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how to build count and say problem in javascript

I am trying to solve the below problem in JavaScript
The count-and-say sequence is the sequence of integers beginning as follows:
1, 11, 21, 1211, 111221, ...
1 is read off as one 1 or 11.
11 is read off as two 1s or 21.
21 is read off as one 2, then one 1 or 1211.
Given an integer n, generate the nth sequence.
Note: The sequence of integers will be represented as a string.
Example:
if n = 2,
the sequence is 11.
So I want to create a function which pass N integer and gives it value
Here is my code:
let countAndSay = function (A) {
if (A == 1) return "1"
if (A == 2) return "11"
let str ="11"
if(A > 2){
// count
}
}
I don't understand the logic for how to build this.

You need to be able to dynamically determine the number and type of chunks a string has, which can be done pretty concisely with a regular expression. To come up with the string to deconstruct on index n, recursively call countAndSay to get the result of n - 1:
let countAndSay = function (count) {
if (count === 1) {
return '1';
}
const digitsArr = countAndSay(count - 1).match(/(\d)\1*/g);
// You now have an array of each chunk to construct
// eg, from 1211, you get
// ['1', '2', '11']
return digitsArr // Turn the above into ['11', '12', '21']:
.map(digitStr => digitStr.length + digitStr[0])
.join(''); // Turn the above into '111221'
};
console.log(
countAndSay(1),
countAndSay(2),
countAndSay(3),
countAndSay(4),
countAndSay(5),
);

Here's a function that generates the next string of numbers based on the previous string you feed it:
function next(s) {
s = s + "*"; // add a flag value at the end of the string
var output = "";
var j = 0;
for (var i = 1; i < s.length; i++) {
if (s.charAt(i) != s.charAt(i - 1)) { // if the character changes, concatenate the amount (i - j) and the digit
output += (i - j) + s.substring(j, j+1);
j = i;
}
}
return output;
}
Then you'd need to recursively run next N times.

Highest occurrence in javascript

Hi my knowledge on javascript is very limited and basic. Basically below is I will prompt a pop-up that displays the answer to the value. The thing is from the coding I found below if I had to insert an array lets say 1,2,3,2 the output would be , since it has the highest occurrence in the array. Is there a way to edit this code so that the answer to the input above would be 2
I have done my fair share of research:
Here are the links:
Highest occurrence in an array or first selected
Get the element with the highest occurrence in an array
https://www.sitepoint.com/community/t/numbers-only-input-values-in-text-box-how/3029
Code:
<script type="text/javascript">
function evaluate() {
var input = prompt("Please enter your input");
var array = new Array();
function mode(array)
{
if(array.length == 0)
return null;
var modeMap = {};
var maxEl = array[0], maxCount = 1;
for(var i = 0; i < array.length; i++)
{
var el = array[i];
if(modeMap[el] == null)
modeMap[el] = 1;
else
modeMap[el]++;
if(modeMap[el] > maxCount)
{
maxEl = el;
maxCount = modeMap[el];
}
}
return maxEl;
}
document.writeln("Your calculation is: ");
document.writeln(mode(input) + " with a starting input string of: " + input);
}
</script>
<script type="text/javascript">
evaluate();
</script>

You want to convert the string '1,2,3,2' into the array [ 1, 2, 3, 2 ] first. this can be done with the split function. You probably also want to trim each element in case someone formats them with a space.
function evaluate() {
const input = prompt("Please enter the array of integers")
.split(',')
.map(item => item.trim());
function mode(items) {
const counts = items
.reduce((counts, item) => {
const currentItemCount = counts.get(item) || 0;
return counts.set(item, currentItemCount + 1);
}, new Map());
const maxEntry = Array.from(counts.entries())
.reduce((maxEntry, entry) => {
return entry[1] > maxEntry[1] ? entry : maxEntry;
});
return maxEntry[0];
}
document.writeln("Your calculation is: ");
document.writeln(mode(input) + " with a starting input string of: " + input);
}
evaluate();

Your issue stems from the fact that you never convert your input (which you receive from prompt as a string) into an actual array.
When mode is called directly on the string, the comma is returned as being the most common because comma is the most common character in the string.
To fix this, you need to convert your string into an actual array so you're operating on the elements of the array instead of the characters of the string.
You can use the split function to split your string ("1,2,3,2") at commas into an array (["1", "2", "3", "2"]) which you can then pass to the mode function:
mode(input.split(","))

Using two for loops to compare two strings

I am working through exercises on exercism.io and the third one asks us to compare two DNA strings and return the difference (hamming distance) between them.
So for example:
GAGCCTACTAACGGGAT
CATCGTAATGACGGCCT
^ ^ ^ ^ ^ ^^
There are 7 different characters lined up in that comparison. My question is whether I'm taking the right approach to solve this. I created two empty arrays, created a function that loops through both strings and pushes the different letters when they meet.
I tried running it through a console and I always get an unexpected input error.
var diff = [];
var same = [];
function ham(dna1, dna2) {
for (var i = 0; i < dna1.length; i++)
for (var j = 0; j < dna2.length; i++){
if (dna1[i] !== dna2[j]) {
console.log(dna1[i]);
diff.push(dna1[i]);
}
else {
console.log(dna1[i]);
same.push(dna1[i]);
}
return diff.length;
}
ham("GAGCCTACTAACGGGAT", "CATCGTAATGACGGCCT");
console.log("The Hamming distance between both DNA types is " + diff.length + ".");

Do not use globals.
Do not use nested loops if you don't have to.
Do not store useless things in arrays.
function ham(dna1, dna2) {
if (dna1.length !== dna2.length) throw new Error("Strings have different length.");
var diff = 0;
for (var i = 0; i < dna1.length; ++i) {
if (dna1[i] !== dna2[i]) {
++diff;
}
}
return diff;
}
var diff = ham("GAGCCTACTAACGGGAT", "CATCGTAATGACGGCCT");
console.log("The Hamming distance between both DNA types is " + diff + ".");

The first problem is that you're missing a closing }. I think you want it right before the return statement.
secondly, there's a problem with your algorithm. You compare every item in dna1 (i) with every item in dna2 instead of coparing the item in the same position.
To use a shorter example so we can step through it, consider comparing 'CAT' and 'CBT'. you want to compare the characters in the same position in each string. So you don't actually want 2 for loops, you only want 1. You'd compare C to C ([0]), A to B ([1]), and T to T ( [2] ) to find the 1 difference at [1]. Now step through that with your 2 for loops in your head, and you'll see that you'll get many more differences than exist.
Once you use the same offset for the characters in each string to compare, you have to stat worrying that one might be shorter than the other. You'll get an error if you try to use an offset at the end of the string. So we have to take that into account too, and assumedly count the difference between string length as differences. But perhaps this is out of scope for you, and the the strings will always be the same.

You only need to have one single loop like below:
var diff = [];
var same = [];
function ham(dna1, dna2) {
for (var i = 0; i < dna1.length; i++) {
if (dna1[i] !== dna2[i]) {
console.log("not same");
diff.push(dna1[i]);
} else {
console.log("same");
same.push(dna1[i]);
}
}
return diff.length;
}
ham("GAGCCTACTAACGGGAT", "CATCGTAATGACGGCCT");
console.log("The Hamming distance between both DNA types is " + diff.length + ".");

The edit distance is not really hard to calculate. More code is needed to cover the edge cases in parameter values.
function hamming(str1, str2) {
var i, len, distance = 0;
// argument validity check
if (typeof str1 === "undefined" || typeof str2 === "undefined") return;
if (str1 === null || str2 === null) return;
// all other argument types are assumed to be meant as strings
str1 = str1.toString();
str2 = str2.toString();
// the longer string governs the maximum edit distance
len = str1.length > str2.length ? str1.length : str2.length;
// now we can compare
for (i = 0; i < len; i++) {
if ( !(str1[i] === str2[i]) ) distance++;
}
return distance;
}

Execution of function:
ham( "GAGCCTACTAACGGGAT", "CATCGTAATGACGGCCT" );
of the following function definition:
function ham(A,B){
var D = [], i = 0;
i = A.length > B.length ? A : B;
for( var x in i)
A[x] == B[x] ? D.push(" ") : D.push("^");
console.log( A + "\n" + B +"\n" + D.join("") );
}
will output the log of:
GAGCCTACTAACGGGAT
CATCGTAATGACGGCCT
^ ^ ^ ^ ^ ^^
Is capable of receiving different length strings, which depending on the requirement and data representation comparison can be modified to fill the blank with adequate standard symbols etc.
Demo:
ham("GAGCCTACTAACGGGAT", "CATCGTAATGACGGCCT");
function ham(A, B) {
var D = [],
i = 0;
i = A.length > B.length ? A : B;
for (var x in i)
A[x] == B[x] ? D.push(" ") : D.push("^");
console.log(A + "\n" + B + "\n" + D.join(""));
};

I think that you would want to do something like this:
var dna1 = "GAGCCTACTAACGGGAT";
var dna2 = "CATCGTAATGACGGCCT";
function ham(string1, string2) {
var counter = 0;
for (i = 0;i < string1.length;i++) {
if (string1.slice(i, i + 1) != string2.slice(i, i + 1)) {
counter++
};
};
return(counter);
};
console.log("insert text here " + ham(dna1, dna2));
It checks each character of the string against the corresponding character of the other string, and adds 1 to the counter whenever the 2 characters are not equal.

You can use Array#reduce to iterate the 1st string, by using Function#call, and compare each letter to the letter of the corresponding index in the 2nd string.
function ham(dna1, dna2) {
return [].reduce.call(dna1, function(count, l, i) {
return l !== dna2[i] ? count + 1 : count;
}, 0);
}
var diff =ham("GAGCCTACTAACGGGAT", "CATCGTAATGACGGCCT");
console.log("The Hamming distance between both DNA types is " + diff + ".");

Validation for numeric data Javascript

I'm writing a program in Javascript that separates even and odd numbers, puts them into an array, adds the sum of numbers, and finds the average.
I'm having an issue not allowing zeros not to count. Because its adding to the array, and when the user types in 6+6, sum is 12, average is calculating to 4 because of the extra 0 in the array.
Is there anyway to not allow the zeros to count? Here is what I have so far..
var evenarray = [];
var oddarray = [];
var avgEven = 0;
var avgOdd = 0;
var isValid;
function numberFunction(){
do
{
var numbers = prompt("Please enter numbers. Enter empty string to exit.");
if(numbers % 2 == 0)
{
evenarray.push(numbers);
var sumEven = 0;
for (var i=0; i < evenarray.length; i++)
{
sumEven = sumEven + Number(evenarray[i]);
}
var avgEven = sumEven/evenarray.length;
//alert("even");
}
if(numbers % 2 !== 0)
{
oddarray.push(numbers);
var sumOdd = 0;
for (var i=0; i < oddarray.length; i++)
{
sumOdd = sumOdd + Number(oddarray[i]);
}
var avgOdd = sumOdd/oddarray.length;
//alert("odd");
}
//if(isNaN(numbers)){
//alert("Only numeric data only");
//}
}
while(numbers !== "");

Just do nothing when the number is actually 0:
if (numbers == 0)
{
}
else if(numbers % 2 == 0)
{
evenarray.push(numbers);
var sumEven = 0;
for (var i=0; i < evenarray.length; i++)
{
sumEven = sumEven + Number(evenarray[i]);
}
var avgEven = sumEven/evenarray.length;
}
else // only odds remain
{
oddarray.push(numbers);
var sumOdd = 0;
for (var i=0; i < oddarray.length; i++)
{
sumOdd = sumOdd + Number(oddarray[i]);
}
var avgOdd = sumOdd/oddarray.length;
}

You can do :
if(numbers % 2 == 0 && numbers !=0) ...
if(numbers % 2 != 0 && numbers !=0) ...
so that you don't do anything when numbers == 0;
It's a little strange to call your variable numbers instead of number.

your function should be,
function numberFunction(){
do
{
var numbers = prompt("Please enter numbers. Enter empty string to exit.");
if(numbers !=0 && !isNaN(numbers))
(numbers %2 == 0)? (evenarray.push(parseInt(numbers))) : (oddarray.push(parseInt(numbers)));
}while(numbers !== "");
for(var i = 0; i < evenarray.length; i++)
sumEven += evenarray[i];
for(var i = 0; i < oddarray.length; i++)
sumOdd += oddarray[i];
avgEven = sumEven / evenarray.length;
avgOdd = sumOdd / oddarray.length;
document.getElementById("even").innerHTML = evenarray.toString();
document.getElementById("sumEvenTotal").innerHTML = sumEven.toString(); //displays sum of even numbers.
document.getElementById("averageOdd").innerHTML = avgOdd; //displays average of odd numbers.
document.getElementById("averageEven").innerHTML = avgEven; //diplays average of even numbers.
document.getElementById("odd").innerHTML = oddarray.toString(); //displays all odd numbers that were entered.
document.getElementById("sumOddTotal").innerHTML = sumOdd.toString();
}

As you already have other answers with solutions to your particular issue, I would suggest a different approach. Think of the data you're manipulating: an array. Try to solve the issue only with data, no user input, no DOM manipulation; just data. This helps to separate concerns, and make your code easier to understand.
Since we're working with arrays, we can make use of some of the built-in JavaScript methods that are present in modern browsers, such as filter and reduce. These methods are in a way, alternatives to for loops, with some pre-defined behavior, and a callback function.
Now, let's think of the steps involved in solving your problem.
Get numbers from the user. We can represent this data as an array, as you were already doing.
We want all odd numbers, their sum and average.
We want all even numbers, their sum and average.
We display the data to the user.
In this solution I'm assuming you already have an array with the data, and will be focusing on points 2 and 3. Remember, think of data, user interaction shouldn't be mixed with your data logic. Instead of asking the user for a number on each loop, you could ask the user for a list of numbers directly; you avoid multiple prompts this way, and it lets you separate data and interaction nicely. Ideally you'd validate all user input to match your requirements.
// Helpers to work with numbers
var odd = function(x) {
return x % 2 === 0;
};
var even = function(x) {
return x % 2 !== 0;
};
var add = function(x, y) {
return x + y;
};
function solve(ns) {
// Solve the problem
// with odd or even numbers
var result = function(fn) {
var xs = ns.filter(fn); // odd or even
var sum = xs.reduce(add);
return {
numbers: xs,
sum: sum,
average: sum / xs.length
};
};
// Return an object
// with odd and even results
return {
odd: result(odd),
even: result(even)
};
}
var numbers = [1,2,3,4]; // from user input
var result = solve(numbers);
console.log(result.odd);
//^ {numbers: [2,4], sum: 6, average: 3}
console.log(result.even);
//^ {numbers: [1,2], sum: 4, average: 2}

C scanf equivalent in nodejs

If I want to read from the input stream in C I write scanf., Is there equivalent method in NodeJS to do the same?
For example, here's the code in C
int n,
m,
i;
scanf("%d", &n);
for (i = 0; i < n; i++) {
scanf("%d", &m);
............
}
Here's where I'm starting from in Node... TODO indicates where I'm stuck:
process.stdin.resume();
process.stdin.setEncoding("ascii");
process.stdin.on("data", function (input) {
var n = +input;
for (var i = 0; i < n; i++) {
// TODO
}
});

For starters, calling scanf and the data event for a readable stream in NodeJS are not equivalent. In the NodeJS example, you will need to parse the chunk of the input you've received.
You can examine how these chunks are sent to you by replacing the body of your on method with a simple:
process.stdout.write('onData: ' + input + '\n');
Given how input then contains your data you'll need to use some method to extract the string of interest and then use parseInt. Perhaps a naive approach to your problem, assuming 1 integer per input:
var n = 0;
var m = 0;
var state = 0;
process.stdin.on('data', function (input) {
switch (state)
{
case 0:
// we're reading 'n'
n = parseInt(input.trim(), 10);
state++;
break;
default:
// we're reading 'm'
m = parseInt(input.trim(), 10);
if (state++ == n)
{
// we've read every 'm'
process.exit();
}
break;
}
});
I'm not a terribly large fan of this means of getting data to your NodeJS event loop, you should instead look to command line arguments, configuration/input files, or some other means.

Check sget.
var sget = require('./sget');
var width = sget('Width?'),
height = sget('Height?'),
area = width * height;
console.log('Area is', area);

This function will do what you asked for:
function readNums(s) {
// Split it up into numbers and spaces
var array = s.split(/(\d+)/);
// Keep just the numbers
array = array.filter(function(i) {return "" + +i == i});
// Convert back to a number
array = array.map(function(i) {return +i});
// How many elements should there have been?
var maxLen = array.shift();
if (array.length < maxLen) {
throw "Not enough enough numbers";
} else {
array.length = maxLen;
}
return array;
}
console.log(readNums("4 10 20 30 40 50 60 70"));
Result:
[10, 20, 30, 40]