CODEWITHSUNDEEP
javascript
A.B.
A.B.

Reputation: 31

misprinted number in prime function

function primeSieve() {
  for(i = 0; i <= 100; i++){
  let flag = true
    for(let j = 2; j < i/2; j++){
      if(i % j === 0){
        flag = false
      }
    }
    if(flag){
      console.log(i)
    }
  }
}

primeSieve();

Hi,

I'm studying some algos and ran into a prime sieve problem. I'm trying to print all prime numbers between 0 and 100 and it's working for the most part. However, i realized that 4 slipped in somehow and i can't figure out why for the life of me. Wondering if i can get a few pairs of eyes and see how 4 ended up being logged to the console and why that's the case.

thank you!

Upvotes: 2

Views: 36

Answers (2)

Nina Scholz
Nina Scholz

Reputation: 386746

Beside the including the value for j to check with j <= i / 2, you could omit the use of a flag and use continue with a label for the outer loop.

function primeSieve() {
    outer: for (var i = 2; i <= 100; i++) {
        for (var j = 2; j <= i / 2; j++) {
            if (i % j === 0) {
                continue outer;
            }
        }
        console.log(i);
    }
}

primeSieve();

Upvotes: 0

CertainPerformance
CertainPerformance

Reputation: 371059

Your condition in the inner loop:

for (let j = 2; j < i / 2; j++) {

is

j < i / 2

This means that when i is 4, once j gets to 2 (or, since j is always initialized to 2, before the first iteration), the loop breaks. So, without any iterations, there's never any chance for an i of 4 to get to flag = false.

Change to

for (let j = 2; j <= i / 2; j++) {

Also, per wikipedia:

A prime number (or a prime) is a natural number greater than 1 that cannot be formed by multiplying two smaller natural numbers.

So you should probably start i at 2, not 0.

Also, just like your let j, it would be good to declare i with let as well so as not to implicitly pollute the global scope:

function primeSieve() {
  for (let i = 2; i <= 100; i++) {
    let flag = true
    for (let j = 2; j <= i / 2; j++) {
      if (i % j === 0) {
        flag = false
      }
    }
    if (flag) {
      console.log(i)
    }
  }
}

primeSieve();

Upvotes: 2

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