CODEWITHSUNDEEP
javascriptif-statementfor-loopprimes
book_lover63
book_lover63

Reputation: 25

for loop unintentionally breaking at if statement

I am trying to determine the largest prime factor of a number by working backwards from the largest possible factor. Once a factor is found, I test to see if it is prime by using the PrimeTest3 function within the original function.

However, it is not giving me the answer I am expecting for the number 13195. When I test the code as shown above using the 'this passed the test' statement, I can see that it is skipping from the first factor found (2639) to the last factor found (5), and strangely, when logging the the result of passing i through PrimeTest3, it shows up as false, even though it had to have been true to pass the if statement in the first place. 

var largestPrimeFactor3 = function (num) {
    function PrimeTest3(a){
        if (a<=1 || a%1!=0) 
            return false;

        limit = Math.ceil(Math.pow(a,.5));

        if (a%2==0 || a%3==0) 
            return false;
        if (a==2 || a==3) 
            return true;

        for (i=6;i<limit;i+=6){
            if (a%(i-1)==0) 
                return false;
            if (a%(i+1)==0) 
                return false;
        }
        return true;
    }
    for(var i = Math.floor(num/2); i>0; i--){
        console.log(i);
        if(num % i === 0 && PrimeTest3(i)){
            console.log('this passed the test:' + PrimeTest3(i));
            return true;
        }
    }
}
console.log(largestPrimeFactor3(13195));

Would really appreciate any help or clarification. Thanks!!

Upvotes: 1

Views: 80

Answers (2)

Barmar
Barmar

Reputation: 782130

The for loop inside PrimeTest3 is using the same variable i as the loop in largestPrimeFactor3. You need to declare this variable as local to the internal function, with a var declaration.

var largestPrimeFactor3 = function (num) {
    function PrimeTest3(a){
        if (a<=1 || a%1!=0) 
            return false;

        limit = Math.ceil(Math.pow(a,.5));

        if (a%2==0 || a%3==0) 
            return false;
        if (a==2 || a==3) 
            return true;

        for (var i=6;i<limit;i+=6){
            if (a%(i-1)==0) 
                return false;
            if (a%(i+1)==0) 
                return false;
        }
        return true;
    }
    for(var i = Math.floor(num/2); i>0; i--){
        console.log(i);
        if(num % i === 0 && PrimeTest3(i)){
            console.log('this passed the test:' + PrimeTest3(i));
            return true;
        }
    }
}
console.log(largestPrimeFactor3(13195));

Upvotes: 2

David Sherret
David Sherret

Reputation: 106800

You're having unexpected results because you're changing the value of i in PrimeTest3.

if(num % i === 0 && PrimeTest3(i)){ // i changed in PrimeTest3
    // now i is not what it was when originally passed in PrimeTest3
    console.log('this passed the test:' + PrimeTest3(i));

A fix would be to change:

for (i=6;i<limit;i+=6){

To:

for (var i=6;i<limit;i+=6){

This will localize the i variable to not change the other i variable outside PrimeTest3's function scope.

While you're at it, localize limit to the scope of PrimeTest3 by doing var limit = ... instead of just limit = ....

Upvotes: 0

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