For Loops Understanding

1

hi, I think I have a misunderstanding for the for loop or there is a technical issue. here are 2 pieces of code where fol loop is considered an infinite loop and I don’t know why
1 -

 function sumPrimes(num) {
 
  var nums = [2];
  var ss =[];
  // Not to add all numbers , but add only Primes in the Nums Array by checking if the added number / every number in the Nums array addd before it isn't out of 1 and the number itself
  for (var i = 3 ; i <= num ; i++){
    for (var j = 0 ; j < nums.length; j++){
        var di = i / nums[j];
         if (di !== Math.floor(di)){
          nums.push(i) ;
    }
  }
    //nums.push(i);
  }
  
  
  var res = nums.reduce(function (a,b){return a+b;},0);
  return nums;
}

sumPrimes(20);

2-

function sumFibs(num) {
  var fiboNums = [1,1];
  var firstFibo = 1 ;
  var secondFibo = 1 ;
  var sum = firstFibo + secondFibo;
  
  fiboNums.push(sum);
  for ( var i = 0 ; i < num ; i++){
   /* 1 + 1 == 2 ;
    1 + 2 == 3 
    2 + 3 == 5 
    3 + 5 == 8*/
    sum = sum + secondFibo;
    fiboNums.push(sum);
  }
  return fiboNums;
}

sumFibs(4);
2

So on your first function, what your doing is mutating the condition nums.length for the for() loop with nums.push(). Every time that you push to nums it increases the length and j won’t trigger the condition.

1 Like
3

In sumPrimes, the inner for-loop pushes values to the nums array. This keeps increasing the value of nums.length, and the j variable can’t catch up, so you get an infinite loop.

1 Like
4

Well, I got what you mean thank you. What about the second one ?! why it is considered as n infinite loop?

5

That’s right. What about the second piece of code ?!

6

The second one runs fine (although I presume it gives the wrong answer).

1 Like
7

What if I till U that it stops my browser from working !! the second for loop stops the browser until i open this url and change it


Cn u till me what’s wrong ?!

9

My code is in the Original Post Content. What I linked was exactly this link : “https://www.freecodecamp.org/challenges/sum-all-odd-fibonacci-numbers?run=disabled”

11

Ok that’s it

function sumFibs(num) {
  var fiboNums = [1,1];
  var firstFibo = 1 ;
  var secondFibo = 1 ;
  var sum = firstFibo + secondFibo;

  fiboNums.push(sum);
  for ( var i = 0 ; i < num ; i++){
   /* 1 + 1 == 2 ;
    1 + 2 == 3 
    2 + 3 == 5 
    3 + 5 == 8*/
    sum = sum + secondFibo;
    fiboNums.push(sum);
  }
  return fiboNums;
}

sumFibs(4);

Is this ok ?

13

Great Explanation! :smiley: I got my wrong in the logic but what i need to make sure that it is right that if the only reason of it considered infinite was it takes too long in 400000 ?!!

15

Actually, I had solved it with for loop :smiley: Thank You at all for your help and your time :slight_smile: