I've written a code that outputs even and odd numbers from 1 to 20, i need help in writing prime numbers, anybody aware?

Question

Prime numbers between 1 and 20, C

Answer 1

//prime numbers from 1 to n in C
int n = 20;
for (int i =2;i<n;i++){
int isprime=1;
for (int j=2;j<i;j++){
if (i!=j && i%j==0)
isprime=0;
}
if(isprime) printf("%d ",i);

}

Answer 2

You can search Code Playground for examples and inspirations 👍

Answer 3

i made a program once to calculate prime numbers. basically, i had a “primes” list with just the number 2 in it, and then i started at 3 and went through all the numbers up to 20 (or whatever). on each number, i checked through all the numbers currently in “primes” to see if it was integer divisible by any of them. if it was, i broke the loop and moved on. if it made it through, it got appended to the primes list.

Answer 4

Thank you, but I'm afraid that doesn't satisfy my question...

Answer 5

Sorry about that, but usually we search Code Playground for codes, and here in Q&A forum we ask questions.

Answer 6

It got it, thank you...

Answer 7

AssasinStudent that code works :-)

Answer 8

https://code.sololearn.com/cxqEhwzYiOss/?ref=app
https://code.sololearn.com/cIwb0TDRuWm3/?ref=app

Answer 9

Prime numbers

Only 2 is the even prime number so it needs to be included.

From odd prime number only those are primes whose factors are integer apart from 1 and odd number it self.

9 is not prime it has integer factor 3 apart from 1 and 9

15 is not prime it has integer factor 3 and 5 apart from 1 and 15.

DHANANJAY

Answer 10

… I did not understand that. Are you trying to define prime numbers, or is this actually a formula for getting prime numbers more easily than we’ve been suggesting? I’ve been wondering if there was such a formula for a long time.

Answer 11

Wilbur Jaywright
I believe this is what he's talking about.

https://code.sololearn.com/cw3lO1sgEYGP/?ref=app

Answer 12

Which languange specifically Ochieng?

I think concept on modulus can serve it best😊

Answer 13

hey, what do you know? had it on my phone! good ole’ program 😊. https://code.sololearn.com/cVc3Vay3utXI/?ref=app

Answer 14

@deleted
you can make your code faster using the observation that the highest divisor of a number besides itself is smaller or equal to the square root of the number. In your code you can replace:
for (int j=2;j<i;j++){
with
for (int j=2; j*j<=i;j++){

Answer 15

Eddy M you just explained the theory to me. 😄

Answer 16

@Wilbur Jaywright I'm happy if you found it useful.

Answer 17

Tabular method is better than

1 x 1 => 1
3 x 1 => 3
7 x 1 => 7
9 x 1 => 9
1 x 3 => 3
3 x 3 => 9
7 x 3 => 1
9 x 3 => 7
1 x 7 => 7
3 x 7 => 1
7 x 7 => 9
9 x 7 => 3
1 x 9 => 9
3 x 9 => 7
7 x 9 => 3
9 x 9 => 1

After 1, 2, 5 as prime numbers

Last digits for decimal primes are 1, 3, 7 and 9

It can be used for big primeness search

DHANANJAY

Answer 18

35 will return as prime with that code. :-(

Answer 19

Philip Ochieng

If small prime needs with range the Tabular method is good

Table needs to be made using digit sum and last digit

1 3 7 9
1
2
3
4
5
6
7
8
9

columns are last digit
And row says the digit some for each round of numbers

table needs to be prepared ones and used till application is running.

DHANANJAY

Answer 20

Use the c language