.
var A = Array(12).fill(Array(81).fill('q'));
The Array.from() method creates a new, shallow-copied Array instance
from an array-like or iterable object.
function createAndFillTwoDArray({
rows,
columns,
defaultValue
}){
return Array.from({ length:rows }, () => (
Array.from({ length:columns }, ()=> defaultValue)
))
}
console.log(createAndFillTwoDArray({rows:3, columns:9, defaultValue: 'q'}))
var A=[[], []];
^ This line declares a two dimensional array of size 1x2. Try this instead:
var A = [];
for (var i = 0; i < 12; i++) {
A[i] = [];
for (var j = 0; j < 81; j++) {
A[i][j] = 'q';
}
}
Since fill() is the most succinct and intuitive, and it works as intended for immutable values, my preference would be an outer from() and an inner fill():
Array.from({length: 12}, _ => new Array(81).fill('q'));
The best approach to fill up 2D array would be like the following
let array2D = [], row = 3, col = 3, fillValue = 1
for (let i = 0; i < row; i++){
let temp = []
for (let j = 0; j < col; j++){
temp[j] = fillValue
}
array2D.push(temp)
}
You need to initialise a new array for i each time the first loop runs, and you don't need to set the layout of the array before you create it (Remove the [], [] inside the declaration of A). Try this:
var A = [];
for (var i = 0; i < 12; i++) {
A[i] = [];
for (var j = 0; j < 81; j++) {
A[i][j] = 'q';
}
}
console.log(A);
.as-console-wrapper {
max-height: 100% !important;
top: 0;
}
I tried to get this to work, but the outer loop stops after second iteration, and everything that's after it does not execute(just like it was the end of the script). I want to fill two dimensional array with any character(here i used 'q' as an example)
var A=[[],[]];
for(var i=0;i<12;i++){
for(var j=0;j<81;j++){
A[i][j]='q';
}
}
It didn't work, so i put alert(i+' '+j); to see if it's even executing, and, as i wrote before, it stops after second iteration of outer loop, and then ignores rest of the script.
All I want is to have this array filled with same character in the given range(12 rows, 81 columns in this specific case), so if there's no hope in this method, i'll be glad to see one that works.
This does the job in one line.
var A = Array(12).fill(null).map(()=>Array(81).fill('q'))
This is an array of references and a bad idea as harunurhan commented.
var A = Array(12).fill(Array(81).fill('q'));
The Array.from() method creates a new, shallow-copied Array instance
from an array-like or iterable object.
function createAndFillTwoDArray({
rows,
columns,
defaultValue
}){
return Array.from({ length:rows }, () => (
Array.from({ length:columns }, ()=> defaultValue)
))
}
console.log(createAndFillTwoDArray({rows:3, columns:9, defaultValue: 'q'}))
var A=[[], []];
^ This line declares a two dimensional array of size 1x2. Try this instead:
var A = [];
for (var i = 0; i < 12; i++) {
A[i] = [];
for (var j = 0; j < 81; j++) {
A[i][j] = 'q';
}
}
Since fill() is the most succinct and intuitive, and it works as intended for immutable values, my preference would be an outer from() and an inner fill():
Array.from({length: 12}, _ => new Array(81).fill('q'));
The best approach to fill up 2D array would be like the following
let array2D = [], row = 3, col = 3, fillValue = 1
for (let i = 0; i < row; i++){
let temp = []
for (let j = 0; j < col; j++){
temp[j] = fillValue
}
array2D.push(temp)
}
You need to initialise a new array for i each time the first loop runs, and you don't need to set the layout of the array before you create it (Remove the [], [] inside the declaration of A). Try this:
var A = [];
for (var i = 0; i < 12; i++) {
A[i] = [];
for (var j = 0; j < 81; j++) {
A[i][j] = 'q';
}
}
console.log(A);
.as-console-wrapper {
max-height: 100% !important;
top: 0;
}
I have a JS function with for loops. Inside the nested for loops, str element prints all of the intended elements. But, outside it doesn't print all of it. I would appreciate any help. Here is my code:
function getResearchersFullName(allDataJson){
var str = [];
var myarr = [];
var c = 0;
for(var i = 0; i < allDataJson.length; i++){
myarr[i] = allDataJson[i].Researchers.split(", ");
for(var j = 0; j < myarr[i].length; j++){
str[c] = myarr[i][j];
//console.log(str[c]); //prints as expected
}
}
return str;
}
I am trying to use the returned value as follows but it only prints one of the str values.
var fullnames = getResearchersFullName(allDataJson);
for(var i = 0; i <fullnames.length; i++){
console.log(fullnames[i]); //returns only 1 object
}
Your code never increments c. The only element of str that's ever modified is element 0.
Use str.push(myarr[i][j]); and you won't need c at all.
javascript:
for (i = 0; i < n; i++)
{
v = fn(obj);
}
function fn(o)
{
for(i = 0; i < o.length; i++)
{
...
}
}
The first index get modify from second FOR inside the function fn(o);
How is it possible? what's the scope of index?
It doesn't look like you're declaring i as a local variable, so what's happening is that i being treated as a global variable. You need to write the var keyword in order for a variable to become a local variable.
for (var i = 0; i < n; i++)
{
v = fn(obj);
}
function fn(o)
{
for(var i = 0; i < o.length; i++)
{
}
}
I don't see any var so it looks like you're not varing anything, but I'll add those in and answer the question
Is it possible to modify the i in one for loop from another in a different scope?
When using functions in JavaScript, all primitive arguments are passed ByVal and all object arguments are passed ByRef. This also happens when using =.
Therefore, if you want to modify i out-of-scope, you will need to always access it through an Object.
function foo() { // scope this away from fn
for (var c = {i: 0}; c.i < n; c.i++) { // i wrapped in object
v = fn(obj, c); // pass wrapping object into `fn`, too. Now see `fn`
// c.i is now the result of change in `fn`
}
}
function fn(o, c) {
for(var i = 0; i < o.length; i++) {
// ...
}
// make change to c.i
c.i = 0;
}
foo();
I'm trying to implement a duplicate method to the js Array prototype which concats a duplicate of the array to itself like so:
[11,22,3,34,5,26,7,8,9].duplicate(); // [11,22,3,34,5,26,7,8,9,11,22,3,34,5,26,7,8,9]
Here's what I have, but it causes the browser to crash:
var array = [11,22,3,34,5,26,7,8,9];
Array.prototype.duplicate = function() {
var j = this.length;
for(var i = 0; i < this.length; i++) {
this[j] = this[i];
j++;
}
return this;
}
I'm trying to do this using native JS as practice for iterations and algorithms so I'm trying to avoid built-in methods, if possible, so that I can get a clearer understanding of how things are being moved around.
Any ideas on why it is crashing and how I can optimize it?
The code inside the loop changes the length of the array, so it will just keep growing and you will never reach the end of it. Get the initial length of the array in a variable and use in the loop condition. You can use that as offset for the target index also instead of another counter:
var array = [11,22,3,34,5,26,7,8,9];
Array.prototype.duplicate = function() {
var len = this.length;
for (var i = 0; i < len; i++) {
this[len + i] = this[i];
}
return this;
}
The length of the array is increasing with each element added so you can not use this as a terminator. Try this.
var j = this.length;
for(var i = 0; i < j; i++) {
this[i+j] = this[i];
}
Here's simplest code
Array.prototype.duplicate = function () {
var array = this;
return array.concat(array);
};
Using Spread syntax;
In a method, this refers to the owner object.
Array.prototype.duplicate = function () {
return [...this, ...this]
};
let array = [1,2,3,4,5];
Array.prototype.duplicate = function () {
return [...this, ...this]
};
console.log(array.duplicate());