Array Filter not filtering

Question

I am trying to filter an array of random numbers to pull out primes.

I have a working function (isPrime) that appears to return true or false correctly but every number in the array is being "filtered" into the array returned by the filter.

I looked at the documentation for filter and watched some videos but I haven't figured it out, I think it may have to do with the function call inside of the filter. Can you tell me where my error is?

numArray = [];

for(i=0; i<1000; i++){
  numArray.push(Math.random(1000));
}

const isPrime = (num) => {
  for(i=2;i<num;i++){
    if(num % i === 0){
      return false;
    }
  }
  return true;
}

const results = numArray.filter(value => isPrime(value));
  
for(num in results){
  console.log(num);
  console.log(isPrime(num)) //added to demonstrate function is working and should return false to the filter                        
}

Math.random(1000) doesn't return integer, you have to round it. — Chris Li, Mar 05 '20 at 19:54
Math.random doesn’t work like that. All your numbers will be floats between 0 and almost 1 — James, Mar 05 '20 at 19:55
@ChrisLi Thank you, yes that does appear to be the reason that my program is not working but in addition it appears I fumbled checking my results as well in the last for loop by referencing the index of the number stored in numArray instead of the value as pointed out in the accepted answer — DeSync, Mar 05 '20 at 20:16

score 3 · Accepted Answer · edited Jun 20 '20 at 09:12

You are generating random number in wrong way. Your code will only create an array of floating points less than 1 so according to your isPrime they all will be prime.

Note: Don't use variables without let or cosnt it can cause many problems in the code.

How to create random numbers?

Math.random() is function which returns a floating point number between 0 and 1.
You multiply that number with the max range you want to have.
After multiplying it will still be a floating number. So use Math.floor() to round the random float to an integer

let numArray = [];

for(let i=0; i<10; i++){
  numArray.push(Math.floor(Math.random() * 30));
}

const isPrime = (num) => {
  for(let i=2;i<num;i++){
    if(num % i === 0){
      return false;
    }
  }
  return true;
}

const results = numArray.filter(value => isPrime(value));

console.log(JSON.stringify(numArray))
console.log(JSON.stringify(results));

Answering OP's question that

"Why bottom for loop used to check the code gives numbers 5, 6 ... 997, 998"

In javascript arrays are basically objects. The for..in is a loop which iterates over the keys of the object not the values. So in case arrays keys are the indexes of the array which are 0...999 in above case. So it was logging those indexes

Oh wow, thank you! Do you know why the bottom for loop that I was using to check the program shows the numbers as 5, 6 ... 997, 998 etc then if they were numbers less than one, and the isPrime function was working as intended in the bottom loop as well? — DeSync, Mar 05 '20 at 19:58
@DeSync Updated the post for explanation of that. If you want anything else feel free to ask? — Maheer Ali, Mar 05 '20 at 20:03

Array Filter not filtering

1 Answers1

How to create random numbers?