How to find Prime Number || Basic Programming Questions Series

In this video, we will learn how to write a program to check the given number is prime or not.

Learn:
1. check the given number is prime or not.
2. print all the prime numbers upto a given a number
3. write different test cases

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[manishpatidar]

perfect coding with all negative test cases

[kshitijagaikwad]

sir you are such a genius...your explanation is too good very easy to understand. Thank u so much sir.
sir how to develop our coding or logic ??? please suggest sir its difficult to write logic. How can I improve it???

[sarakhalifa]

I tried your logic in isPrimeNumber without the for loop, I just put (if(num%2==0). i dont think I need the for loop, please clarify

[dimplekumari]

Thank u so much sir for providing such good collection of videos..

[igoril]

@Naveen AutomationLabs, again huge thanks ! Iknow u didnt use it here but still : if in isPrime() i say i<num/2 in the loop and i pass any number greater than 4 to thegetPrimeNumbers() - 4 gets printed ! And its not prime . Only if i say i<=num/2 in the isPrime() for loop then it works correctly, why ? Im breaking my brain . Im very close to understanding but cant nail it .
p.s Watching all your videos, youre the best !

[jayalakshmig]

Can you please make more videos on programming which will be asked for testers in interview

[samuam]

You are the best Naveen, in all your videos that is! Popular in NYC!! Whats the best way to contact you for some offers?

[Tidda]

One question Naveen, i saw some programs on internet for checking prime numbers and there logic is including i<num/2, so why num/2, has been written in for loop, will u pls explain.

[myamazingvideos]

/**
* This program has two methods, one of which tells if a number is prime or not
* and the second method returns all the list of prime numbers less than the number you've enetered
*/
package introduction;

/**
* @author saksham
*
*/
public class PrimeNumbers {

/**
* @param args
*/
public static void main(String[] args) {

int i = 23;
if (isPrime(i) == true) {
System.out.print(
"Output 1 : The number you've entered is also prime and all the prime numbers less than and equal to the given number are -> \n");
listPrimenumbers(i);
System.out.println("\n");

} else if (isPrime(i) == false) {
System.out.print("Output 1 : Although the number you've entered (" + i + ") is not a prime number, "
+ "all the prime numbers below that number is/are -> \n");
listPrimenumbers(i);
System.out.println("\n");

}

int j = 24;
if (isPrime(j) == true) {
System.out.print(
"Output 2 : The number you've entered is also prime and all the prime numbers less than and equal to the given number are -> \n");
listPrimenumbers(j);
System.out.println("\n");

} else if (isPrime(j) == false) {
System.out.print("Output 2 : Although the number you've entered (" + j + ") is not a prime number, "
+ "all the prime numbers below that number is/are -> \n");
listPrimenumbers(j);
System.out.println("\n");

}

}

public static boolean isPrime(int num) {
if (num <= 1) {
return false;
}

for (int j = 2; j < num; j++) {
if (num % j == 0) {
return false;
}

}
return true;
}

public static void listPrimenumbers(int num) {

for (int i = 1; i <= num; i++) {
if (isPrime(i) == true) {
System.out.print(i + ", ");

}
}
}

}



Output is :
Output 1 : The number you've entered is also prime and all the prime numbers less than and equal to the given number are ->
2, 3, 5, 7, 11, 13, 17, 19, 23,

Output 2 : Although the number you've entered (24) is not a prime number, all the prime numbers below that number is/are ->
2, 3, 5, 7, 11, 13, 17, 19, 23,



Thanks @Naveen

[diptmangupta]

Well explained ! Just one clarification: If we run the loop till num/2 that will also work ? bcz if any number is not divisible by half of its values it won't be divisible anyways ?

[ashwinikunapareddy]

Y did u change it from num to i in second static method.. If it is i how it will be calling the first static method. Please clarify

[shobhitkrishan]

I was getting an error while calling a non-static method in a static method. It seems Java doesn't support this.

[sunilgowda]

how to print prime numbers by using scanner class?

[rqbyhco]

class HelloWorld {
public static boolean isPrime(int num)
{
if(num<=1)
{
return false;
}
else
{
return true;
}
}
public static void main(String[] args) {
System.out.println( "3 is a Prime number : " +isPrime(3));
System.out.println( "0 is a Prime number : " +isPrime(0));
}
}
We could have done like this way?

[nabilelassaad]

Why do u declare the method as static ?

Wrong
If we give num =15
15 % 2 == 0
1 == 0
So the condition is false
then it returns true
Can you please explain me