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What is the most efficient way to reverse an array in Javascript?
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Closed 11 months ago.
I currently use this bit of javascript inside Adobe After Effects to print a customizable list of numbers
x = 0 //starting point (the first number in the list);
y= 20 //increments (the size of the steps);
z= 5 //the number of steps;
Array.from(new Array(z)).map((_, i) => i * y + x).join("\n")
which (in this case) would output
0
20
40
60
80
It would be really cool if I could also generate the list in reverse
80
60
40
20
0
but because I'm not a programmer I have no idea how to do this. Could someone help me with this?
Instead of starting from the first number (x) and adding y for each iteration, you can start from the last number and subtract y for every iteration.
The last number would be the number of times y is added added to x. That gives the formulat (z - 1) * y) + x
x = 0 //starting point (the first number in the list);
y= 20 //increments (the size of the steps);
z= 5 //the number of steps;
const result = Array.from(new Array(z))
.map((_, i) => (((z - 1) * y) + x) - (i * y)).join("\n");
console.log(result);
I think there is no need to optimisation here, as the op has no programming experience, a simple solution is well enough, which would be the Array.reverse().
See the the MDN docs for correct usage.
const x = 0 //starting point (the first number in the list);
const y= 20 //increments (the size of the steps);
const z= 5 //the number of steps;
const array = Array.from({length: z}, ((_, i) => i * y + x)).reverse().join("\n");
console.log(array);
Another possible solution: simply populate your array from the back. Then no need to reverse.
const x = 0 //starting point (the first number in the list);
const y= 20 //increments (the size of the steps);
const z= 5 //the number of steps;
const array = Array.from({length: z}, ((_, i) => (z * y - y) - i * y)).join("\n");
console.log(array);
I have been running through Codility questions and and one of the questions was to count all the possible factors of a number. I looped through the whole number got the answer but it wasn't efficient of course.
I searched for the answer and got this
function solution(N) {
var i;
var NumFactors = 0;
for(i = 1;i*i < N; i++) {
if(N%i == 0) NumFactors += 2;
}
if(i*i == N) NumFactors++;
return NumFactors
}
for anyone who hasn't tried the challenge if you run solution(24) it should return 8 as number of factors which are (1, 2, 3, 4, 6, 8,12, 24)
Since the person who wrote the code didn't leave any explanation, can someone who get what's happening kindly explain to me the i*i and the reason of incrementing NumFactors by 2.
The i*i is for checking until squareRoot(N). Because if you have a divisor for a number N then you actually have two divisor. Because the division result is another divisor. For example, in case of 24,
If you take divisor 2, you will find another divisor which is 12. Because 2 X 12 = 24. If you loop through N i.e. 1 to 24 you will get the divisors like this,
2 X 12 = 24
3 X 8 = 24
4 X 6 = 24
6 X 4 = 24
8 X 3 = 24
12 X 2 = 24
24 X 1 = 24
You see we have got redundant values after squareRoot(N). That is why for optimization we are going from 1 to squareRoot(N).
Now about increase factors by 2 is already described above. For the special case when N is a perfect square number like 36 or 49 you will face a case where 6 X 6 = 36 and 7 X 7 = 49 that is why in that case we are increasing the factor by one. Because there is actually on divisor namely 6 and 7 in our case.
how much 0 at the end of 1*2*3*....*100?
for example1 zero 10,2 zero at the end of 10100,0 zero at 10001
The interviewer gave me 3 mins.
front-end develope intern was the job i went for.
i tried to solve it in JavaScript,it was wrong because 100! is so big for a JS variable.Here is my code.
var factorial = function(x){
if(x<0){
return error;
}else if (x ===1 || x ===0) {
return x;
}else if (x > 1) {
return (factorial(x - 1) * x);
}
}
var counter = 0;
var countZero = function(x){
while((x%10) === 0){
counter = counter + 1;
x = x/10;
}
return counter;
}
console.log(countZero(factorial(100)));
Any ideas?I solved it without computer in a pure-math way,but i want to know how to solve it in JS.
A number gets a zero at the end of it if the number has 10 as a factor. For instance, 10 is a factor of 50, 120, and 1234567890. So you need to find out how many times 10 is a factor in the expansion of 100!.
But since 5×2 = 10, we need to account for all the products of 5 and 2. Looking at the factors in the above expansion, there are many more numbers that are multiples of 2 (2, 4, 6, 8, 10, 12, 14,...) than are multiples of 5 (5, 10, 15,...). That is, if we take all the numbers with 5 as a factor, We'll have way more than enough even numbers to pair with them to get factors of 10 (and another trailing zero on my factorial). So to find the number of times 10 is a factor, all we really need to worry about is how many times 5 is a factor in all of the numbers between 1 and 100
Okay, how many multiples of 5 are there in the numbers from 1 to 100? There's 5, 10, 15, 20, 25,...
Let's do this the short way: 100 is the closest multiple of 5 below or equal to 100, and 100 ÷ 5 = 20, so there are twenty multiples of 5 between 1 and 100.
But wait: 25 is 5×5, so each multiple of 25 has an extra factor of 5 that we need to account for. How many multiples of 25 are between 1 and 100? Since 100 ÷ 25 = 4, there are four multiples of 25 between 1 and 100.
Adding these, We get 20 + 4 = 24 trailing zeroes in 100!
It was more of a math question that a Javascript specific question and you don't need to calculate the actual factorial to find the number of trailing zeros
More details here
Zeros at the end of a number result from 2s and 5s being multiplied together. So, one way to count up the total number of zeros would be to figure out how many total 2s and 5s go into 100! - don't calculate that number, just add up the number of factors - and then return whichever count (2s or 5s) is smallest.
let twos = 0;
let fives = 0;
for (let i = 1; i <= 100; i++) {
let num = i;
while (num % 2 === 0) {
twos++;
num /= 2;
}
while (num % 5 === 0) {
fives++;
num /= 5;
}
}
console.log(Math.min(twos, fives));
For the numbers less or equal than 1000000000 as your case here you could use the following algorithm :
function numberOfZeros(n) {
if (n == 0) return 0;
return parseInt(n * (n - 1) / (4 * n));
}
console.log(numberOfZeros(100));
Lets say I have a list of numbers in the following form(Ignore the | they are there for formating help).
00|00|xx
00|xx|00
xx|00|00
etc.
Rules: XX can be any number between 1 and 50. No XX values can be identical.
Now I select a random set of numbers(no duplicates) from a list qualifying the above format, and randomly add and subtract them. For example
000011 - 002400 - 230000 = -232389
How can I determine the original numbers and if they were added or subtracted solely from -232389? I'm stumped.
Thanks!
EDIT:
I was looking for a function so I ended up having to make one. Its just a proof of concept function so variables names are ugly http://jsfiddle.net/jPW8A/.
There are bugs in the following implementation, and it fails to work in a dozen of scenarios. Check the selected answer below.
function reverse_add_subtract(num){
var nums = [];
while(num != 0){
var str = num.toString(),
L = Math.abs(num).toString().length,
MA = str.match(/^(-?[0-9]?[0-9])([0-9][0-9])([0-9][0-9])*$/);
if(MA){
var num1 = MA[1],
num2 = MA[2];
}else{
var num1 = num,
num2 = 0;
}
if(L%2)L++;
if( num2 > 50){
if(num < 0) num1--;
else num1++;
}
nums.push(num1);
var add = parseInt(num1 + Array(--L).join(0),10);
num = (num-add);
}
return nums;
}
reverse_add_subtract(-122436);
First note that each xx group is constrained from [1, 50). This implies that each associated pair in the number that is in the range [50, 99) is really 100 - xx and this means that it "borrowed from" the group to the left. (It also means that there is only one set of normalized numbers and one solution, if any.)
So given the input 23|23|89 (the initial xx spots from -232389), normalize it -- that is, starting from the right, if the value is >= 50, get 100 - value and carry the 100 rightward (must balance). Example: (23 * 100) + 89 = 2300 * 89 = 2400 - 11 = 2389. And example that shows that it doesn't matter if it's negative as the only things that change is the signs: (-23 * 100) - 89 = -2300 - 89 = -2400 + 11 = -2389
(Notes: Remember, 1 is added to the 23 group to make it 24: the sign of the groups is not actually considered in this step, the math is just to show an example that it's okay to do! It may be possible to use this step to determine the sign and avoid extra math below, but this solution just tries to find the candidate numbers at this step. If there are any repeats of the number groups after this step then there is no solution; otherwise a solution exists.)
The candidate numbers after the normalization are then 23|24|11 (let's say this is aa|bb|cc, for below). All the xx values are now known and it is just a matter of finding the combination such that e * (aa * 10000) + f * (bb * 100) + g * (cc * 1) = -232389. The values aa, bb, cc are known from above and e, f, and g will be either 1 or -1, respectively.
Solution Warning: A method of finding the addition or subtraction given the determined numbers (determined above) is provided below the horizontal separator. Take a break and reflect on the above sections before deciding if the extra "hints" are required.
This can then be solved by utilizing the fact that all the xx groups are not dependent after the normalization. (At each step, try to make the input number for the next step approach zero.)
Example:
-232389 + (23 * 10000) = -2389 (e is -1 because that undoes the + we just did)
-2389 + (24 * 100) = 11 (likewise, f is -1)
11 - (11 * 1) = 0 (0 = win! g is 1 and solution is (-1 * 23 * 10000) + (-1 * 24 * 100) + (1 * 11 * 1) = -232389)
Happy homeworking.
First, your math is wrong. Your leading zeros are converting the first two numbers to octal. If that is the intent, the rest of this post doesn't exactly apply but may be able to be adapted.
11-2400-230000 = -232389
Now the last number is easy, it's always the first two digits, 23 in this case. Remove that:
-232389 + 230000 = -2389
Your 2nd number is the next 100 below this, -2400 in this case. And your final number is simply:
-2389 + 2400 = 11
Aww! Someone posted an answer saying "brute force it" that I was about to respond to with:
function find(num){for(var i=1;i<50;i++){for(var o1=0;o1<2;o1++){for(var j=1;j<50;j++){for(var o2=0;o2<2;o2++){for(var k=1;k<50;k++){var eq;if(eval(eq=(i+(o1?'+':'-')+j+'00'+(o2?'+':'-')+k+'0000'))==num){ return eq; }}}}}}}
they deleted it... :(
It was going to go in the comment, but here's a cleaner format:
function find(num){
for(var i=1;i<50;i++){
for(var o1=0;o1<2;o1++){
for(var j=1;j<50;j++){
for(var o2=0;o2<2;o2++){
for(var k=1;k<50;k++){
var eq;
if(eval(eq=(i+(o1?'+':'-')+j+'00'+(o2?'+':'-')+k+'0000'))==num){ return eq; }
}
}
}
}
}
}
In JavaScript, how do I get:
The whole number of times a given integer goes into another?
The remainder?
For some number y and some divisor x compute the quotient (quotient)[1] and remainder (remainder) as:
const quotient = Math.floor(y/x);
const remainder = y % x;
Example:
const quotient = Math.floor(13/3); // => 4 => the times 3 fits into 13
const remainder = 13 % 3; // => 1
[1] The integer number resulting from the division of one number by another
I'm no expert in bitwise operators, but here's another way to get the whole number:
var num = ~~(a / b);
This will work properly for negative numbers as well, while Math.floor() will round in the wrong direction.
This seems correct as well:
var num = (a / b) >> 0;
I did some speed tests on Firefox.
-100/3 // -33.33..., 0.3663 millisec
Math.floor(-100/3) // -34, 0.5016 millisec
~~(-100/3) // -33, 0.3619 millisec
(-100/3>>0) // -33, 0.3632 millisec
(-100/3|0) // -33, 0.3856 millisec
(-100-(-100%3))/3 // -33, 0.3591 millisec
/* a=-100, b=3 */
a/b // -33.33..., 0.4863 millisec
Math.floor(a/b) // -34, 0.6019 millisec
~~(a/b) // -33, 0.5148 millisec
(a/b>>0) // -33, 0.5048 millisec
(a/b|0) // -33, 0.5078 millisec
(a-(a%b))/b // -33, 0.6649 millisec
The above is based on 10 million trials for each.
Conclusion: Use (a/b>>0) (or (~~(a/b)) or (a/b|0)) to achieve about 20% gain in efficiency. Also keep in mind that they are all inconsistent with Math.floor, when a/b<0 && a%b!=0.
ES6 introduces the new Math.trunc method. This allows to fix #MarkElliot's answer to make it work for negative numbers too:
var div = Math.trunc(y/x);
var rem = y % x;
Note that Math methods have the advantage over bitwise operators that they work with numbers over 231.
I normally use:
const quotient = (a - a % b) / b;
const remainder = a % b;
It's probably not the most elegant, but it works.
var remainder = x % y;
return (x - remainder) / y;
You can use the function parseInt to get a truncated result.
parseInt(a/b)
To get a remainder, use mod operator:
a%b
parseInt have some pitfalls with strings, to avoid use radix parameter with base 10
parseInt("09", 10)
In some cases the string representation of the number can be a scientific notation, in this case, parseInt will produce a wrong result.
parseInt(100000000000000000000000000000000, 10) // 1e+32
This call will produce 1 as result.
Math.floor(operation) returns the rounded down value of the operation.
Example of 1st question:
const x = 5;
const y = 10.4;
const z = Math.floor(x + y);
console.log(z);
Example of 2nd question:
const x = 14;
const y = 5;
const z = Math.floor(x % y);
console.log(x);
JavaScript calculates right the floor of negative numbers and the remainder of non-integer numbers, following the mathematical definitions for them.
FLOOR is defined as "the largest integer number smaller than the parameter", thus:
positive numbers: FLOOR(X)=integer part of X;
negative numbers: FLOOR(X)=integer part of X minus 1 (because it must be SMALLER than the parameter, i.e., more negative!)
REMAINDER is defined as the "left over" of a division (Euclidean arithmetic). When the dividend is not an integer, the quotient is usually also not an integer, i.e., there is no remainder, but if the quotient is forced to be an integer (and that's what happens when someone tries to get the remainder or modulus of a floating-point number), there will be a non-integer "left over", obviously.
JavaScript does calculate everything as expected, so the programmer must be careful to ask the proper questions (and people should be careful to answer what is asked!) Yarin's first question was NOT "what is the integer division of X by Y", but, instead, "the WHOLE number of times a given integer GOES INTO another". For positive numbers, the answer is the same for both, but not for negative numbers, because the integer division (dividend by divisor) will be -1 smaller than the times a number (divisor) "goes into" another (dividend). In other words, FLOOR will return the correct answer for an integer division of a negative number, but Yarin didn't ask that!
gammax answered correctly, that code works as asked by Yarin. On the other hand, Samuel is wrong, he didn't do the maths, I guess, or he would have seen that it does work (also, he didn't say what was the divisor of his example, but I hope it was 3):
Remainder = X % Y = -100 % 3 = -1
GoesInto = (X - Remainder) / Y = (-100 - -1) / 3 = -99 / 3 = -33
By the way, I tested the code on Firefox 27.0.1, it worked as expected, with positive and negative numbers and also with non-integer values, both for dividend and divisor. Example:
-100.34 / 3.57: GoesInto = -28, Remainder = -0.3800000000000079
Yes, I noticed, there is a precision problem there, but I didn't had time to check it (I don't know if it's a problem with Firefox, Windows 7 or with my CPU's FPU). For Yarin's question, though, which only involves integers, the gammax's code works perfectly.
const idivmod = (a, b) => [a/b |0, a%b];
there is also a proposal working on it
Modulus and Additional Integer Math
Alex Moore-Niemi's comment as an answer:
For Rubyists here from Google in search of divmod, you can implement it as such:
function divmod(x, y) {
var div = Math.trunc(x/y);
var rem = x % y;
return [div, rem];
}
Result:
// [2, 33]
If you need to calculate the remainder for very large integers, which the JS runtime cannot represent as such (any integer greater than 2^32 is represented as a float and so it loses precision), you need to do some trick.
This is especially important for checking many case of check digits which are present in many instances of our daily life (bank account numbers, credit cards, ...)
First of all you need your number as a string (otherwise you have already lost precision and the remainder does not make sense).
str = '123456789123456789123456789'
You now need to split your string in smaller parts, small enough so the concatenation of any remainder and a piece of string can fit in 9 digits.
digits = 9 - String(divisor).length
Prepare a regular expression to split the string
splitter = new RegExp(`.{1,${digits}}(?=(.{${digits}})+$)`, 'g')
For instance, if digits is 7, the regexp is
/.{1,7}(?=(.{7})+$)/g
It matches a nonempty substring of maximum length 7, which is followed ((?=...) is a positive lookahead) by a number of characters that is multiple of 7. The 'g' is to make the expression run through all string, not stopping at first match.
Now convert each part to integer, and calculate the remainders by reduce (adding back the previous remainder - or 0 - multiplied by the correct power of 10):
reducer = (rem, piece) => (rem * Math.pow(10, digits) + piece) % divisor
This will work because of the "subtraction" remainder algorithm:
n mod d = (n - kd) mod d
which allows to replace any 'initial part' of the decimal representation of a number with its remainder, without affecting the final remainder.
The final code would look like:
function remainder(num, div) {
const digits = 9 - String(div).length;
const splitter = new RegExp(`.{1,${digits}}(?=(.{${digits}})+$)`, 'g');
const mult = Math.pow(10, digits);
const reducer = (rem, piece) => (rem * mult + piece) % div;
return str.match(splitter).map(Number).reduce(reducer, 0);
}
If you are just dividing with powers of two, you can use bitwise operators:
export function divideBy2(num) {
return [num >> 1, num & 1];
}
export function divideBy4(num) {
return [num >> 2, num & 3];
}
export function divideBy8(num) {
return [num >> 3, num & 7];
}
(The first is the quotient, the second the remainder)
function integerDivison(dividend, divisor){
this.Division = dividend/divisor;
this.Quotient = Math.floor(dividend/divisor);
this.Remainder = dividend%divisor;
this.calculate = ()=>{
return {Value:this.Division,Quotient:this.Quotient,Remainder:this.Remainder};
}
}
var divide = new integerDivison(5,2);
console.log(divide.Quotient) //to get Quotient of two value
console.log(divide.division) //to get Floating division of two value
console.log(divide.Remainder) //to get Remainder of two value
console.log(divide.calculate()) //to get object containing all the values
You can use ternary to decide how to handle positive and negative integer values as well.
var myInt = (y > 0) ? Math.floor(y/x) : Math.floor(y/x) + 1
If the number is a positive, all is fine. If the number is a negative, it will add 1 because of how Math.floor handles negatives.
This will always truncate towards zero.
Not sure if it is too late, but here it goes:
function intdiv(dividend, divisor) {
divisor = divisor - divisor % 1;
if (divisor == 0) throw new Error("division by zero");
dividend = dividend - dividend % 1;
var rem = dividend % divisor;
return {
remainder: rem,
quotient: (dividend - rem) / divisor
};
}
Calculating number of pages may be done in one step:
Math.ceil(x/y)
Here is a way to do this. (Personally I would not do it this way, but thought it was a fun way to do it for an example) The ways mentioned above are definitely better as this calls multiple functions and is therefore slower as well as takes up more room in your bundle.
function intDivide(numerator, denominator) {
return parseInt((numerator/denominator).toString().split(".")[0]);
}
let x = intDivide(4,5);
let y = intDivide(5,5);
let z = intDivide(6,5);
console.log(x);
console.log(y);
console.log(z);