Sum of all primes numbers

[CrxC01]

hello,

Can someone help me please with this algorithm?

my example is 10.
(A prime number is a whole number greater than 1 with exactly two divisors: 1 and itself. For example, 2 is a prime number because it is only divisible by 1 and 2. In contrast, 4 is not prime since it is divisible by 1, 2 and 4.
Rewrite sumPrimes so it returns the sum of all prime numbers that are less than or equal to num.)


function sumPrimes(num) {
var x=2;
var added=2;
while (x<num) {
x++;
for (var i=2;i<x;i++){
if (x%i===0){
break;
}
else if (i===x-1) {
added += x;
}
}
}
return added;
}

but I am lost whem x =4.

x++; (x=3)
i=2;
(i===x-1); (2 === 3 -1 )
added += 3;
...
then
x++; x=4;
i=3;
(i===x-1); (3 === 4 -1 )
added += 4; But... 4 should not be add since is about all primes numbers <10.

Can someone help me please?
Thank you in advance!

 

[feherke]

Hi

I do not get you. For [tt]sumPrimes[teal]([/teal][purple]10[/purple][teal])[/teal][/tt] your code returns 17, which according to my calculation is correct. And no, 4 is not included in that 17, as your code correctly not adds it.


Feherke.
feherke.github.io

 

[CrxC01]

Thank you for your answear.

but I don't understand the part where:
x++; x=4;
i=3;
I don't know how my the code does not add 4 beause the condition of ``else if `` is applied:
(i===x-1); (3 === 4 -1 )
added(which is 5 ) += 4 (is reciving 4 becouse is executing this line of code else if (i===x-1) ; and added it will be now 9).
this is how i use it:

x++; x=4;
i=3;
(i===x-1); (3 === 4 -1 )
added(5)+= 4;


x++; x=5;
i=4;
(i===x-1); (4 === 5 -1 )
added(9)+= 5; and so on...

 

[feherke]

Hi

(is reciving 4 becouse is executing this line of code else if (i===x-1) ; and added it will be now 9).

Huh ? When testing x = 4 the [tt]else if (i===x-1)[/tt] line is not executed. It would be executed when i = 3, but it [tt]break[/tt]s out from the [tt]for[/tt] loop when i = 2.

Here I added some [tt]console.log()[/tt]s to your code :

[b]function[/b] [COLOR=orange]sumPrimes[/color][teal]([/teal]num[teal])[/teal]
[teal]{[/teal]
	[b]var[/b] x [teal]=[/teal] [purple]2[/purple][teal];[/teal]
	[b]var[/b] added [teal]=[/teal] [purple]2[/purple][teal];[/teal]
	[b]while[/b] [teal]([/teal]x [teal]<[/teal] num[teal]) {[/teal]
		x[teal]++;[/teal]
		[b]for[/b] [teal]([/teal][b]var[/b] i [teal]=[/teal] [purple]2[/purple][teal];[/teal] i [teal]<[/teal] x[teal];[/teal] i[teal]++) {[/teal]
			[b]if[/b] [teal]([/teal]x [teal]%[/teal] i [teal]===[/teal] [purple]0[/purple][teal]) {[/teal]
console[teal].[/teal][COLOR=orange]log[/color][teal]([/teal][i][green]'skipped'[/green][/i][teal],[/teal] x[teal],[/teal] [i][green]'because divisible with'[/green][/i][teal],[/teal] i[teal])[/teal]
				[b]break[/b][teal];[/teal]
			[teal]}[/teal] [b]else if[/b] [teal]([/teal]i [teal]===[/teal] x [teal]-[/teal] [purple]1[/purple][teal]) {[/teal]
console[teal].[/teal][COLOR=orange]log[/color][teal]([/teal][i][green]'ADDED  '[/green][/i][teal],[/teal] x[teal])[/teal]
				added [teal]+=[/teal] x[teal];[/teal]
			[teal]}[/teal]
		[teal]}[/teal]
	[teal]}[/teal]
	[b]return[/b] added[teal];[/teal]
[teal]}[/teal]

console[teal].[/teal][COLOR=orange]log[/color][teal]([/teal][i][green]'Result '[/green][/i][teal],[/teal] [COLOR=orange]sumPrimes[/color][teal]([/teal][purple]10[/purple][teal]));[/teal]

Then when running it produces this output :

[navy]master #[/navy] nodejs CrxC01.js 
ADDED   3
skipped 4 because divisible with 2
ADDED   5
skipped 6 because divisible with 2
ADDED   7
skipped 8 because divisible with 2
skipped 9 because divisible with 3
skipped 10 because divisible with 2
Result  17

Feherke.
feherke.github.io

 

[CrxC01]

THANK YOU SO MUCH!!!!

I thought that i is increased with every iteration and that’s why I thought i = 3, because when x = 4, was the second iteration and that it would be i = 3.

but now I still don't understand, if x = 5, the i is still 2? beacouse if i is still 2, the condition of (i === x - 1) is not met, (2 === 5-1) and added will not recive 5.

I am sorry if I miss something, you explanation is very good.

 

[feherke]

Hi

Unsure what exactly are you not understanding there.

There are 2 loops : the outer [tt]while[/tt] loop and the inner [tt]for[/tt] loop :
[ul]
[li]The [tt]while[/tt] loop iterates once over the values from 2 to num - 1[/li]
[li]The [tt]for[/tt] loop iterates once for each x value over the values from 2 to x - 1[/li]
[/ul]

[pre]
[silver]╔════════════╦════════════════════════════════════════╗[/silver]
[silver]║[/silver] while loop [silver]║[/silver] for loop [silver]║[/silver]
[silver]║[/silver] value of x [silver]║[/silver] value of i [silver]║[/silver] condition met [silver]║[/silver]
[silver]╠════════════╬════════════╬═══════════════════════════╣[/silver]
[silver]│[/silver] 2 [silver]│[/silver] 2 [silver]│[/silver] x divisible by i -> break [silver]│[/silver]
[silver]├────────────┼────────────┼───────────────────────────┤[/silver]
[silver]│[/silver] 3 [silver]│[/silver] 2 [silver]│[/silver] last i tested -> is prime [silver]│[/silver]
[silver]├────────────┼────────────┼───────────────────────────┤[/silver]
[silver]│[/silver] 4 [silver]│[/silver] 2 [silver]│[/silver] x divisible by i -> break [silver]│[/silver]
[silver]├────────────┼────────────┼───────────────────────────┤[/silver]
[silver]│[/silver] 5 [silver]│[/silver] 2 [silver]│[/silver] [silver]│[/silver]
[silver]│[/silver] [silver]│[/silver] 3 [silver]│[/silver] [silver]│[/silver]
[silver]│[/silver] [silver]│[/silver] 4 [silver]│[/silver] last i tested -> is prime [silver]│[/silver]
[silver]├────────────┼────────────┼───────────────────────────┤[/silver]
[silver]│[/silver] 6 [silver]│[/silver] 2 [silver]│[/silver] x divisible by i -> break [silver]│[/silver]
[silver]├────────────┼────────────┼───────────────────────────┤[/silver]
[silver]│[/silver] 7 [silver]│[/silver] 2 [silver]│[/silver] [silver]│[/silver]
[silver]│[/silver] [silver]│[/silver] 3 [silver]│[/silver] [silver]│[/silver]
[silver]│[/silver] [silver]│[/silver] 4 [silver]│[/silver] [silver]│[/silver]
[silver]│[/silver] [silver]│[/silver] 5 [silver]│[/silver] [silver]│[/silver]
[silver]│[/silver] [silver]│[/silver] 6 [silver]│[/silver] last i tested -> is prime [silver]│[/silver]
[silver]├────────────┼────────────┼───────────────────────────┤[/silver]
[silver]│[/silver] 8 [silver]│[/silver] 2 [silver]│[/silver] x divisible by i -> break [silver]│[/silver]
[silver]├────────────┼────────────┼───────────────────────────┤[/silver]
[silver]│[/silver] 9 [silver]│[/silver] 2 [silver]│[/silver] [silver]│[/silver]
[silver]│[/silver] [silver]│[/silver] 3 [silver]│[/silver] x divisible by i -> break [silver]│[/silver]
[silver]├────────────┼────────────┼───────────────────────────┤[/silver]
[silver]│[/silver] 10 [silver]│[/silver] 2 [silver]│[/silver] x divisible by i -> break [silver]│[/silver]
[silver]└────────────┴────────────┴───────────────────────────┘[/silver]

[/pre]

Feherke.
feherke.github.io

 

[CrxC01]

After I analyzed again what you told me I understand, thank you so much for taking time to explain me !!!