Take the following function to generate a square multiplication table:
function getMultTable(n) {
/* generate multiplication table from 1 to n */
let table = new Array(n);
for (let i=1; i<=n; i++) {
table[i-1] = new Array(n);
for (let j=1; j<=n; j++) {
table[i-1][j-1] = i*j;
}
}
return table;
}
console.log(getMultTable(3));
// [
// [ 1, 2, 3 ],
// [ 2, 4, 6 ],
// [ 3, 6, 9 ]
// ]
Why can't the following be done instead?
function getMultTable(n) {
let table = new Array(n);
for (let i=1; i<=n; i++) {
for (let j=1; j<=n; j++) {
table[i-1][j-1] = i*j;
}
}
return table;
}
In other words, why can't a multi-dimensional array be created on-the-fly in javascript, or is it possible to do that some other way?
Let's start from the basics, why you can't do the following
let arr;
arr[0] = 1
The answer is because arr is undefined. Well, when an array is initialized it is initialized with all its entries undefined. The operation table[i-1][j-1] = i*j is equivalent to
const row = table[i-1];
row[j-1] = i*j
So, when the table is created all its items are undefined, and the statement row[j-1] = i*j, is trying to set a property of row, in other words setting a property of undefined.
The reason is similar to why you can't run this
function getMultTable(n) {
let table;
for (let i=1; i<=n; i++) {
for (let j=1; j<=n; j++) {
table[i-1][j-1] = i*j;
}
}
return table;
}
Only that in this case your problem is trying to read a property from undefined
You can do it something like this:
let arr = new Array(n).fill().map(el => new Array(n).fill());
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'm trying to make my 2d matrix to have numbers which continue on the new row
var myMatrix = [];
var row = 5;
var colom = 3;
for (var i = 0; i < rows; i++) {
var toto = 1;
myMatrix[i] = [i];
for (var j = 0; j < colom; j++) {
myMatrix[i][j] = [i + j];
}
}
console.log(myMatrix);
I'm trying to make it print numbers like this:
123
456
789 and etc...
but without success:/
can someone help and also give a video or site with examples where i can learn more about that kind of stuff?
First, a look at what your code is doing:
const myMatrix = [];
const rows = 5;
const columns = 3;
for (var i = 0; i < rows; i++) {
myMatrix[i] = [i];
for (var j = 0; j < columns; j++) {
myMatrix[i][j] = [i+j];
}
}
console.log(myMatrix);
You have a typo in your row/rows variable name. Ignoring that though...
Your myMatrix[i] line is creating an array at i, which is then being set to an array with a value of i. Just this creates a wonky mash-up , where each "row" gets an array with it's row number as the first value, something like this:
[[0], [1], [2], [3], [4]]
Your inner loop then adds a value to that array at the place and adds i+j together, but puts that inside of an array, which isn't what you want, so you get something like this:
[
[[0], [1], [2]], // i = 0
[[1], [2], [3]], // i = 1
[[2], [3], [4]], // i = 2
// ... etc
]
Also note that you are replacing that first [i] anyways, so don't set it like that, just make it an empty array [].
What you want is something like this:
const myMatrix = [];
const rows = 5;
const columns = 3;
for (var i = 0; i < rows; i++) {
myMatrix[i] = [];
for (var j = 0; j < columns; j++) {
myMatrix[i][j] = (i*columns)+j;
}
}
console.log(myMatrix);
There were three changes to your code:
Make the [i] and []. It doesn't hurt anything, but [i] also doesn't make sense.
Take the i+j part out of the array, you just want a value there.
When you add i, multiply it by columns so it doesn't reset every time: (i*columns)+j
This will give you a nice output, starting with 0. If you want it start at 1, just add one to your value:
const myMatrix = [];
const rows = 5;
const columns = 3;
for (var i = 0; i < rows; i++) {
myMatrix[i] = [];
for (var j = 0; j < columns; j++) {
myMatrix[i][j] = (i*columns)+j+1;
}
}
console.log(myMatrix);
Use i * columns + j ... and I have to add up to 30 chars for padding
I want to write a function that takes an array such as:
var columns = ['distance', 'times', 'acceleration']
Then from this array, I want to generate something like this:
[{id: id_0, distance: 0, times: 0, acceleration: 0}, {id: id_1, distance: 1, times: 1, acceleration: 1}]
Notice that we have 2 objects here, but I want it to be whatever number I pass in to my parameter. Here is what I have:
generateData: function(rows, columns) {
var generatedData = [];
for (var i = 0, rowLen = rows.length; i < rowLen; i++) {
for (var n = 0; i < columns.length; n++) {
// not sure how to construct an object here from looping through my columns array
generatedData.push({
id: 'id_ + n',
// confused here
});
}
return generatedData;
}
}
This is the perfect place to dynamically create your own function. Try this:
function createArrayOfObjects(columns, count) {
var objectProps = new Array(columns.length);
for (var i = 0; i < columns.length; i++){
//":j" will be the variable j inside the dynamic function
objectProps[i] = columns[i] + ":j";
}
var funcBody = "var arr = new Array(count);" +
"for(var j = 0; j < count; j++){" +
"arr[j] = {" + objectProps.join(',') + "};" +
"}" +
"return arr;";
//Create a new function and call it with count as the parameter, returning the results
return new Function("count", funcBody)(count);
}
var count = 10;
var columns = ['distance', 'times', 'acceleration'];
createArrayOfObjects(columns.concat('id'), count);
This has the benefit of only having to loop over the columns array once where other solutions require nested loops.
JSPerf
I am giving you away the initial non-optimized solution. Its upto you to do the optimizations.
generateData: function(rows, columns) {
var generatedData = [];
for (var i = 0; i < rows.length; i++) {
var myObj = {};
myObj["id_" + i] = i;
for (var n = 0; n < columns.length; n++) {
myObj[columns[n]] = i;
}
generatedData.push(myObj);
}
return generatedData;
}
A functional approach that will take the object properties from the passed in array, instead of hard-coding them, might look something like this inside the for loop to populate an array named 'rows' with property names coming from the values of an array named 'cols':
cols.forEach(function(cv, ci, ca) { rows[ri][cv] = ri; });
See the snippet for a full example. Note that, in this example, I'm just shoving the current index of "rows" into the object as the property value.
var columns = ['distance', 'times', 'acceleration'];
function generateData(numRows, cols) {
rows = new Array(numRows);
for(ri=0; ri < rows.length; ri++) {
rows[ri] = { id: ri };
cols.forEach(function(cv, ci, ca) {
rows[ri][cv] = ri;
});
}
return rows;
}
data = generateData(5, columns);
console.log(data);
I have got a little function in javascript and I want to split an array A into a 2d array.
I will be used for square matrices. Obviously,I want it to be 2x2 if a square matrix of 2x2 is in the input and so on for 3x3 and. But I'm stuck after having read a first row.So my arr rows are repeated. Does anyone have any ideas about how I can get the next rows read properly.So,for instance,lets say I do have an array
A = [2,1,4,5,1,2,3,1,9]
Then I want my array arr to look like this:
arr = [[2,1,4],[5,1,2],[3,1,9]]
This will later be used for calculation a determinant of a matrix.
function create2Darray(clname) {
var A = document.getElementsByClassName(clname);
var arr = new Array();
var rows = Math.sqrt(A.length);
for (var i = 0; i < rows; i++) {
arr[i] = new Array();
for (var j = 0; j < rows; j++) {
arr[i][j] = A[j].value;
}
}
}
You are assigning always the same value. It should be something like:
arr[i][j] = A[i*rows+j].value;
EDIT: Here's a complete function without the DOM manipulation (that is, A is a simple array of integers):
function create2Darray(A) {
var arr = [];
var rows = Math.sqrt(A.length);
for (var i = 0; i < rows; i++) {
arr[i] = [];
for (var j = 0; j < rows; j++) {
arr[i][j] = A[i * rows + j];
}
}
return arr;
}
console.log(create2Darray([1, 2, 3, 4, 5, 6, 7, 8, 9]))