CODEWITHSUNDEEP
javascriptfor-loopif-statementprimes
LebeneBou
LebeneBou

Reputation: 29

(Javascript) Prime numbers in a given interval: for loop doesn't iterate

In the code below is a prime test function (returns true if prime and false if not). I am using it to print out prime numbers between 5 and 20.

The expected output is obviously 5,7,11,13,17,19

The for loop doesn't seem to iterate and just stays at 5. The output is as if the script is stuck inside an infinite while loop.

I don't know what I'm doing wrong, maybe the if statement isn't written correctly? Still a bit new to javascript.

function is_prime(number){

    if (number==2 || number==3){
        return true;
    }
    
    if (number==0 || number==1 || number%2==0){
        return false;
    }

    if ((number+1)%6!=0 && (number-1)%6!=0){
        return false;
    }
    
    for(i=3; i-1<=parseInt(number)**.5; i+=2){

        if (number%i==0){
            return false;
        }
    }
    
    return true;
}


for(i=5;i<=20;i++){

    if(is_prime(i)){

        console.log(i)
    }
}

Upvotes: 1

Views: 180

Answers (6)

Joseph Mark
Joseph Mark

Reputation: 1

    const lowerNumber = 2;
    const higherNumber = 100;

        for(let i = lowerNumber; i <= higherNumber; i++){
            let flag = 0;
            for(let j = 2; j < i; j++){
                if(i % j == 0){
                    flag = 1;
                    break;
                }
            }
            if(i > 1 && flag == 0){
                console.log(i);
            }
        }

Upvotes: -1

zcoop98
zcoop98

Reputation: 3087

In addition to using let in your for loop initializers, which is just good practice anyway, it's often well-advised to use different names for different variables, for a number of reasons:

  • It makes your variable usage less ambiguous
  • Helps prevent confusion later on (by both yourself and other readers of your code!)
  • Makes debugging easier (If you're only using i in one place, then you know where to look if it you get an unexpected value in i)

As an example, if I'm nesting loops, or have multiple loops in a script, I'll often start with i and work my way down the alphabet, j, k, l, etc.

Essentially any pattern you use is fine, as long as it helps you keep track of your data!

Upvotes: 0

Matiiv Vasyl
Matiiv Vasyl

Reputation: 127

Because you create a global variable in your loop. Declare the variable with let and everything will work fine. P.S. use strict mode will not allow you to do some stupid mistakes again

Upvotes: 0

user9613905
user9613905

Reputation:

You're not declaring your variable i, so JavaScript is creating it in the global scope. In your function you also haven't declared it so it's using the same variable, and the loops don't execute as you expect.

Declare the variables with let so that they don't clash, as in

for (let i=5;...

See Let on MDN

Upvotes: 2

Med Mhamdi
Med Mhamdi

Reputation: 3

Just add (let i = 5 in the for loop you need to declare that i

for(let i=5;i<=20;i++){

    if(is_prime(i)){

        console.log(i)
    }

Upvotes: 0

Randy Casburn
Randy Casburn

Reputation: 14175

You've created a global variable named i in your for() loop - then you change that same variable in the function.

Use let i in both cases.

Upvotes: 3

Related Questions