Prime numbers #1 : C program.

First be clear.

What is prime number?

"A prime number is a whole number greater than 1, whose only two whole-number factors are 1 and itself. "

simply, a number that can be divided by 1 and itself only.

How to start thinking ?
Clearly , every number is divisible by 1. So , our task is to check whether the number has any other divisor except itself or not, and If not , the number is prime number.

Program #1 : Print prime numbers up to n , where n is entered by user .

So we need to check every number falling in the domain 2<=i<=n,
where i is arbitrary number starting from 2 and ending with n itself .

Now,
How to check-

• Consider numbers from 2 to that number.
• Check for divisibility of the number from 2 to the number under consideration by using %(division modulo).
• If there is a divisor apply break!
• Means number may not be prime unless the number under consideration is divisor itself.
• So if the divisor is the considered number .
• The number is prime .
• Print that number.

Now try to follow this steps for building a program and if you are unable to build copy the code given here. The code contains comments for explanation purpose . 

How to follow these procedure in c program :

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C Program : Prime numbers !

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#include < stdio.h > 
int main()
{
    int i,j,n;/*declaration of required variables , i for considering number ,j for checking divisibility and n for user entry.*/
    printf("Enter n upto which you want prime number !\n");
    scanf("%d",&n);
    printf("Required prime numbers are:\n");
    for(i=2;i < = n;i++)/*loop for considering number from 2 to n*/
    {
        for(j=2;j < = i;j++)/*loop for checking divisibility of number*/
        {
            if(i%j==0)/*the number has divisor , break here*/
                break ;
        }
            if(i==j)/*the divisor is number itself, the number is prime according to definition*/
                printf("%d ",i);/*printing the prime number*/
    }
}

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Thanking you !